Deck board calculator
Board count depends on one number most calculators hide: the coverage width of a single row. That is the actual face width plus the gap. Get it wrong by half an inch and a 16 ft deck is off by a board or two in either direction.
Your deck
If you buy this length instead
| Stock length | Boards |
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How this is calculated
Rows are counted across the deck width, then multiplied by the run length. Three formulas, in order:
- Coverage width = face width + gap. A 5.5″ board with a 1/4″ gap covers 5.75″ per row.
- Rows = ⌈(deck width in inches) ÷ coverage width⌉. Rounded up, because a partial row still needs a board.
- Linear feet = rows × run length × angle factor, then × (1 + waste%).
Worked example
A 16 ft × 12 ft deck, 5.5″ boards, 1/4″ gap, straight layout, 10% waste:
- Coverage = 5.5 + 0.25 = 5.75″
- Rows = ⌈(12 × 12) ÷ 5.75⌉ = ⌈25.04⌉ = 26 rows
- Linear feet = 26 × 16 = 416, × 1.10 = 457.6 lf
- At 16 ft stock: ⌈457.6 ÷ 16⌉ = 29 boards
Note what that 10% actually bought: 41.6 extra linear feet, which is 2.6 boards. On a rectangular deck with a straight layout that is usually enough. Add a picture frame and it is not.
Where the waste actually goes
A 10% allowance is not padding — it disappears quickly. Butt joints have to land on a joist, which forces cuts that leave offcuts too short to reuse. A picture-frame border consumes a full board width on every side plus four mitred corners. Boards arrive with crook, checks and mill damage. And if you are laying diagonal, every board is cut at both ends, which is why the angle factor is √2 and the recommended waste is 15%.
What this calculator will not plan
- It does not plan your joints. Where boards butt, and whether the joint lands on a joist, changes both the count and the look. Plan splices before you order.
- It does not include stairs, fascia, or a picture-frame border. Those are separate stock and separate cuts.
- It does not size your framing. Joist spacing and depth are structural — see the joist calculator for layout, and a span table for sizing.
- It assumes rectangular geometry. L-shapes and angled bays must be split into rectangles and added.
Questions people actually ask
How many deck boards do I need for a 12×16 deck?
With 5.5″ boards, a 1/4″ gap and 10% waste, you need 26 rows and about 458 linear feet, which is 29 boards at 16 ft stock. Change the gap to 1/8″ and the row count stays at 26; change the board to a 3.5″ face and it jumps to 42 rows.
Is a 1/4″ gap right, or 1/8″?
1/4″ is the common spec for pressure-treated wood and most composite. 1/8″ is used for hardwood such as ipe installed dry, because it expands less. Grooved composite with hidden clips usually specifies 3/16″. Follow the manufacturer — the gap is part of the product warranty.
Does the gap really change the board count?
Yes, and it accumulates. Across a 12 ft width, 26 rows at 5.75″ coverage versus 5.625″ coverage is a difference of about one row. Small per board, one full board per deck.
Why does diagonal decking need so much more material?
Two reasons: the run across the deck is the diagonal (1.414× longer than the width), and both ends of every board are cut at an angle, so offcuts are rarely reusable. Plan on 15% waste, not 10%.
Can I use this for composite decking?
Yes, if you enter the actual face width and the gap the manufacturer specifies for their clip system. Composite profiles are not always 5.5″ — some are 5.25″ — and that difference alone moves a 12 ft deck by a row.