How to use the deck calculator
Length is the direction your deck boards run; width is the distance across them. On most attached decks the boards run parallel to the house, so the length is the side along the house.
Enter the board's actual face width: a nominal 6 in deck board, wood or composite, is about 5.5 in wide. Choose the board length you'll buy. Longer boards mean fewer butt joints but cost more per foot and are harder to haul. The gap is the space between boards; composite makers usually specify it, often 3/16 in.
How the boards are counted
- Rows = deck width (in) ÷ (board width + gap), rounded up
- Boards: each row is filled with as few full boards as possible. When a row needs a short piece, an offcut of 2 ft or longer from an earlier row is used first.
- Extra: 5% more boards for splits, warped boards and a bad cut
- Joists = deck length (in) ÷ joist spacing + 1, not counting rim or doubled joists
- Screws = rows × joists × 2 (two screws where each board crosses each joist)
Butt joints between boards must land on a joist, and many builders double the joist there. If you plan joints in advance, a pattern of 16 ft and 8 ft boards can stagger them neatly.
Boards for common deck sizes
5.5 in boards, 3/16 in gaps, joists 16 in on center, 5% extra:
| Deck | Board length | Rows | Boards | Joists | Screws |
|---|---|---|---|---|---|
| 8 × 8 ft | 8 ft | 17 | 18 | 7 | 238 |
| 10 × 10 ft | 10 ft | 22 | 24 | 8 | 352 |
| 12 × 12 ft | 12 ft | 26 | 28 | 10 | 520 |
| 16 × 12 ft | 16 ft | 26 | 28 | 13 | 676 |
| 20 × 12 ft | 20 ft | 26 | 28 | 16 | 832 |
| 16 × 16 ft | 16 ft | 34 | 36 | 13 | 884 |
| 24 × 16 ft | 16 ft | 34 | 54 | 19 | 1,292 |
Joist spacing for deck boards
The IRC limits how far deck boards may span between joists (Table R507.7, boards over three or more joists):
| Decking | Boards square to joists | Boards on a 45° diagonal |
|---|---|---|
| 5/4 wood (1 1/4 in) | 16 in on center | 12 in on center |
| 2x wood (2 in) | 24 in on center | 24 in on center |
Joists themselves have span limits by size, species and spacing. For southern pine at a 40 psf live load, the 2021 IRC lists:
| Joist | 12 in on center | 16 in on center | 24 in on center |
|---|---|---|---|
| 2x6 | 9'-11" | 9'-0" | 7'-7" |
| 2x8 | 13'-1" | 11'-10" | 9'-8" |
| 2x10 | 16'-2" | 14'-0" | 11'-5" |
| 2x12 | 18'-0" | 16'-6" | 13'-6" |
Board counts are for buying materials. Ledger attachment, footings, beams, joist spans and railings are set by your local building code, and most decks need a permit. Confirm the structure with your building department before you build.
Worked example: a 20 × 12 ft deck with 16 ft boards
- Rows: 12 ft = 144 in. (144 + 3/16) ÷ (5.5 + 3/16) = 25.4, rounded up = 26 rows.
- Each row is 20 ft: one 16 ft board plus a 4 ft piece. The first row's 4 ft piece comes from a new board, leaving a 12 ft offcut; that offcut supplies the next three rows' 4 ft pieces.
- Boards: 33 before extras, 35 with 5%. With 20 ft boards it would be 28 boards and no butt joints at all.
- Joists at 16 in: 240 ÷ 16 + 1 = 16, each 12 ft long.
- Screws: 26 × 16 × 2 = 832. Buy a 1,000-count box, or hidden fasteners if your boards use them.
Questions people ask
How many deck boards do I need for a 12 × 12 deck?
28 twelve-foot boards, with 5.5 in boards, 3/16 in gaps and 5% extra.
Should deck boards run parallel or perpendicular to the house?
Usually parallel, so the joists run out from the house and hang from the ledger. Diagonal boards look good but need closer joist spacing and about 10% more material.
How far apart should deck joists be?
16 in on center is the most common choice and works for 5/4 wood and most composite boards laid straight. Diagonal decking usually needs 12 in.