The math: rows across, boards along
Deck boards run one direction — normally the long way, parallel to the house. Divide the deck’s width by the board width plus its gap and you get the number of rows; each row is one board running the full length (scarf-jointed where the deck is longer than your stock). The exact formula this page’s calculator uses is:
rows = (deck width in inches + gap) ÷ (board width + gap), rounded up. Boards before waste equal the row count, and linear feet equal rows × length. Everything below is that arithmetic, with the real numbers filled in.
The gap is not optional
Pressure-treated lumber and composites both move with moisture and temperature, so the boards are never butted tight. Gapping at 1/8–1/4 inch lets water drain and lets the boards breathe; tight boards cup, hold debris and rot early. Kiln-dried and composite decking usually takes a 1/8 in gap, while wet pressure-treated boards — which shrink as they dry — are commonly gapped at 3/16–1/4 in. Composite brands publish their own required gap and it changes with the temperature on install day, so follow the manufacturer’s number rather than a rule of thumb.
Board width drives the row count more than the deck size does
The biggest single lever is board width, because narrower boards mean more rows. And nominal sizes lie: a “6-inch” deck board is actually 5½ inches wide and a “4-inch” board is 3½. The calculator works from the actual width, so use those figures when you measure.
| Board (nominal) | Actual width | Row pitch (board + 1/4" gap) | Rows across a 12 ft deck |
|---|---|---|---|
| 2×6 or 5/4×6 | 5½ in | 5¾ in | 26 |
| 5/4×4 or 2×4 | 3½ in | 3¾ in | 39 |
Switching the same 12 ft deck from 6-inch to 4-inch boards adds 13 rows — on a 16 ft deck that is 14 more boards and 208 more linear feet of material for identical coverage.
Buy 10% extra — and 15–20% on diagonals
The calculator adds a 10% waste factor for crooked boards you reject, end cuts, and the fascia or stair boards you forgot you needed. Straight-lay decks genuinely run close to 10%; anything with angles needs more, so adjust before you buy.
| Layout | Waste to add | Why |
|---|---|---|
| Straight (parallel to the house) | 10% | End cuts, rejected boards and a few spares |
| Diagonal (45°) | 15% | Every board now crosses the rim at an angle, one cut each |
| Herringbone or parquet | 20% | Two cut angles per board and short offcuts |
On the same 12 ft × 16 ft deck, 10% waste is 29 boards, 15% is 30 and 20% is 32 — the difference between two spare boards and five.
Three worked examples
1 — A standard 12 × 16 ft deck, 6-inch boards. Width 12 ft = 144 in. Every row plus its gap takes 5½ + ¼ = 5¾ in, so 144.25 ÷ 5.75 = 25.1, rounded up to 26 rows. Add 10% and you buy 29 boards, running 26 × 16 = 416 linear feet. This is the size most homeowners build, and it fits in one truck load of 16 ft stock.
2 — A large 16 × 20 ft deck, 6-inch boards. Now width 16 ft = 192 in, so 192.25 ÷ 5.75 = 33.4, rounded up to 34 rows. With 10% waste that is 38 boards and 34 × 20 = 680 linear feet. Notice the count rose faster than the area: a deck 1.67× the size needs 1.31× the boards per row but longer ones, so material climbs steeply.
3 — The same 12 × 16 ft deck in 4-inch boards. The row pitch drops to 3½ + ¼ = 3¾ in, so 144.25 ÷ 3.75 = 38.5, rounded up to 39 rows — 13 more boards and 624 to 416 linear feet for exactly the same floor. Narrow boards look finer but cost more in both material and labour, and every extra gap is another place water can sit.
Common mistakes
- Using nominal width instead of actual. A 6-inch board is 5½ inches wide. Calculating rows on 6 inches, or on 5.5 without the gap, is off by several boards.
- Leaving the gap out of the row pitch. The divisor is board width plus gap; skipping the gap under-counts rows on a wide deck.
- Measuring in feet and forgetting the ×12. The row formula works in inches. A 12 ft width is 144 in, not 12.
- Adding waste to area only. The 10% applies to the board count, since it covers rejects and end cuts, not extra ground.
- Ignoring board length. Linear feet is material, not stock. A 20 ft span needs 20 ft boards or a scarf joint over a joist; buying 8 ft boards changes both the count and the look.
- Budgeting only the walking surface. Decking is one part of the bill. Joists, beams, posts, footings, hangers, screws, railing and stairs are all separate, and the railing alone can cost more than the decking.
- Using 10% waste on a diagonal layout. Angled boards need 15–20%; 10% will leave you short with a half-laid deck and a colour-lot mismatch at the store.
Tools for laying deck boards
The board count is only useful if the rows go down straight and evenly gapped. These are the tools the build actually needs, from the first end cut to the last screw.
- Deck board spacer — The eighth-to-quarter-inch gap the row count assumes has to be held consistently, or the boards you bought will not all fit the width you measured.
- Circular saw — Every board gets end-cut and the rim boards get ripped. A sharp framing blade and a straight edge do the cuts the waste factor is covering.
- Speed square — For marking square crosscuts and the stair and fascia angles that a plain board count leaves out.
- Deck screws — Boards are counted, fasteners are not. Coated or stainless screws suit the treated and composite stock this page sizes, and the fascia and stairs will need them too.
As an Amazon Associate I earn from qualifying purchases. The links above are affiliate links — if you buy through one, the price you pay is unchanged and a small commission helps keep these calculators free. Tools are listed because the job calls for them, not because of commission rates.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below, and each worked example is computed by the same deck engine the calculator runs.
- American Wood Council — the DCA 6 prescriptive residential wood deck construction guide, span tables and lumber design values.
- APA — The Engineered Wood Association — panel span ratings and installation guidance for decking and sheathing.
- WoodWorks — Wood Products Council — deck framing details, fastening and span guidance for wood decks.
Figures, formulas and reference tables last verified: 27 September 2026.