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.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
rows = deck width (in) ÷ (board width + gap), rounded up
boards = rows × ⌈deck length ÷ board length⌉ × (1 + waste ÷ 100)
lineal feet = rows × deck length × (1 + waste ÷ 100)
joists = ⌊deck length (in) ÷ joist spacing⌋ + 1
screws ≈ rows × joists × 2, rounded up to the next 100
Assumptions & limits
- Board widths are actual, not nominal: 5/4×6 and 2×6 measure 5.5 in, composite about 5.25 in, 2×4 is 3.5 in.
- Default gap is 1/4 in. Pressure-treated boards installed wet can be butted tighter because they shrink as they dry; composite follows the maker’s spacing.
- Joists span the deck width and are spaced along its length at your chosen on-centre spacing. 16 in OC suits most wood decking; composite and diagonal layouts usually need 12 in OC.
- Two screws per board-to-joist crossing, which is standard face-screwed practice. Hidden-fastener systems use a different count entirely.
- Default waste is 10%. Diagonal or picture-frame layouts need more.
- This counts the deck surface and its joists. It does not size beams, posts, footings or the ledger connection — span tables in AWC DCA 6 govern those.
Worked example
A 16 ft × 12 ft deck using 16 ft 5/4×6 boards, 1/4 in gap, joists 16 in OC, 10% waste
- Rows = 12 × 12 ÷ (5.5 + 0.25) = 144 ÷ 5.75 = 25.04 → 26 rows
- Boards = 26 × ⌈16 ÷ 16⌉ × 1.10 = 28.6 → 29 boards
- Lineal feet = 26 × 16 × 1.10 = 458 ft
- Joists = ⌊16 × 12 ÷ 16⌋ + 1 = 13
- Screws = 26 × 13 × 2 = 676 → 700
Result:29 boards, 13 joists and about 700 deck screws for 192 sq ft of deck.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on August 17, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
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.
Deck board sizes — nominal vs actual
| Board | Actual width | Coverage with 1/4″ gap |
|---|---|---|
| 5/4×6 (standard decking) | 5.5 in | 5.75 in per row |
| 2×6 | 5.5 in | 5.75 in per row |
| 2×4 | 3.5 in | 3.75 in per row |
| Composite (typical) | 5.25–5.5 in | per maker’s gap spec |
The row count runs on actual width plus gap — using the nominal 6 inches undercounts rows by nearly 9% and leaves you short a few boards on every deck.
Common mistakes
- Nominal-width math. A “6 inch” board covers 5.75 inches with its gap — the difference compounds across forty rows.
- 16″ joists under diagonal decking. Diagonal and most composite layouts want 12″ on center; check the decking’s span chart before framing.
- Butting composite tight. Composite moves with temperature and needs the manufacturer’s gap — side AND end gaps. Wood installed wet gets butted tighter because it shrinks.
- Screwing board ends without pilots. Ends split; pre-drill or use hidden fasteners rated for your boards.
- Mixing composite batches. Color varies by production run — order the waste allowance up front, from one batch.
This counts the surface. The structure underneath — beams, posts, footings, ledger — follows the span tables in the AWC deck guide cited below, and the money side lives in the deck cost calculator. Buying lumber by volume? The board foot calculator converts your stack.
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.