Category: Guides

Start with the room, not the square footage

A 400 sq ft living room is almost never a 20 by 20 square. The one we are working through here is 19 ft 6 in by 20 ft 6 in, which is 399.75 sq ft. That half foot matters, because laminate is bought in whole boxes and installed in whole rows, and both of those round in ways plain area math hides.

The box count is the easy half. The row layout is the half that ruins a Saturday.

The box formula

Boxes = round up ( room area x (1 + waste factor) / coverage per box )

Waste for a straight lay in a simple rectangular room runs about 7 percent. Coverage per box is printed on the carton and it is not standardized. Laminate cartons commonly land anywhere from 18 to 24 sq ft depending on plank thickness, width and how many boards the manufacturer stuffs in. Two products that look identical on the rack can be three boxes apart on the same floor.

For our room: 399.75 x 1.07 = 427.7, call it 428 sq ft of material needed.

Coverage printed on the box	428 sq ft divided by coverage	Boxes to buy	Actual material
18.0 sq ft	23.78	24	432.0 sq ft
20.15 sq ft	21.24	22	443.3 sq ft
22.0 sq ft	19.45	20	440.0 sq ft
24.0 sq ft	17.83	18	432.0 sq ft

We will run the rest of this example on a common 12 mm plank, 7-1/2 in wide by 47-3/4 in long, 8 planks per carton, 20.15 sq ft per box. That is 22 boxes.

The first and last row problem

Here is what the box count does not tell you. Rows accumulate across the room in one direction, and the last row is whatever is left over. If that leftover is a sliver, the floor looks wrong and the sliver is a nightmare to click in.

Run the planks the long way, so the rows stack across the 19 ft 6 in dimension. That is 234 in. Take off the expansion gap at both walls, commonly 1/4 to 3/8 in for laminate, so 3/8 in each side:

234 – 0.75 = 233.25 in of floor to fill with 7.5 in rows

233.25 / 7.5 = 31.1 rows. So 31 full rows cover 232.5 in and the last row is 0.75 in wide. Three quarters of an inch. You cannot cut that, you cannot lock it, and even if you glued it in it would read as a mistake from the doorway.

The fix costs no extra material. Take one full plank width plus the leftover and split it between the first and last row:

(7.5 + 0.75) / 2 = 4.125 in

Rip the first row to 4-1/8 in, run 30 full rows in the field, and the last row also lands at 4-1/8 in. Check: (30 x 7.5) + 4.125 + 4.125 = 233.25. Same 32 rows, same material, two balanced edges instead of one ugly one.

The working rule is that no edge row should finish under about 2 in. Under that width the tongue or groove has almost nothing left to hold, the locking profile tends to break out during assembly, and the strip flexes underfoot. If your leftover comes out between 2 in and a full plank, leave it alone. If it comes out under 2 in, balance it.

Rows, planks and end cuts

Now the other direction. The room is 20 ft 6 in, or 246 in, minus the same 3/8 in at each end is 245.25 in of run per row. At 47-3/4 in per plank, that is 5 full planks (238.75 in) plus a 6-1/2 in end piece.

Item	Number
Rows across the room	32 (30 full width, 2 ripped to 4-1/8 in)
Full planks per row	5
End piece per row	6-1/2 in
Planks in 22 boxes	176
Planks consumed if every row started fresh	192

That second-to-last line is why offcut discipline matters. Straight through, 32 rows x 6 planks is 192 planks and you only bought 176. You close the gap by starting each new row with the offcut from the previous row. That also handles the stagger requirement, since most manufacturers want end joints in adjacent rows separated by a minimum distance, commonly 8 to 12 in. Check the sheet in the box for the number your product wants, because a 6-1/2 in offcut is below some minimums and cannot legally start a row under those instructions.

Transitions and the perimeter

A floating laminate floor needs a gap at every vertical obstruction, not just the four walls. That means door jambs, pipe penetrations, the fireplace hearth, and the island toe kick. Undercut the door jambs and casing with a jamb saw and slide the plank under, rather than scribing around them, and you get the gap and a clean line at the same time.

Doorways get a transition unless the same floor continues through. Measure each opening and order separately: a reducer where laminate meets something lower, a T-molding where the same floor continues but the run is broken, a stair nose at any drop. Nobody counts these until the floor is down.

