Deck Board Calculator
Work out how many deck boards to buy for a rectangular deck, how many linear feet that is, and how many joists and screws go with it. It works for pressure-treated, cedar and composite boards: just enter the actual board width and the gap.
How to use the deck board calculator
- Enter the deck length in the direction the boards will run, and the width across the boards.
- Enter the board's actual width (5.5 in for a nominal 6 in board) and the gap between boards.
- Choose the board length you will buy. If the deck is longer than one board, the calculator adds a second board per row.
- Set the joist spacing (16 in is common; 12 in for diagonal layouts and many composites) and the number of screws per board at each joist.
- Enter your board and screw prices under Cost estimate.
How the deck board count works
- Rows = deck width across the boards (in) ÷ (board width + gap), rounded up
- Boards per row = deck length ÷ board length, rounded up
- Boards = rows × boards per row × (1 + waste), rounded up
- Linear feet = rows × deck length
- Joists = deck length (in) ÷ joist spacing, rounded up, + 1
- Screws = rows × joists × screws per board at each joist
Nominal and actual sizes differ: a "2×6" or "5/4×6" deck board is 5.5 in wide (American Softwood Lumber Standard, PS 20). Always use the actual width, or the row count comes out about 8% low.
Worked example: a 16 ft × 12 ft deck
This is the deck loaded above, with 5.5 in boards, ⅛ in gaps and 16 ft boards running the long way.
- Rows: 144 in ÷ (5.5 + 0.125) in = 25.6, rounded up to 26 rows.
- Boards per row: 16 ÷ 16 = 1, so 26 boards before waste.
- With 10% waste: 26 × 1.10 = 28.6, so 29 boards.
- Linear feet: 26 × 16 = 416 ft, useful when boards are priced per foot.
- Joists at 16 in: 192 ÷ 16 + 1 = 13.
- Screws: 26 rows × 13 joists × 2 = 676.
The last row usually has to be ripped narrower to fit; that's included in the rounding up.
Joists, gaps and fasteners
The American Wood Council's DCA 6 deck guide calls for each deck board to be fastened to every joist with two 8d threaded nails or two #8 screws, boards spaced about ⅛ in apart, laid anywhere from square to the joists up to 45°, and each board bearing on at least three joists. Composite and PVC boards have their own gap and spacing rules (often 12 in joist spacing, or less for diagonal layouts), so follow the maker's installation guide. The joist count here is for planning only; joist size, span, beams and footings must follow IRC R507 or your local code.
Deck boards for common deck sizes
| Deck (along × across boards) | Rows | Board length | Boards + 10% | Joists at 16 in | Screws (2 per joist) |
|---|---|---|---|---|---|
| 10 × 10 ft | 22 | 10 ft | 25 | 9 | 396 |
| 12 × 12 ft | 26 | 12 ft | 29 | 10 | 520 |
| 16 × 12 ft | 26 | 16 ft | 29 | 13 | 676 |
| 16 × 16 ft | 35 | 16 ft | 39 | 13 | 910 |
| 20 × 12 ft | 26 | 10 ft | 58 | 16 | 832 |
| 20 × 16 ft | 35 | 10 ft | 77 | 16 | 1,120 |
| 20 × 20 ft | 43 | 10 ft | 95 | 16 | 1,376 |
5.5 in boards with a ⅛ in gap. 20 ft decks use two 10 ft boards per row (or 20 ft boards where you can get them).
Deck board tips
- Buy board lengths that suit the deck. A 16 ft deck uses 16 ft boards with no joints; a 20 ft deck uses two 10 ft boards per row (or 20 ft boards if available) to avoid wasting 12 ft offcuts.
- Stagger the butt joints. Where boards meet end to end, land each joint on a joist and offset joints from row to row.
- Allow more waste for diagonal decking. Angled boards create more offcuts; 15% is a safer allowance than 10%.
- Picture-frame borders and stairs add boards. Count border boards (deck perimeter ÷ board length) and stair treads separately; the deck stair calculator gives the tread count.
Frequently asked questions
How many deck boards do I need for a 12×16 deck?
With 5.5 in boards and ⅛ in gaps running along the 16 ft side, you need 26 rows of 16 ft boards: 26 boards, or 29 with 10% waste. That is 416 linear feet.
How many screws do I need for a deck?
About two per board at every joist. A 12 × 16 ft deck with 26 rows over 13 joists needs 26 × 13 × 2 = 676 screws. Hidden fastener clips are usually one per board per joist.
What gap should I leave between deck boards?
AWC's DCA 6 guide says about ⅛ in for wood decking. Wet pressure-treated boards are often laid tight because they shrink as they dry. Composite boards: use the gap in the maker's instructions.
How many joists do I need for a deck?
Divide the deck length in inches by the joist spacing, round up and add one. A 16 ft deck at 16 in spacing has 13 joists; at 12 in it has 17. Joist size and span must follow your local code.
How wide is a 6 inch deck board?
5.5 in. Lumber is sold by nominal size: a 2×6 or 5/4×6 board is actually 5.5 in wide once dried and planed (American Softwood Lumber Standard).
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