★ CONSTRUCTION · DECKS · REFERENCE
5.5-inch deck boards at a 1/4-inch gap need 208.70 lineal feet per 100 sq ft — so a 12 ft × 10 ft deck takes 250.43 lineal feet, 21 boards, and 420 screws. Pick your board, gap, and deck size below and the lookup does the rest, or read the chart for every common board and gap.
A 12 ft × 10 ft deck with 5.5-inch boards at a 1/4-inch gap needs 250.43 lineal feet, which is 20.87 boards — buy 21 — and 420 screws at 16 inches on centre. The chart below covers every common board and gap.
A deck is boards laid side by side, each board covering its own width plus one gap. A 5.5-inch board at a 1/4-inch gap covers 5.75 inches of deck width, so one square foot of deck needs 12 / 5.75 = 2.087 lineal feet of board — and 100 square feet needs 208.70 lineal feet. That is the whole chart: lineal feet per 100 sq ft is 1200 divided by the board width plus the gap, both in inches. Multiply by your deck area, divide by the board length, round up, and you have the board count. Screws are two per joist crossing, and a 12-ft board on joists 16 inches on centre crosses 10 joists.
| Gap | 3.5 in (2x4) | 5.5 in (2x6) | 7.25 in (2x8) | 9.25 in (2x10) | 11.25 in (2x12) |
|---|---|---|---|---|---|
| 1/4 in | 320.00 | 208.70 | 160.00 | 126.32 | 104.35 |
| 3/16 in | 325.42 | 210.99 | 161.34 | 127.15 | 104.92 |
| 1/8 in | 331.03 | 213.33 | 162.71 | 128.00 | 105.49 |
Read across for your board, down for your gap. A tighter gap always needs a little more board: at 5.5 inches, a 1/8-inch gap needs 213.33 lineal feet per 100 sq ft against 208.70 at 1/4 inch. Wider boards need far less board overall: an 11.25-inch board at a 1/4-inch gap needs only 104.35 lineal feet per 100 sq ft, about half what a 5.5-inch board needs.
| Material | Gap | Notes |
|---|---|---|
| Pressure-treated wood | 1/8–1/4 in | Wet boards butt tight or 1/16 in — they shrink as they dry. Dry boards go in at 1/4 in. |
| Cedar / redwood | 1/8–1/4 in | Stable and dry at the yard; 1/8 in is the usual pick, 1/4 in if the boards are wet. |
| Composite | 3/16–1/4 in | Follow the maker’s instructions — composite moves with temperature more than wood does. |
| PVC | 1/8–3/16 in | PVC is the most stable of the synthetics; the maker’s gap is usually 1/8 in. |
| Hidden fasteners | 3/16 or 1/4 in | The fastener sets the gap for you, usually 3/16 in or 1/4 in — no spacer needed. |
The rule behind the numbers: the gap is for drainage and expansion. Too tight and water sits on the deck, leaves rot, and boards swell against each other; too wide and heels catch, small things fall through, and the deck looks gappy. A 1/4-inch gap is the safe default for wood; composite and PVC follow the maker, because the warranty follows the maker’s gap.
| Deck size | Area | Lineal feet | Boards (12 ft) |
|---|---|---|---|
| 10 ft × 10 ft | 100.00 sq ft | 208.70 lf | 18 |
| 12 ft × 10 ft | 120.00 sq ft | 250.43 lf | 21 |
| 12 ft × 12 ft | 144.00 sq ft | 300.52 lf | 26 |
| 16 ft × 12 ft | 192.00 sq ft | 400.70 lf | 34 |
| 20 ft × 12 ft | 240.00 sq ft | 500.87 lf | 42 |
These counts are before waste. Add 10% for cuts and angles: the 12 ft × 10 ft deck goes from 21 boards to 23, and the 16 ft × 12 ft deck from 34 to 37.
Setup: deck 12 ft × 10 ft; board 5.5 in; gap 1/4 in; board length 12 ft; joists 16 in on centre.
Line 1 — area. 12 × 10 = 120.00 sq ft.
Line 2 — coverage per board. 5.5 + 0.25 = 5.75 in of deck width per board.
Line 3 — lineal feet per 100 sq ft. 1200 / 5.75 = 208.70 lf.
Line 4 — lineal feet. 120.00 × 208.70 / 100 = 250.43 lf.
Line 5 — boards. 250.43 / 12 = 20.87 → 21 boards.
Line 6 — joist crossings. A 12-ft board is 144 in; 144 / 16 = 9 spaces, so 10 crossings (one at each end, eight between).
Line 7 — screws. 10 crossings × 2 screws × 21 boards = 420 screws.
Line 8 — waste. 250.43 × 1.1 = 275.47 lf, which is 275.47 / 12 = 22.96 → 23 boards.
3.5-inch boards, same deck: 1200 / 3.75 = 320.00 lf per 100 sq ft, so 120.00 sq ft needs 384.00 lf, which is 384.00 / 12 = 32 boards. Narrow boards mean a third more board and a third more screws than 5.5-inch boards.
7.25-inch boards, same deck: 1200 / 7.50 = 160.00 lf per 100 sq ft, so 120.00 sq ft needs 192.00 lf, which is 192.00 / 12 = 16 boards. Five fewer boards than the 5.5-inch case, and 80 fewer screws.
1/8-inch gap, 5.5-inch boards: 1200 / 5.625 = 213.33 lf per 100 sq ft, so 120.00 sq ft needs 256.00 lf, which is 256.00 / 12 = 21.33 → 22 boards. One more board than the 1/4-inch gap for the same deck.