What we would actually buy

Twenty-two boxes at 20.15 sq ft, plus one spare box set aside in the closet for future repairs. Laminate runs change, and a plank pulled from a box with the same run number three years from now will match. One pulled from a new run probably will not.

Let the material acclimate in the room before installation. Many manufacturers specify 48 to 72 hours with the boxes flat and the room at normal living temperature and humidity, and skipping it is a common warranty exclusion. Read the specific instructions in your carton, and if the room sits over a slab or a crawlspace, check what that product requires for moisture control before you open a single box.

Sources and Further Reading

Run your own numbers with the Flooring Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.

Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.

Last updated: August 7, 2026

The short version

A 10×12 shed slab poured 4 inches thick with a thickened perimeter edge takes about 92 bags of 80 lb concrete mix, or about 122 bags of 60 lb mix, once you allow 10 percent for waste. That is roughly 2 cubic yards of concrete. A plain flat 4 inch rectangle with no edge is only about 67 bags of 80 lb, which is why so many people come up short on pour day.

A shed slab is not just a rectangle

The number that matters is not the floor area, it is the total volume of the hole you are filling. On a shed slab that hole usually has three parts most calculators ignore.

• The thickened edge. Most shed slabs are poured monolithic, meaning the perimeter is turned down deeper than the field of the slab so the wall plates bear on something more substantial than 4 inches of concrete over gravel. A turndown adds real volume.
• Subgrade slop. Hand grading is never dead flat. Half an inch of extra depth across 120 square feet is 5 cubic feet, or eight extra bags of 80 lb mix.
• Form spring and spillage. Wet concrete pushes forms outward, and every wheelbarrow leaves something behind.

Turndown depth and width are code items in a lot of jurisdictions, and frost depth drives them. Before you dig, call your building department and ask for the local frost depth and whether a shed this size needs a permitted, frost protected footing instead of a floating slab. In many areas a small storage shed can float, but that is not universal and we would never assume it.

The formula

Work in cubic feet, then convert once at the end.

Field slab (cu ft) = length x width x (thickness in inches / 12)

Thickened edge (cu ft) = perimeter x (extra depth in inches / 12) x (edge width in inches / 12)

Total bags = (field + edge) x 1.10 / bag yield in cubic feet

Cubic yards is the total divided by 27. Note that the “extra depth” in the edge formula is the depth below the bottom of the field slab, not the full edge depth, or you will double count the top 4 inches.

Worked example, 10×12 with an 8 inch turndown

Field slab: 10 x 12 = 120 sq ft, times 4/12 of a foot, gives 40 cubic feet.

Perimeter: 2 x (10 + 12) = 44 linear feet.

Turndown: 8 inches total edge depth minus the 4 inch slab leaves 4 inches of extra depth, and we will make the turndown 8 inches wide. So 44 x (4/12) x (8/12) = 44 x 0.2222 = 9.8 cubic feet.

Total before waste: 40 + 9.8 = 49.8 cubic feet, which is 1.84 cubic yards. Measuring the perimeter on the outside face slightly overcounts the four corners, and we are fine with that. It is free contingency.

Component	Cubic feet	Cubic yards	80 lb bags	60 lb bags
Field slab, 120 sq ft at 4 in	40.0	1.48	67	89
Thickened edge, 44 lf	9.8	0.36	17	22
Subtotal	49.8	1.84	83	111
With 10% waste	54.8	2.03	92	122

Bag yields, and why 45 bags equals a yard

A bag of concrete mix yields far less finished concrete than its dry weight suggests, because the aggregate packs tight once water goes in. An 80 lb bag makes about 0.60 cubic feet. Divide 27 cubic feet in a yard by 0.60 and you get exactly 45. That is where the 45 bags per cubic yard rule comes from, and it is worth memorizing.

Bag size	Approx yield (cu ft)	Bags per cubic yard	Bags for this shed slab
40 lb	0.30	90	183
50 lb	0.375	72	147
60 lb	0.45	60	122
80 lb	0.60	45	92

Base prep, and the number it quietly changes

Under the slab you want compacted crushed stone, commonly 4 inches of clean angular material, placed and compacted in lifts of 3 to 4 inches rather than dumped all at once. Rounded pea gravel will not lock up. A plate compactor rental is cheap next to the cost of a slab that settles.

