How it works
Rows = ceil(deck width in inches ÷ (board width + gap)). Boards per row = ceil(deck length ÷ board length): a run longer than one board needs boards joined end to end. Total boards = ceil(rows × boards per row × (1 + waste ÷ 100) − 1e-9). Total linear feet = total boards × board length.
The starting board width of 5.5 in and composite gap of 3/16 in, from the Southern Pine Inspection Bureau and Trex, are editable starting values. Wood decking gaps vary: enter the gap you plan to use.
Fasteners aren't estimated here. Follow your decking manufacturer's installation guide.
Worked example
A deck 12 ft across is 144 in. Each row takes 5.5 + 0.1875 = 5.6875 in, so 144 ÷ 5.6875 = 25.3 → 26 rows.
With 12 ft boards on a 12 ft run, each row takes 1 board: 26 × 1 = 26 boards before waste, or 312 linear ft.
Tips and common mistakes
- Length runs along the boards; width runs across them. Swapping the two changes the count.
- A nominal 6 in deck board is narrower in actual width. Measure your board or check the product listing.
- Off-cuts from butt-jointed rows are not reused in this count, so the result may leave spare pieces.
- For diagonal decking, add your own extra waste. No default diagonal allowance is assumed.
- Waste starts at 0% so the worked example holds. Raise it for cuts and damaged boards.
Questions about this calculator
How do I calculate how many deck boards I need?
- Rows = deck width in inches ÷ (board width + gap), rounded up.
- Boards per row = deck length ÷ board length, rounded up.
- Total = rows × boards per row, plus waste.
Example: a deck 12 ft across with 5.5 in boards and a 3/16 in gap needs 144 ÷ 5.6875 = 25.3, so 26 rows. With 12 ft boards on a 12 ft run, that is 26 boards before waste.
How do you calculate deck board spacing?
The gap comes from the board maker, not a formula; the calculator then adds it to the board width to count rows. Trex recommends a 3/16 in side-to-side gap for its composite deck boards in all weather, and up to 3/8 in in heavily wooded areas. Wood decking gaps vary with the wood and its moisture, so enter your own gap for wood.
How wide are deck boards?
Lumber is sold by nominal size. A nominal 2×6 is 5½ inches wide, because 2×4s and 2×6s are ½ in narrower than their names (Southern Pine Inspection Bureau). Composite and other boards vary. The calculator uses 5.5 in by default; measure your boards and change it if they differ.
What is the 3-4-5 rule for decks?
It is a way to check that a corner is square. Measure 3 ft along one side and 4 ft along the other; if the diagonal between those marks is exactly 5 ft, the corner is 90°. This works because 3² + 4² = 5². It is a layout check, not a structural rule.
Sources
Values marked with a link come from the linked source, which opens in a new tab. Unit conversions and our own defaults are listed separately. Every value the site uses is in the methodology source register.
Where the numbers come from 3
- Editable deck board actual width default
- 5.5 in
- Southern Pine Inspection Bureau, Nominal vs actual lumber sizes (cites PS 20-20) (opens in a new tab)blog.spib.org
- Meters per international foot
- 0.3048 m/ft
- NIST, international foot conversion (opens in a new tab)nist.gov
Unit conversions and our defaults8
- Inches per foot
- 12 in/ft
- Unit definition: one foot contains twelve inches
- Centimeters per meter
- 100 cm/m
- Metric prefix definition: centi is one hundredth
- Percent denominator
- 100 %
- Definition of percent: parts per hundred
- Deck board lengths offered
- 12, 16, 20 ft
- Owner specification; editable selection
- Deck board calculator starting dimensions
- 12, 12 ft run, ft width
- Owner worked example; editable starting measurements
- Deck board calculator starting board length
- 12 ft
- Owner worked example; editable selection
- Deck board waste default
- 0 %
- Owner build choice preserves the worked example; editable
- Count rounding tolerance
- 1e-9 unitless
- Owner specification: ceil(x − 1e-9)