16 ft × 12 ft deck, 5.5-inch boards, 1/4-inch gap, 16-ft boards: 192.00 sq ft × 208.70 / 100 = 400.70 lf, which is 400.70 / 16 = 25.04 → 26 boards. Longer boards help here: the same deck in 12-ft boards would need 34.
Reverse — what 30 boards cover: 30 boards of 12-ft 5.5-inch at a 1/4-inch gap is 360.00 lf, and 360.00 × (5.75 / 12) = 172.50 sq ft. So 30 boards deck about a 12 ft × 14 ft area, or a 10 ft × 17 ft strip.
The defaults are a rectangular 12 ft × 10 ft deck, 5.5-inch boards (a 2x6) at a 1/4-inch gap, 12-ft boards, and joists 16 inches on centre. The board count is the exact coverage rounded up — a deck that needs 20.87 boards needs 21 boards, because you cannot buy 0.87 of a board. The screw count assumes two screws at every joist crossing, which is the standard for face-screwed boards; hidden fasteners use one fastener per crossing instead, so the screw count does not apply to them.
Rounding convention, stated plainly: lineal feet are rounded to two decimals — 250.43, 208.70 — and boards and screws are whole numbers, always rounded up. The lineal-feet figure is the exact coverage; the board count is the ceiling of that figure divided by the board length.
| Output | Formula | What it means |
|---|---|---|
| Deck area | 12 ft × 10 ft | 120.00 sq ft to cover. |
| Lineal feet per 100 sq ft | 1200 / (5.5 + 0.25) | 208.70 lf of board per 100 sq ft. |
| Lineal feet needed | 120.00 × 208.70 / 100 | 250.43 lf of board in total. |
| Boards needed | 250.43 / 12, rounded up | 21 boards of 12 ft. |
| Joist crossings per board | 144 / 16 + 1 | 10 crossings on a 12-ft board. |
| Screws needed | 21 × 10 × 2 | 420 screws, two per crossing. |
| With 10% waste | 250.43 × 1.1, rounded up | 23 boards to buy. |
The coverage math is exact for square and rectangular decks with no waste: lineal feet per 100 sq ft is 1200 divided by the board width plus the gap, and the board count is that figure divided by the board length, rounded up. But real decks are not all rectangles. Angled cuts, picture-frame borders, stair treads, and decking around posts all use extra board that this math does not count — add 10% for cuts and angles, and more for a complex shape. The screw count assumes face screws, two per joist crossing; hidden fasteners use one clip per crossing, and the clip count is the board count times the crossings, not times two. Board widths are the face widths: a 2x6 is 5.5 inches, a 2x4 is 3.5 inches, and the chart uses those real widths, not the nominal ones. Always follow the board maker’s gap instructions over this chart: the maker’s gap is the one the warranty is written against, and it changes with the product, the climate, and whether the boards are wet or dry at installation.
Bright Side Kit. "Deck Board Spacing Chart: Gaps, Coverage & Board Count" https://brightoolbox.com/deck-board-spacing-chart/ Accessed October 24, 2026.1/8 to 1/4 inch, depending on the material. Pressure-treated wood takes 1/8 to 1/4 inch — wet boards go in tight, at 1/16 inch or butt tight, because they shrink as they dry. Cedar and redwood take 1/8 to 1/4 inch. Composite takes 3/16 to 1/4 inch, and PVC takes 1/8 to 3/16 inch. Hidden fasteners set the gap for you, usually 3/16 or 1/4 inch.
21 boards of 12 ft, or 23 with 10% waste. A 12 ft × 10 ft deck is 120.00 sq ft, and 5.5-inch boards at a 1/4-inch gap need 208.70 lineal feet per 100 sq ft, which is 250.43 lineal feet — 250.43 divided by 12 is 20.87, so 21 boards.
420 for the 12 ft × 10 ft deck at the defaults. Each 12-ft board crosses 10 joists at 16 inches on centre, and each crossing takes 2 screws, so 21 boards × 10 crossings × 2 is 420 screws. With 10% waste, 23 boards need 460 screws.
208.70 for 5.5-inch boards at a 1/4-inch gap. The formula is 1200 divided by the board width plus the gap, both in inches: 1200 / (5.5 + 0.25) = 208.70. A 3.5-inch board needs 320.00, and an 11.25-inch board needs only 104.35.
Yes, if they are wet at installation. Wet pressure-treated boards shrink as they dry, which is why they go in tight — butt tight or a 1/16-inch gap — and open up to about 1/4 inch as they dry. Dry boards go in at 1/4 inch straight away. That is why the gap table gives a range for pressure-treated wood rather than one number.
10% for a rectangular deck, more for angles. The 12 ft × 10 ft deck needs 250.43 lineal feet, and 10% extra is 275.47 lineal feet, which is 23 boards instead of 21. Angled cuts, picture-frame borders, and stairs need more — count those boards separately.
3/16 to 1/4 inch, and follow the maker. Composite moves with temperature more than wood does, so the maker’s gap is the one the warranty is written against. At 5.5 inches, a 3/16-inch gap needs 210.99 lineal feet per 100 sq ft against 208.70 at 1/4 inch — the gap barely changes the board count, so there is no reason to guess.
Not directly — count the border separately. A picture-frame border runs boards around the edge at right angles to the field, so its boards are a different length and a different count. Work out the field with this chart, then add the perimeter boards for the border on top.