Here is the practical link to your bag count. If your base is compacted flat and screeded to grade with a straightedge, 10 percent waste is plenty. If you dug by hand and eyeballed it, use 15 percent. On this slab that is the difference between 92 and 96 bags of 80 lb.

Vapor barrier and anchor bolts

If the shed will store anything that can rust, warp, or mildew, put a vapor barrier under the slab. Six mil poly is the common minimum, thicker 10 or 15 mil sheet is far more puncture resistant when you are dragging rebar chairs and boots across it. Lap seams at least 6 inches and tape them. Run the sheet over the top of the gravel, not under it.

Anchor bolts go in wet, not drilled later, if you can help it. Half inch J bolts are the common choice, set within about a foot of each plate end, hooked end down, with enough thread exposed for a plate, washer, and nut. Diameter, embedment, and spacing are code items that vary, so confirm what your jurisdiction requires before the mixer shows up. Keep a tape and a chalked plate layout at the edge of the pour while the concrete is still plastic.

Two cubic yards is a decision point

Ninety two bags is roughly 7,400 lb of material to buy, haul, and mix, and it is more than one person can place in front of a setting edge on a warm day. At this size we would price a ready-mix delivery before committing to a pallet of bags. If the shed will carry anything heavier than shelving and a mower, or if the ground is soft or filled, that is engineer territory rather than a volume calculation.

Sources and Further Reading

Run your own numbers with the Concrete Slab Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.

Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.

Last updated: August 7, 2026

The short answer, then the reason it is not 40

A standard 8 by 10 foot bathroom floor is 80 square feet. A 12×24 tile covers 2 square feet, so the bare count is 40 pieces. Order 47 tiles, which for most 12×24 lines means 6 cartons at 8 pieces each. The gap between 40 and 47 is not padding for clumsiness. It comes from the way a two foot long tile lands in a small rectangular room.

We ran this floor both ways before settling on the layout below, and the deciding factor was not tile count. Turning the 24 inch dimension to run with the long wall cut the number of edge pieces almost in half, because the field lands on a full tile at one end instead of a two inch sliver at both. The box count barely moved. The cut list, and the day, got a lot shorter.

With a 4 inch tile you can nudge a layout anywhere and nobody notices. With 12×24 the module is coarse. Two inches of error at one wall shows up as a two inch sliver at the other, and there is no hiding it behind a toilet.

Work the layout module, not just the area

The number you actually need is how many tile positions fit, row by row. That depends on the tile plus the grout joint, which tile setters call the module.

Module = tile dimension + joint width

Positions per run = ceiling of (wall length / module)

Last cut size = wall length minus (full positions x module)

For a rectified 12×24 porcelain at a 1/8 inch joint, the modules are 12.125 and 24.125 inches. Our 8 by 10 room is 96 by 120 inches. Running the long dimension of the tile down the 10 foot direction:

Direction	Wall length	Module	Full tiles	Total positions	Final cut
Across (8 ft)	96 in	12.125 in	7 (84.875 in)	8 rows	11.1 in strip
Along (10 ft)	120 in	24.125 in	4 (96.5 in)	5 per row	23.4 in piece

Eight rows of five is 40 positions, which matches the area math. But look at the final cut in the long direction. It is 23.4 inches off a 24 inch tile. That tile is consumed for a piece nearly its own size, and the sliver that falls off is trash. That happens eight times, once per row.

The worked example

1. Floor area: 8 x 10 = 80 sq ft. We tile under the vanity and behind the toilet, so nothing gets deducted.
2. Tile coverage: 12 x 24 / 144 = 2.0 sq ft per piece.
3. Base count: 80 / 2.0 = 40 pieces.
4. Cut loss on a 1/3 offset in a room with two cut walls: 12 percent. 40 x 0.12 = 4.8 tiles.
5. Handling, culling and bad cuts on large format: 5 percent. 40 x 0.05 = 2.0 tiles.
6. Total: 40 + 4.8 + 2.0 = 46.8, round to 47 tiles.
7. Cartons at 8 pieces (16 sq ft): 47 / 8 = 5.875, so 6 cartons, 48 tiles, 96 sq ft.

One spare tile is thin attic stock for a 12×24. Buy a seventh carton and store it. Dye lots do not come back.

