Decking Gap CalculatorDecking Board & Gap Spacing Calculator
Calculate how many decking boards and rows you need for any deck width, with the correct gap for drainage and timber movement. Gap guidance adjusts to the moisture content of your boards at installation.
Why this calculation matters
The gap decides drainage and whether boards buckle in the wet season. It is set by moisture content on the day of fixing, not by a standard table.
Decking Gap Calculator
Decking Board & Gap Spacing Calculator
Across the boards.
Along the boards.
Determines recommended gap.
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About Decking Gap Calculator
Decking boards must be laid with a deliberate gap between them for drainage, air circulation and to allow for the timber's natural expansion and contraction. Get the gap wrong and the deck either traps water and rots, or buckles as boards swell. This tool calculates how many boards and rows you need for a given deck width, and recommends the right gap based on the timber's moisture content at installation. It answers the most-asked question on any deck build — the right gap between decking boards — and totals boards and screws while it's at it.
Where Is This Used?
Class temperature is species-specific. The ITWA handbook sets the Thermo-D temperature per species, not once for the class: Pine and Spruce 212 ±3 °C, Radiata Pine 230 ±3 °C, Hemlock 225 ±3 °C, Ayous 208 ±3 °C, Iroko 190 ±3 °C, Ash and Frake 212 °C (−2/+5). Thermo-S is 190 ±3 °C for all species. Enter the temperature your supplier actually used — a class name alone does not fix it.
Formulas Used
Coverage per unit = Board width + GapBoard rows = ceil((Deck width + Gap) / Coverage per unit)Boards per row = ceil(Deck length / Board length)Total boards = Rows x Boards per rowActual coverage = (Rows x Board width) + ((Rows-1) x Gap)How the calculation works
The gap belongs between boards, not after the last one, which is why one gap is added before dividing and one fewer gap is counted in the coverage. Getting that off by one is how a deck ends up with a sliver board at the edge.
Show the formula
Board pitch = board width + gap. Boards = ceiling((deck width + gap) / pitch), since n boards carry n-1 gaps. Actual coverage = boards x width + (boards - 1) x gap. Boards per row = ceiling(deck length / board length).The recommended gap assumes boards fixed at the moisture content stated. Boards fixed wet need a smaller gap because they will shrink; boards fixed dry need a larger one.
References and what each one provides
- USDA Wood Handbook (FPL GTR-190)USDA Forest Products LaboratoryPublished density, moisture relationships, shrinkage coefficients and mechanical properties for a wide range of species. The reference dataset behind most wood-property figures on this site.
- EN 350CENDurability classification of wood and wood-based materials. Classes 1–5 describe heartwood resistance to fungal decay.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Assumes kiln-dried boards at the moisture content entered. It does not account for boards that will wet up before fixing, joist spacing effects, fastener type, or the seasonal range at the specific site.
Do not use this for Warranty compliance where the decking manufacturer specifies its own gap and fixing method.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- Board/gap geometry: n boards + (n−1) gaps
Formula checked against the sources above by an automated regression test (tests/test-calculators.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter your project details below, then press Calculate.
- Pick your timber moisture at install from the dropdown.
- Fill in deck width, deck length, board width, board length.
- Press Calculate.
Worked example
Common mistakes to avoid
- Setting summer gaps in the dry season. Boards installed dry and gapped tight close up completely in monsoon.
- Using the same gap for ThermoWood and untreated wood. ThermoWood moves less — its gaps can be smaller.
- Forgetting end-to-end (butt) gaps. Boards move along their length too, a little.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers
Full listing in the durability class reference.