Published August 11, 2026 · Updated August 23, 2026 · By Tupep
Rows, not square feet
Dividing deck area by board area gets you close and then leaves you short. Decking is installed in rows, and a row is either there or it is not. What you actually need to know is how many rows fit across the deck’s width, and how you are going to make up each row’s length out of the stock lengths your yard sells. Everything else, the screw count, the waste factor, the blocking, falls out of those two numbers.
The formula
- Module = board face width + gap
- Rows (first pass) = deck width in inches / module, rounded to the nearest whole number
- Exact gap for that row count = (deck width minus rows x face width) / (rows minus 1)
- Boards = rows x pieces needed per row at your stock length
The second line is where the craft is. You pick the row count first and then let the gap float slightly to make it come out exact, instead of letting a ripped sliver land at the last row.
Worked example: 12 ft x 16 ft, 5/4×6 decking
Set the deck up with joists running the 12 ft direction and boards running the 16 ft direction, parallel to the house. Boards therefore have to cover 12 ft of width, which is 144 in, and each row is 16 ft long.
5/4×6 decking is 5.5 in of actual face, not 6 in. Nominal sizes are milled down, and a 5/4×6 comes out 1 in thick by 5.5 in wide. Add a 1/4 in gap and the module is 5.75 in. 144 / 5.75 = 25.04, so try 25 rows.
Now check it exactly. 25 rows of 5.5 in face is 137.5 in of wood. That leaves 144 minus 137.5 = 6.5 in to be split across 24 gaps, which is 0.271 in each, or a hair over 9/32. That is a perfectly normal deck gap and it lands the last board dead on the edge with no rip.
Twenty six rows does not work: 26 x 5.5 = 143 in of wood leaves 1 in across 25 gaps, or 0.04 in. That is effectively no gap, which traps water and debris and gives the boards nowhere to swell.
| Row count | Wood face total | Gaps | Gap width required | Verdict |
|---|---|---|---|---|
| 24 | 132.0 in | 23 | 0.522 in | Too wide, looks like a boardwalk |
| 25 | 137.5 in | 24 | 0.271 in | Use this one |
| 26 | 143.0 in | 25 | 0.040 in | Effectively no gap, rejected |
Turning rows into boards
Twenty five rows at 16 ft each is 400 lineal feet of decking. How many boards that is depends entirely on the stock length you buy, and the stock length also determines how many butt joints you inherit.
| Stock length | Boards to buy | Lineal ft purchased | Butt joints | Waste |
|---|---|---|---|---|
| 16 ft | 25 | 400 | 0 | Essentially none |
| 20 ft | 25 | 500 | 0 | 100 ft of 4 ft cutoffs |
| 12 ft | 34 | 408 | 25 | 8 ft, but 25 joints to block |
| 8 ft | 50 | 400 | 25 | None, but 25 joints to block |
The 12 ft row works out to 25 full boards plus 25 four-foot pieces, and you get three 4 ft pieces out of each extra 12 ft board, so nine more boards covers it: 25 + 9 = 34. Buy 16 ft stock if it is available and straight. A joint-free deck surface is stronger, looks better and skips the blocking work entirely.
Butt joints need blocking, always
If you do end up with butt joints, every one of them needs solid wood underneath that gives both board ends full bearing and its own fasteners. A single joist is 1.5 in wide, so two board ends sharing it get 3/4 in each, and the screws land less than 3/8 in from the end grain. That splits.
The fix is either a doubled joist at the joint line or a block sistered alongside, so each board end lands on its own 1.5 in of wood. Then stagger the joints so they do not line up across adjacent rows. Joints stacked in a straight line across the deck read as a seam from twenty feet away and concentrate movement in one place. We shift the joint line at least two joists between neighboring rows.
Fasteners
Joists at 16 in on center across a 16 ft face means 16 x 12 / 16 = 12 spaces, so 13 joists. Two screws per board per joist is the standard. 25 rows x 13 joists x 2 = 650 screws. Add 10 percent for drops and drivers and buy about 715. Deck screw boxes are sold by count, so read the box rather than guessing at a pounds-to-screws conversion.
Two things worth knowing before you start
Gap depends on how wet the lumber is. Green pressure treated decking is heavy with moisture and shrinks across its width as it dries. Lay that tight or nearly tight and the gaps open up on their own to about where you wanted them. Kiln dried treated stock and cedar barely move, so you gap those at install. Getting this backwards gives you either a solid water-trapping deck or gaps you can lose a phone through.
Put the rip at the house, not the outside edge. If your math does not come out clean and one row has to be ripped narrow, run it against the house wall where a threshold, trim board or skirt covers the seam. The outside edge is the one everybody looks at, and it should be a full width board. We also snap a chalk line every fourth or fifth row and measure the remaining distance to the far edge, because boards crown and wander and a sixteenth per row becomes an inch by row 16.
Related guides
- Deck Framing Lumber List for a 10×12 Deck
- Composite vs Wood: Board Length and Waste
- Picture Frame Border: Extra Boards and Blocking
- How Many Posts and Pickets for a 100 ft Privacy Fence
Sources and Further Reading
Run your own numbers with the Deck Board 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 23, 2026
Leave a Reply