The 1/3 offset rule, and why half offset ruins 12×24

Nearly every 12×24 manufacturer prints a maximum 33 percent offset in the installation instructions, and it is there for a physical reason. Pressed porcelain in that shape almost always has a slight crown, meaning the middle of the tile sits a hair higher than the ends. In a half offset brick pattern, the high middle of one tile lands right against the low end of its neighbor, and the two errors add. Pull the offset back to a third and the mismatch drops to roughly a third of the crown.

If you want the brick look anyway, dry lay two rows and run a straightedge across the joint. If you can feel a ledge with a fingernail, grout will not fix it.

Flatness and mortar, the two things that decide lippage

Large format tile does not bend to follow a hump. Most manufacturers ask for a substrate flat within 1/8 inch in 10 feet and 1/16 inch in 2 feet, which is tighter than the 1/4 inch in 10 feet people are used to for small tile. Check your floor with a 6 foot level before you buy a single tile. A bag of patch or a small pour of self leveler costs less than a day of fighting lippage.

Use a large and heavy tile mortar, sometimes labeled medium bed, rather than a standard thin set. Standard thin set is formulated to be under about 3/16 inch after the tile is beaten in, and it shrinks if you build it thicker. Medium bed holds a thicker bed without slumping, which is exactly what you need when you are dialing a 24 inch tile flat.

Trowel notch	Typical use	Approx coverage per 50 lb bag
1/4 x 1/4 square	Mosaic, small wall tile	90 to 100 sq ft
1/4 x 3/8 square	12×12 floor tile	85 to 95 sq ft
1/2 x 1/2 square	12×24 and larger	40 to 50 sq ft
1/2 x 1/2 plus back butter	12×24, wet areas	30 to 35 sq ft

For our 80 square foot bathroom with back buttering, that is roughly 80 / 32, so three 50 pound bags. Back buttering is not optional on this size tile if you want the coverage a bathroom floor deserves.

Practical cut planning

• Split the difference. If a wall leaves you an 11 inch strip on one side and a full tile on the other, shift the field so both sides get about 5.5 inches. Symmetry reads as intentional.
• Never leave a cut under 2 inches. Slivers snap on the saw and they telegraph every out of square wall.
• Use leveling clips. Two per long edge, one per short edge. On 12×24 they are cheap insurance, not a crutch.

Comb your mortar ridges in one direction and set the tile across them so trapped air can escape. Pull a tile after the first few and look at the back. If you are not seeing near full contact, change trowels before you go any further.

Sources and Further Reading

Run your own numbers with the Tile Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.

Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.

Last updated: August 7, 2026

Why a Garage Isn’t Just a Big Bedroom

A two-car garage looks like the easiest drywall job in the house. Big open box, no closets, no soffits, maybe one window. Then you start the takeoff and run into three things that never come up in a bedroom. The walls are usually taller than eight feet, one entire wall is mostly overhead door opening, and the surfaces shared with the living space are typically part of a fire separation assembly with its own board requirement.

That last item drives your whole material list, so settle it before you measure anything.

Settle the Fire Separation Question First

In most jurisdictions, the wall between an attached garage and the house, and the garage ceiling below habitable space, have to be covered with a specific board. The most commonly cited version is 1/2-inch gypsum board on the separation wall and 5/8-inch Type X on a garage ceiling with living space above. Plenty of local amendments go further and call for 5/8 Type X everywhere in the garage. We have seen two townships in the same metro area read this differently, so call your building department before you buy a pallet and get the answer for your exact situation: attached or detached, living space above or open trusses, and whether the ceiling has to be finished at all.

Detached garages past a certain setback often have no board requirement whatsoever. If you are finishing purely for comfort and dust control, that changes your list completely. Verify locally either way. Nothing below substitutes for what your inspector tells you.

Surface	Commonly specified board	What changes it
Wall shared with the house	1/2″ gypsum, frequently amended to 5/8″ Type X	Local amendment, whether the wall supports the rated ceiling
Garage ceiling, habitable space above	5/8″ Type X	Attic access, storage-only space above
Garage ceiling, open attic or trusses	Often nothing required	Whether the attic connects to the house attic
Exterior garage walls	No fire requirement in most cases	Lot line proximity
Detached garage	Often no requirement at setback distance	Distance to the dwelling

The Formula

Wall area = perimeter x wall height, minus every opening larger than about 20 sq ft

Ceiling area = length x width for a flat ceiling, or length x width x 1.15 if you are following a shallow gable or scissor truss

Sheets = (wall area + ceiling area) x 1.10, divided by sheet area, rounded up

The opening deduction is where garage math splits from room math. In a bedroom you ignore doors and windows because cutting waste eats the difference. A 16 by 7 overhead door is 112 square feet. Ignore that and you have paid for three and a half extra sheets that will sit in the corner until you throw them out.

