How to calculate deck boards
Deck boards are laid in rows across the joists. To count them, figure out how many rows fit across the width of the deck, then how many boards each row takes along the length. The calculator does both, reusing leftover pieces from one row to finish another when they are long enough, then adds your waste allowance.
Boards = rows × boards per row (offcuts reused where they fit)
Joists = deck length in inches ÷ joist spacing, rounded up, + 1
Screws = 2 × rows × joists
Adding one gap to the deck width accounts for the fact that the last board has no gap after it.
Worked example: a 16 × 12 ft deck in 5/4 decking
The boards run along the 16 ft side, and rows are laid across the 12 ft width. The decking is 5/4×6 (5-1/2 in. wide) with 3/16 in. gaps, joists are 16 in. on center, and you buy 16 ft boards:
- Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 144.19 ÷ 5.69 = 25.35, so 26 rows. The last row is ripped narrower.
- Boards: a 16 ft board covers one full row (frame the deck an inch or two short of 16 ft if you want to trim the ends), so 26 boards. Add 10% waste: 26 × 1.10 = 28.6, so buy 29 boards.
- Linear feet installed: 26 × 16 = 416 ft.
- Joists: 192 in. ÷ 16 = 12, plus 1 = 13 joists, each about 12 ft long.
- Screws: 2 × 26 × 13 = 676 screws.
Choose board lengths that match your deck
Board length matters more than most people expect. When the board length matches the deck length (or divides into it evenly), there's almost no waste. When it doesn't, you pay for offcuts you can't use. Here's the same 12 ft wide deck (26 rows) built 20 ft long with different board lengths, before waste:
| Board length | Pieces per row | Boards to buy | Linear feet bought |
|---|---|---|---|
| 8 ft | 8 + 8 + 4 ft | 65 | 520 |
| 10 ft | 10 + 10 ft | 52 | 520 |
| 12 ft | 12 + 8 ft (4 ft offcut each) | 52 | 624 |
| 16 ft | 16 + 4 ft (one board makes four 4 ft pieces) | 33 | 528 |
| 20 ft | one full board | 26 | 520 |
The 20 ft boards cost more each and are harder to haul, but they leave no joints at all. The 12 ft boards are the worst choice here, buying 20% more decking than you need. A few more things to keep in mind:
- Joints must land on framing. With 16 in. joist spacing, joints at 4, 8, 12, 16 and 20 ft land on a joist, but 10 ft does not (120 ÷ 16 = 7.5), so 10 ft boards need extra blocking at every joint.
- Butt joints need double support. A single 1-1/2 in. joist leaves each board end only 3/4 in. to bear on. Add a doubled joist or blocking at the joint so both ends can be fastened without splitting.
- Stagger the joints. Don't line up joints in neighboring rows. A random pattern or a breaker board (a perpendicular board across the deck over a doubled joist) looks cleaner.
- Leave room to trim. Boards are usually installed a little long and cut flush or with a small overhang. If the deck is exactly 16 ft, 16 ft boards leave no room for that.
Joist spacing by decking type
Thinner boards need joists closer together. The figures below are the common maximums under the International Residential Code, which most US towns adopt. Composite and PVC decking follow the manufacturer's installation guide, which is also what keeps the warranty valid.
| Decking | Actual size | Straight (perpendicular to joists) | Diagonal (45°) |
|---|---|---|---|
| 5/4×6 wood | 1 × 5-1/2 in. | 16 in. | 12 in. |
| 2×6 wood | 1-1/2 × 5-1/2 in. | 24 in. | 16 in. |
| Composite or PVC | about 1 × 5-1/2 in. (varies) | Check manufacturer; often 16 in. | Check manufacturer; often 12 in. |
Even where 24 in. is allowed, many builders frame at 16 in. because the deck feels stiffer underfoot. Joist size, beam size, post spacing and footing depth depend on your spans and loads, so use your local code's span tables or get the plan reviewed with your permit. The concrete calculator estimates bags for footings.
Gaps between boards
- Pressure-treated pine is often sold wet and shrinks as it dries, so many builders lay it tight or with about 1/8 in. and let the gaps open up.
- Kiln-dried wood and cedar are usually gapped about 1/8 to 3/16 in. for drainage.
- Composite and PVC typically need 3/16 to 1/4 in. side gaps, plus end gaps that depend on the temperature at installation. Hidden clips usually set the side gap automatically.
Screws and hidden fasteners
Face-screwing uses two screws per board at every joist, which works out to about 3 screws per square foot at 16 in. spacing, plus extras at butt joints. Use deck screws rated for treated lumber (coated or stainless): about 2-1/2 in. long for 5/4 boards and 3 in. for 2× boards. Screws are sold by weight or count, so check the count on the box.
Hidden clip systems fit in the gap between boards, with roughly one clip at each place a board crosses a joist, plus starter clips for the first row. Composite makers often require their own clips or screws for the warranty, so buy fasteners made for your decking.
Need the area of an L-shaped or multi-level deck? Split it into rectangles with the square footage calculator and run each section here. For a matching fence, see the fence calculator.
Frequently asked questions
How many deck boards do I need for a 12×12 deck?
With 5-1/2 in. boards and 3/16 in. gaps, a 12 ft wide deck takes 26 rows. Using 12 ft boards, that is 26 boards, or about 29 with 10% for waste.
How many deck screws do I need per square foot?
About 3.2 screws per square foot (roughly 320 per 100 sq ft) for 5-1/2 in. boards on joists 16 in. apart, with two screws at each joist. Add extra for butt joints and dropped screws.
What joist spacing do I need for composite decking?
Most composite and PVC decking allows joists up to 16 in. on center for straight boards and 12 in. for diagonal layouts or stairs, but limits vary by product. Follow the manufacturer's installation guide.
How many joists do I need for a 16 ft deck?
At 16 in. on center, 192 in. ÷ 16 = 12 spaces, so 13 joists including both ends. At 12 in. on center you need 17, and at 24 in. you need 9. Add doubled joists wherever boards butt together.
How much gap should be between deck boards?
About 1/8 to 3/16 in. for dry wood, and typically 3/16 to 1/4 in. for composite, per the manufacturer. Wet pressure-treated boards are often installed tight because they shrink as they dry.