Paint Calculator: How Much Paint Do You Really Need?

Buying paint sounds simple until you’re standing in the store aisle wondering whether one gallon will cover your bedroom — or whether you’ll be driving back tomorrow for a second can that doesn’t quite match the first batch. Our paint calculator above takes the guesswork out: enter your room dimensions, subtract doors and windows, choose your number of coats, and you’ll know exactly how many gallons to put in your cart.

How the Paint Calculation Works

The math behind paint estimation is straightforward, but skipping a step is what leads to bad guesses. Here’s the formula our calculator uses:

Paintable area = (2 × (Length + Width) × Wall height) − (Doors × 21 sq ft) − (Windows × 15 sq ft)

Gallons needed = (Paintable area × Number of coats) ÷ Coverage per gallon

A standard interior door takes up about 21 square feet of wall, and an average window about 15 square feet. Unless you’re painting them the same color, that’s area you shouldn’t pay for.

Most interior latex paints cover about 350 square feet per gallon on a smooth, previously painted surface. That’s the number printed on the back of nearly every can — but real-world coverage varies, and that’s where most estimates go wrong.

What Changes Your Coverage Rate

Not every wall drinks paint at the same rate. Adjust the coverage field in the calculator if any of these apply to you:

New drywall or bare plaster soaks up paint like a sponge. Expect 250–300 sq ft per gallon on the first coat, and always prime first — primer is cheaper than paint and seals the surface.

Textured walls (orange peel, knockdown, popcorn) have more surface area than flat walls. Reduce your coverage estimate by 15–25% depending on how heavy the texture is.

Dramatic color changes — going from navy to white, or white to deep red — usually demand an extra coat. A tinted primer can save you a full coat of the more expensive finish paint.

Paint quality matters too. Premium paint-and-primer-in-one products often genuinely cover in fewer coats than bargain paint, which can make the more expensive can the cheaper choice per square foot of finished wall.

One Coat or Two?

For almost every repaint, the honest answer is two coats. One coat rarely produces an even sheen, even when the color technically “covers.” Two coats also give the paint film its full durability and washability — something manufacturers assume in their warranties. The main exception: repainting a wall the identical color and sheen to freshen it up, where one careful coat can be enough.

Don’t Forget the Extras

Your walls aren’t the whole job. When you’re at the store, remember:

• Ceilings: length × width of the room, divided by coverage. A 12 × 10 room ceiling is 120 sq ft — about a third of a gallon per coat.
• Trim and baseboards: a quart of trim paint handles the average room.
• Primer for bare surfaces, stains, or big color changes.
• Leftover for touch-ups: keeping a well-sealed quart of your wall color makes future scuff repairs painless.

Frequently Asked Questions

How much paint do I need for a 12×12 room? With 8-foot ceilings, a 12×12 room has about 384 sq ft of wall before openings. Subtract a door and two windows (about 51 sq ft) and you’re near 333 sq ft. At 350 sq ft per gallon, that’s roughly 1 gallon per coat — so buy 2 gallons for a proper two-coat job.

Is it better to buy a 5-gallon bucket? If your project needs 4 gallons or more, a 5-gallon bucket is usually 10–15% cheaper per gallon and guarantees perfect color consistency across the whole job.

Does the second coat use less paint? Yes — typically 10–20% less, because the sealed first coat absorbs less. Our calculator uses the full rate for both coats, which builds in a small safety margin.

Should I round up or down? Round up. Leftover paint stores for years and rescues you from touch-up emergencies; running out mid-wall leaves a visible lap line where old and new paint meet.

Estimates are for planning purposes. Always check the coverage rate printed on your specific paint can.