Worked Example: 22 x 22 Garage, 9-Foot Walls

Perimeter is 2 x (22 + 22) = 88 feet. At 9 feet tall, gross wall area is 88 x 9 = 792 sq ft.

Item	Math	Sq ft
Gross wall	88 lf x 9 ft	792
Overhead door	16 x 7	-112
Service door	3 x 7	-21
Window	3 x 4	-12
Net wall		647
Flat ceiling	22 x 22	484
Total surface	647 + 484	1,131
With 10% waste	1,131 x 1.10	1,244

Divide 1,244 by your sheet size and round up. That is 39 sheets of 4×8, 26 sheets of 4×12, or 24 sheets of 54-inch by 12-foot board.

Why 54-Inch Board Exists

Nine-foot walls are the standard in modern garages, and 54-inch board is the reason they are not a nightmare. Two courses of 54-inch board stacked horizontally measure exactly 108 inches, which is nine feet, with no rip and no third joint. Hang 4-foot board on a 9-foot wall and you get two full courses plus a 12-inch strip, which means an extra taped seam running the length of every wall and a rip that has no factory edge on either side. Working this through on the 22-foot garage, we cut roughly 90 linear feet of filler seam out of the job just by changing sheet width.

Sheet size	Sq ft	Sheets needed	Approx weight, 5/8 Type X	Seams on a 9-ft wall
4 x 8	32	39	70 lb	Two horizontal, plus filler strip
4 x 12	48	26	106 lb	Two horizontal, plus filler strip
54″ x 12	54	24	119 lb	One horizontal, no filler

Fasteners and What a Rated Assembly Changes

Standard field practice is screws every 16 inches on walls and every 12 inches on ceilings, hitting every framing member, and never closer than 3/8 inch to a sheet edge. For 5/8 board on wood framing use 1-5/8 inch coarse thread, because the screw needs about 5/8 inch of penetration past the board.

Here is the part people miss. A fire-rated assembly is a tested sandwich, not a thickness. The listing covers framing size and spacing, board type, fastener type, fastener spacing, and joint treatment. Fastener spacing in a listed assembly is often tighter than field practice, sometimes 7 or 8 inches. If your inspector is looking for a rated assembly, follow the listing rather than habit, and ask which listing they expect to see.

Practical Notes Before You Order

• Rent a panel lift for the ceiling. A 54-inch by 12-foot sheet of 5/8 Type X is around 120 pounds and two people will not hold it flat.
• Hang the ceiling first, then walls. Wall sheets pushed tight support the ceiling perimeter.
• Hold every sheet a half inch off the slab. Garage floors get wet, and gypsum wicks.
• Screw count runs about one per square foot as a planning figure, so budget roughly 1,200 screws for this garage and buy a 5-pound box.
• Order 5 percent over your count if the store is far. Corner damage on 12-footers is common in transit.

Confirm every fire separation detail with your local building department before ordering. Requirements vary by jurisdiction and this article is a planning guide, not a code citation.

Sources and Further Reading

Run your own numbers with the Drywall Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.

Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.

Last updated: August 7, 2026

One room, three separate estimates

A 12×12 bedroom looks like a single job. It is really three jobs stacked on top of each other, and each one uses a different product at a different coverage rate. Walls get a matte or eggshell wall paint. The ceiling gets a dead flat ceiling paint. Trim gets an enamel that costs two to three times as much per gallon and gets applied in much smaller quantities. If you walk into the store and ask for “enough paint for a bedroom,” you will walk out with too much wall paint and not enough enamel to finish the door.

We estimate each surface separately, then round up to the container sizes that actually exist. Here is how each one works.

Walls: perimeter times height, minus the holes

The wall formula is simple:

• Gross wall area = room perimeter x ceiling height
• Net wall area = gross area – openings
• Gallons = (net area x number of coats) / coverage rate

The deduction step is where people go wrong in both directions. The trade convention is to only deduct openings larger than about 20 square feet, because you still have to cut in carefully around every window, and that cutting eats paint faster than rolling does. A brushed cut line lays down a thicker film than a rolled field, so a room with a lot of small windows uses more paint per square foot than a plain box would.

