Decking Calculator
Decking boards, joists and fasteners for a deck of a given size.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Board rows = ⌈ Deck width ÷ (board width + gap) ⌉
Deck boards = (rows × deck length) ÷ board length, rounded up
Joists = ⌊ deck length ÷ spacing ⌋ + 1; screws ≈ 2 × boards × joists
Run the calculation to see each step with your own numbers.
A 16 ft × 12 ft deck, 5.5 in boards with 3/16 in gaps, 16 ft boards, joists 16 in o.c., 10% waste.
Rows = ⌈144 ÷ 5.6875⌉ = 26; deck-board LF = 26 × 16 = 416 ft
Boards = 416 ÷ 16 = 26 → order 29 (with 10%)
Joists = ⌊192 ÷ 16⌋ + 1 = 13
- Rectangular deck with boards running the full length direction.
- Joists span the deck width; the beam, posts, ledger, hangers, railing and stairs are separate.
- Screw estimate is 2 per board per joist — hidden-fastener systems use a clip count from the manufacturer instead.
- No allowance for a picture-frame border beyond the waste percentage.
- Why 16 in joists for composite?
- Many composite boards require joists at 16 in o.c. for a straight layout and 12 in for a diagonal one, because composite is less stiff than wood. Always check the board's installation guide.
- What gap should I leave between deck boards?
- About 3/16 in for pressure-treated wood, which shrinks as it dries. Composite boards have their own spacing, often set by hidden fasteners — check the installation guide and enter that figure.
- How many screws will I need?
- Roughly two per board at every joist it crosses — a 16 × 12 ft deck at 16 in joist spacing is around 700 screws. Hidden-fastener systems use a clip count from the manufacturer instead.
Layout
Standard deck framing (IRC / prescriptive deck guide DCA 6) and manufacturer installation instructions. Standard geometry.
About the Decking Calculator
The Decking Calculator gives the deck boards, joists and fasteners for a rectangular deck, from the deck size, the board face width and gap, and the joist spacing. Board count depends on how many rows fit across the width once the gap between boards is included. The beam, posts, ledger, railing and stairs are separate calculations.
What you enter
- Deck length — The direction the deck boards run.
- Deck width
- Board face width — A "5/4×6" or 2×6 board is 5.5 in wide.
- Gap between boards — ≈ 3/16 in for wood; check the manufacturer for composite.
- Deck board length
- Joist spacing
- Joist length (ft) — Usually equal to the deck width.
- Waste allowance — Default 10%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Deck boards — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Joists — the exact result of the calculation.
- Deck screws — ≈ 2 screws per board at every joist. Buy the next box size up.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Floor Joist Calculator — Floor joist count and a span estimate — an estimate, not an engineering design.
- Board Foot Calculator — Board feet for a piece or a list of lumber from its dimensions.
- Lumber Calculator — Lineal feet and piece count of dimensional lumber for a job.
- Wood Beam Span Calculator — Allowable span estimate for a sawn-lumber beam under a uniform load — an estimate, not an engineering design.
Part of the Lumber & Wood calculators.