Decks & Fences
Decking Calculator
Enter your deck length and width in feet to get the number of deck boards and joists to buy, using editable standards of 5.5 in boards and 16 in on-center joist spacing.
Formula tested · Local units · No sign-up
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Enter measurements
Use your preferred units. Results update automatically.
Show the calculation methodFormula, conversions, rounding, and assumptions+
Rows of boards = deck width ÷ (board width + gap), rounded up. A standard 5/4 or composite deck board is 5.5 in wide; with a 1/8 in drainage gap each row takes 5.625 in of width.
Boards per row = deck length ÷ board length, rounded up — a 16 ft board on a 16 ft deck is exactly one board per row.
Boards to buy adds your wastage allowance and rounds up; joists = deck length ÷ joist spacing (16 in on-center is the common standard) rounded up, plus one closing joist.
Real-world example
Worked example: 16 ft × 12 ft deck, 5.5 in boards, 1/8 in gap, 16 ft boards, 16 in joists
- Row width: 5.5 + 0.125 = 5.625 in. Rows: 144 in ÷ 5.625 = 25.6 → round up to 26 rows.
- Boards per row: 16 ÷ 16 = 1 board.
- Boards to buy: 26 × 1 × 1.10 (10% wastage) = 28.6 → round up to 29 boards.
- Joists: 192 in ÷ 16 = 12, + 1 closing joist = 13 joists.
- Example cost: 29 boards × $40 = $1,160; 13 joists × $15 = $195; materials $1,355; 7% tax $94.85.
Buy 29 boards and 13 joists. With the example prices above and 7% sales tax, the estimate is $1,449.85 — enter your supplier's quote and local rate.
Before you start
How to measure
- Measure the finished deck length (the direction the boards run) and width in feet.
- Use the actual board width, not the nominal size — a 'six-inch' deck board actually measures 5.5 in; composite boards state their true width on the spec sheet.
- Enter the board length you'll actually buy (12, 16 and 20 ft are common stock lengths) — buying boards that span the deck avoids butt joints.
Local guidance
Notes for United States
- 16 in on-center is the common joist spacing for wood decking laid perpendicular to joists; many composite brands require closer spacing (often 12 in) for diagonal installs — follow the manufacturer's span chart.
- Most US jurisdictions require a permit for attached or elevated decks, and ledger attachment and footing details are inspected — check with your building department before building.
- Pressure-treated southern yellow pine is the standard framing lumber; decking itself may be treated wood, cedar or composite — the calculator works for any of them since you enter the board size.
- Sales tax varies by state and locality and is entered manually.
Quick reference
Common US decking planning values
| Item | Common value |
|---|---|
| Board width (actual) | 5.5 in |
| Drainage gap | 1/8 in (composite: per manufacturer) |
| Board stock lengths | 12, 16, 20 ft |
| Joist spacing | 16 in on-center (12 in for some composite/diagonal) |
Planning values only — manufacturer span charts and local code govern.
Good to know
Common mistakes to avoid
- Using the nominal 6 in width instead of the actual 5.5 in — the row count comes out wrong.
- Ignoring the manufacturer's joist-spacing requirement for composite boards.
- Forgetting wastage: cuts, crowned boards and damaged pieces routinely eat 10%.
- Treating this estimate as a structural design — footings, beams and ledger connections are not sized here and usually need a permit.
Need help?
Frequently asked questions
How many boards for a 12 ft × 12 ft deck?
Rows: 144 in ÷ 5.625 = 25.6 → 26 rows. With 12 ft boards that's one board per row, and 26 × 1.10 = 28.6 → 29 boards with 10% wastage. Joists at 16 in: 144 ÷ 16 = 9, + 1 = 10 joists.
What changes if I frame at 12 in on-center?
Only the joist count: for the 12 ft-long deck above, 144 ÷ 12 = 12, + 1 = 13 joists instead of 10. Board count is unchanged. Closer spacing is often required for composite or diagonal decking.
Does the calculator work for composite decking?
Yes — enter the composite board's actual width, required gap and stock length from its spec sheet. Composite brands set their own joist-spacing and gapping rules, which override the wood defaults shown here.
Keep planning
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- United States
- Spotted an error?
- Report a correction
Methodology
- Boards are assumed to run parallel to the deck length; joists run parallel to the width, spaced along the length. All dimensions are converted to metres internally before any arithmetic.
- Rows of boards = deck width ÷ (board width + gap), rounded UP. Boards per row = deck length ÷ board length, rounded UP — part boards count as whole boards because that's what you buy.
- Boards to buy = rows × boards per row × (1 + wastage%), rounded UP. Joists = deck length ÷ joist spacing, rounded UP, plus one closing joist. Exact multiples are not bumped up.
- The cost estimate multiplies the board and joist counts by the prices you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Board dimensions: 5.5 in is the actual width of a nominal 6 in deck board; 1 in = 25.4 mm (exact).
- Joist spacing: 16 in on-center is common US practice for perpendicular wood decking; manufacturer charts govern composites.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not size joists, bearers, posts, footings or ledger connections — deck structure is an engineering and permit matter. Confirm the structural design with a qualified person and quantities with your supplier before ordering.