Structure calculator

Deck Board Calculator

Plan deck boards with board width, spacing, orientation, and waste. Switch between US customary and metric units, adjust the assumptions, and keep every step visible.

01 Measure02 Allow03 Order

Project inputs

Use one unit system throughout. Defaults show a realistic example; replace them with measurements from your project.

Unit system

Deck lengthExample input — replace with your project value.ftDeck widthExample input — replace with your project value.ftActual board widthPlanning default — verify the selected product or project specification.inBoard gapPlanning default — verify the selected product or project specification.inStock board lengthPlanning default — verify the selected product or project specification.ftWaste allowanceAdd for cuts, breakage, spillage, and uneven site conditions.%

Measure the finished shape. This estimates field decking only. It does not include framing, picture-frame borders, stairs, blocking, or fastening requirements.

Estimate matches the inputs shown.

ReviewedNovember 2, 2026

ByBuildGauge Editorial Team

ScopeFormula, defaults & sources

Report an issue ↗

The calculation

A visible formula, not a black box.

Boards & length · Mathematical derivationboards = deck width ÷ (board width + gap) × deck length ÷ stock length × waste factor

Inputs are converted to a common base unit before calculation. Purchase quantities are rounded only at the final buying boundary.

Example inputDefaultStatus

Deck length20 ftReplace with measurement

Deck width12 ftReplace with measurement

Actual board width5.5 inPlanning default — verify

Board gap0.25 inPlanning default — verify

Stock board length16 ftPlanning default — verify

Waste allowance10 %Planning default — verify

Worked example

Example: a 20 × 12 ft deck

Using 5.5 in field boards, a 1/4 in gap, 16 ft stock, and 10% allowance, the sample rounds to 36 boards.

Use this to understand the workflow—not as a recommendation for your project.

Sources & scope

What supports this calculation.

Manufacturer dataTrex decking installation guide ↗

Product-specific gapping requirements; board size, gap, and stock length must match the selected product.

Mathematical derivationNIST SI conversion factors ↗

Exact length, area, and volume unit conversions; the project formula is derived from standard geometry.

Common mistakes

Check the job, not just the number.

01Leaving the example deck boards inputs unchanged instead of entering measured project values.
02Treating an editable coverage, size, density, or allowance as universal instead of checking the selected product label, project documents, and local requirements.
03Ordering from the rounded result without reviewing exclusions: This estimates field decking only. It does not include framing, picture-frame borders, stairs, blocking, or fastening requirements.

Questions, answered

Deck boards FAQ

What does the Deck Board Calculator estimate?

Plan deck boards with board width, spacing, orientation, and waste. The primary output is boards & length, with the intermediate assumptions kept visible.

How is the deck boards result calculated?

The visible formula is: boards = deck width ÷ (board width + gap) × deck length ÷ stock length × waste factor. Inputs are converted to a common base unit before the final purchase boundary is rounded.

Are the default values recommendations?

No. Dimensions are worked examples and product-dependent presets are planning defaults. Replace them with the selected product label, project documents, and local requirements.

Is the result ready to order from?

Use it as a checked planning estimate, then confirm packaging, minimum orders, installation instructions, site conditions, and any code or professional-design requirements.