Deck Board Calculator
Boards & linear feet for a deck surface
Building a deck surface? Enter the deck dimensions to estimate the linear feet of decking and the number of boards in three common lengths. Below: how the gap between boards changes the count, a size chart, joist spacing rules, and the fasteners nobody remembers to order.
Deck boards per square foot
Every decking calculator turns on one number: how much deck width a single board actually covers.
A standard deck board is nominally 6 inches wide and actually 5½ inches. Laid with the ⅛ inch gap that drainage and expansion require, each board covers 5⅝ inches of deck width — 0.469 ft² per linear foot, or 2.13 linear feet of board per square foot of deck.
That eighth of an inch matters more than it looks. Ignoring the gap and treating a board as 5½ inches makes you buy about 2% more board than you need, and this calculator used to do exactly that. The gap is now in the coverage figure, where it belongs, with waste kept as a separate allowance.
| Board length | Covers | Boards per 100 ft² |
|---|---|---|
| 12 ft | 5.63 ft² | 18 |
| 16 ft | 7.50 ft² | 14 |
| 20 ft | 9.38 ft² | 11 |
Boards by deck size
Including the 10% the calculator adds for cuts and the occasional warped board.
| Deck | Area | 12 ft boards | 16 ft boards | 20 ft boards | Linear ft |
|---|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 20 | 15 | 12 | 235 |
| 12 × 12 ft | 144 ft² | 29 | 22 | 17 | 338 |
| 16 × 12 ft | 192 ft² | 38 | 29 | 23 | 451 |
| 16 × 20 ft | 320 ft² | 63 | 47 | 38 | 751 |
| 20 × 24 ft | 480 ft² | 94 | 71 | 57 | 1,126 |
Buy the longest boards that span your deck without a joint where you can. A 16 ft deck decked in 16 ft boards has no butt joints at all; the same deck in 12 ft boards has a seam line down every run, and every seam needs blocking underneath and doubles the end cuts.
Which gap, and does the wood shrink?
The gap depends on the material and its moisture content, and getting it wrong is one of the few decking mistakes you cannot fix afterwards.
| Material | Gap at install | Why |
|---|---|---|
| Wet pressure-treated | Butt tight | Shrinks to a gap as it dries |
| Kiln-dried or seasoned wood | ⅛ in | Already stable |
| Cedar and redwood | ⅛ to ¼ in | Minor movement |
| Composite | ⅙ to ¼ in | Expands with heat, per manufacturer |
| End-to-end joints | ⅛ in | All materials |
Freshly delivered pressure-treated lumber is often soaking wet and will shrink noticeably across its width as it dries, so it goes down tight and opens up on its own. Gapping wet PT boards gives you inch-wide slots a year later. Composite does the opposite — it expands in heat, and manufacturers void warranties over incorrect gapping, so follow their number rather than a rule of thumb.
Board direction and joist spacing
Decking runs perpendicular to the joists, and the joist spacing you frame to limits what you can lay on top:
| Layout | Joist spacing | Extra boards |
|---|---|---|
| Straight, wood decking | 16 in on centre | — |
| Straight, most composite | 16 in on centre | — |
| 45° diagonal | 12 in on centre | about 15% |
| Herringbone or picture frame | 12 in, with blocking | 15 to 20% |
Diagonal decking looks excellent and costs twice: 15% more boards from the angled end cuts, and a third more joists because the span between supports increases when boards run at 45 degrees. Decide the pattern before framing, because retrofitting 12-inch spacing under a finished frame is not realistic. A 12 ft wide deck at 16 in on centre needs 10 joists; at 12 in it needs 13.
Fasteners and what else you need
This count covers surface boards only. Used as a deck fastener calculator, the rule is one fastener pair per board per joist:
- Face screws: 2 per board per joist. A 16 × 12 ft deck at 16 in centres is roughly 350 screws, so buy 400.
- Hidden clips: 1 per board per joist, sold by coverage — check the box, which usually states ft² per bucket.
- Joists, beams, posts and a ledger, none of which are in this count.
- Joist tape over every joist top. Cheap, and it roughly doubles the life of the framing by keeping water out of screw holes.
- Blocking at every butt joint and at mid-span on long joists.
Set the posts and footings first: the concrete calculator with the shape set to Circle handles deck footings, and stain or seal is priced separately with the deck stain calculator.
How this deck board calculator works out its numbers
Board counts are deck area × (1 + waste) ÷ coverage per board, rounded up. Coverage uses a 5½ in actual board width plus a ⅛ in gap, so each board covers 5⅝ in = 0.469 ft² per linear foot.
- The gap is inside the coverage figure, which it previously was not. The old constant of 0.458 ft²/ft was 5.5 ÷ 12, meaning boards butted with no gap at all, and it over-ordered by about 2%. Waste belongs in the waste field, not baked into the geometry.
- A different gap shifts the count by roughly 2% either way — tight-butted wet PT needs about 2% more board, a ¼ in composite gap about 2% less. Both sit comfortably inside the 10% waste allowance.
- Board widths vary. 5½ in is standard for 5/4×6 and 2×6 decking, but some composite runs 5¼ or 5⅝ in. Check the actual width before ordering a large deck.
- Not included: joists, beams, posts, ledger, footings, fascia, railings and stairs. This is the walking surface only.
- Diagonal layouts add roughly 15% in boards and need 12 in joist spacing, which is a framing decision rather than a decking one.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Deck Board Calculator: frequently asked questions
How many deck boards do I need?
Divide your deck area by the area each board covers, then add about 10% for cuts. A 16 ft board 5½ in wide laid with a ⅛ in gap covers 7.5 ft², so a 192 ft² deck needs 29 of them including waste.
How many deck boards per square foot?
About 2.13 linear feet of board per square foot of deck, for a 5½ in board with a ⅛ in gap. Put another way, 100 ft² of deck takes 14 sixteen-foot boards, 18 twelve-foot boards or 11 twenty-foot boards.
What gap between deck boards?
⅛ inch for kiln-dried wood, ⅙ to ¼ inch for composite per the manufacturer, and butt tight for wet pressure-treated lumber, which shrinks to a gap on its own as it dries. Gapping wet PT boards leaves inch-wide slots a year later.
Which way should deck boards run?
Perpendicular to the joists, so each board is supported across the framing. A 45° diagonal pattern hides the board ends and looks excellent, but needs 12 in joist spacing rather than 16 and about 15% more boards.
How many screws do I need for a deck?
Two per board per joist. A 16 × 12 ft deck framed at 16 in on centre works out to roughly 350 screws, so buy 400. Hidden fastener clips use one per board per joist and are sold by area coverage.
Should I use 12 or 16 foot deck boards?
Use whatever spans your deck without a butt joint. A 16 ft deck decked in 16 ft boards has no seams; the same deck in 12 ft boards has a seam line down every run, each needing blocking underneath and twice the end cuts.
Do I need blocking under deck boards?
Yes at every butt joint, so both board ends land on solid framing, and at mid-span on long joists to stop them twisting. Add joist tape over the joist tops while you are there.