How Many Deck Boards Do I Need? Chart for Common Sizes
Divide the deck's width by one board plus one gap. With standard 5.5 in boards and a 3/16 in gap, a 12 ft wide deck needs 26 rows. If your boards are as long as the deck, that's 26 boards, plus a few spares for splits and miscuts.
Deck board chart
Boards run along the deck's length and are laid side by side across its width. All rows use 5.5 in boards with a 3/16 in gap on joists at 16 in centres, with boards as long as the deck (20 ft for the largest), and no spares. Screws are two per board at each joist.
| Deck (length × width) | Rows | Boards | Screws |
|---|---|---|---|
| 8 × 8 ft | 17 | 17 × 8 ft | 238 |
| 10 × 10 ft | 22 | 22 × 10 ft | 396 |
| 12 × 12 ft | 26 | 26 × 12 ft | 520 |
| 16 × 12 ft | 26 | 26 × 16 ft | 676 |
| 20 × 16 ft | 34 | 34 × 20 ft | 1,088 |
| Deck (length × width) | Rows | Boards | Screws |
|---|---|---|---|
| 3 × 3 m | 21 | 21 × 3.0 m | 336 |
| 3.6 × 3.6 m | 25 | 25 × 3.6 m | 450 |
| 4.8 × 3.6 m | 25 | 25 × 4.8 m | 600 |
| 5.4 × 4 m | 28 | 28 × 5.4 m | 728 |
Worked example
- A 12 × 12 ft deck is 144 in wide. Each board plus its gap takes 5.5 + 0.1875 = 5.6875 in.
- 144 ÷ 5.6875 = 25.3, so 26 rows. The last board is ripped to fit.
- With 12 ft boards, each row is one board: 26 boards. Add 5% for spares: 28.
- Joists at 16 in centres: 144 ÷ 16 = 9 spaces, so 10 joists. Screws: 26 × 10 × 2 = 520.
Make the same deck 16 ft long with 12 ft boards and every row needs two boards joined over a joist: 52 boards, with plenty of offcuts. Buying 16 ft boards would need only 26.
Joist spacing and gaps
Trex, for example, says joists for composite decking should generally be no more than 16 in apart, and 12 in apart if the boards run diagonally. Closer joists mean more screws and a stiffer deck. Gaps between boards depend on the material and the maker's instructions; wood is often gapped at around 1/8 to 1/4 in.
Common mistakes
- Using the nominal width. A "6 inch" board is about 5.5 in wide. Using 6 in undercounts the rows.
- Forgetting the gap. Leaving out a 3/16 in gap on a 12 ft deck adds about one row you don't need; on wider decks the error grows.
- Joins between joists. Every butt joint must sit on a joist. Plan board lengths so joins land there, and stagger them.
- Picture-framing. A border board around the edge adds boards the chart doesn't include.
