Deck Board Calculator
Calculate how many deck boards you need — enter your deck size, board width, board length and gap to get the board count, lineal feet, joists and fasteners, with a shopping list.
How to calculate deck boards
Deck boards run across the joists, so the count depends on your board width and the gap between boards. Divide the deck width by the board coverage — the board width plus the gap — to get the number of rows, then work out how many board lengths fill each row. The calculator above does this for any board size, length and gap, then adds the joists and fasteners and a waste allowance.
rows = deck width ÷ (board width + gap) · boards = rows × (deck length ÷ board length)
Deck board sizes & coverage
| Board | Actual width | Rows per 12 ft width |
|---|---|---|
| 5/4×6 (most common) | 5.5″ | ≈ 26 rows |
| 2×6 | 5.5″ | ≈ 26 rows |
| Composite | 5.25–5.5″ | ≈ 25–26 rows |
| 2×4 | 3.5″ | ≈ 39 rows |
Rows shown at a 1/4″ gap — your gap and waste allowance change the exact count.
Pricing the build instead? Use the deck cost calculator, or plan a ground-level deck with the floating deck calculator.
Frequently asked questions
How many deck boards do I need?
Divide the deck width by the board coverage (board width plus the gap between boards) to get the number of rows, then multiply by the deck length for lineal feet, and divide by your board length for the board count. The calculator above does this for any board size and gap, and rounds up with a waste allowance.
What size are deck boards?
The most common decking is 5/4×6, which is actually about 1 inch thick and 5.5 inches wide. Composite boards are similar at roughly 5.5 inches wide. Standard lengths are 8, 12, 16 and 20 feet. The calculator lets you set the actual board width, length and gap so the count matches your product.
What gap should I leave between deck boards?
A gap of about 1/8 to 1/4 inch is typical, to let the deck drain and the boards expand. Pressure-treated boards installed wet can be butted tighter since they shrink as they dry; composite usually follows the maker’s spacing. The calculator factors your gap into the board count.
How many screws do I need for a deck?
Plan on about 350 deck screws per 100 square feet of decking — roughly two screws where each board crosses a joist. The calculator estimates the total from your board layout and joist spacing so you can buy the right box size.
What joist spacing should I use?
Joists are usually 16 inches on center for wood decking. For composite, or when running boards diagonally, 12 inches on center is often required — check your decking’s instructions. The calculator uses your spacing to estimate the number of joists.