For a bedroom we deduct the door and the windows anyway, then buy with a cushion. It is more honest arithmetic than pretending the door is solid wall.

Worked example

A 12×12 bedroom with an 8 foot ceiling, one standard door, and two 3×4 windows.

• Perimeter: (12 + 12 + 12 + 12) = 48 linear feet
• Gross wall area: 48 x 8 = 384 sq ft
• Door: 3 ft x 6 ft 8 in = 20 sq ft
• Windows: 2 x (3 x 4) = 24 sq ft
• Net wall area: 384 – 44 = 340 sq ft
• Two coats: 340 x 2 = 680 sq ft to cover
• At 350 sq ft per gallon: 680 / 350 = 1.94 gallons

That is two gallons with almost nothing left. If you are changing color rather than refreshing the same color, buy two gallons plus a quart. Most major brands state 350 to 400 sq ft per gallon on smooth, sealed drywall, and the real world number on a first coat over fresh patching runs closer to 300.

Ceiling: the easy one

A 12×12 ceiling is 144 square feet. Two coats is 288 square feet. Flat ceiling paints usually state 350 to 400 sq ft per gallon, so 288 / 375 = 0.77 gallons. One gallon covers it with room to spare.

Do not substitute wall flat here if you can avoid it. Ceiling paint is formulated dead flat with a high extender load specifically so it holds a wet edge longer and hides lap marks. Ceilings are lit at a grazing angle by window light, which is the single worst lighting condition for showing roller laps. When we have cut corners and used leftover wall flat overhead, the laps showed by mid afternoon.

Trim: measure in linear feet, convert to square feet

Trim is the piece everyone guesses at. The fix is to measure the running length of each trim element and multiply by its developed width, meaning the total width of every face you will actually coat, including the top edge of the baseboard and the returns.

Trim element	Typical developed width	Quantity in this room	Area
Baseboard (5 1/4 in plus top edge)	0.5 ft	45 linear ft	22.5 sq ft
Door slab, one face plus latch edge	n/a	3 x 6.67 ft	21 sq ft
Door casing and jamb, one side	0.8 ft combined	17.5 linear ft	14 sq ft
Window casing, stool, apron, jambs	n/a	2 windows	24 sq ft
Total trim			81.5 sq ft

Baseboard runs 48 feet of perimeter minus the 3 foot door opening, so 45 linear feet. Two coats of enamel over 81.5 square feet is 163 square feet of coverage. Enamels state roughly 350 to 400 sq ft per gallon, which is 87 to 100 sq ft per quart, so 163 / 90 = 1.8 quarts. Buy two quarts.

The full buy list

Surface	Net area	Coats	Total coverage	Rate used	Calculated	Buy
Walls	340 sq ft	2	680 sq ft	350 sq ft/gal	1.94 gal	2 gal + 1 qt
Ceiling	144 sq ft	2	288 sq ft	375 sq ft/gal	0.77 gal	1 gal
Trim and door	81.5 sq ft	2	163 sq ft	90 sq ft/qt	1.8 qt	2 qt

Three things that change the number

• Ceiling height. Nine foot ceilings add 48 square feet of wall in this room, roughly 12 percent more paint. Ten foot ceilings push a two gallon job to nearly two and a half.
• Fresh drywall or heavy patching. Bare joint compound and bare paper drink paint at wildly different rates, which is why the first coat over unsealed repairs flashes. Seal patches with a drywall primer or a coat of the wall paint before you count on the label spread rate.
• Sheen. Flat and matte finishes carry more pigment and extender and generally hide in fewer coats than a satin or semi gloss in the same color. Going from a flat wall to a satin wall in the same color still usually takes two coats, because the sheen change itself is visible wherever the coverage is thin.

One last practical note on ordering. Wall paint, ceiling paint, and enamel are three different cans, and the trim can is the one most likely to run short because people forget the door has two faces if you are painting both sides. In this estimate we counted one face. If the hallway side gets painted too, add another 21 square feet and you are into a third quart.

Sources and Further Reading

Run your own numbers with the Paint Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.

Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.

Last updated: August 7, 2026