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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.

Board Rows + CountGap GuidanceLinear MetresBy Moisture LevelPDF Report
DG

Decking Gap Calculator

Decking Board & Gap Spacing Calculator

Deck Area
mm

Across the boards.

mm

Along the boards.

Board & Gap
mm
mm
mm

Determines recommended gap.

OK
Decking Layout Results
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rows
Board Rows
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boards
Total Boards
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mm
Gap Used
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metres
Linear Decking
MetricValueDetail
Decking Calculation
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.
Where Is This Used?
Deck BuildingOutdoor ConstructionLandscapingMaterial EstimationCarpentryPatio + Terrace
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)
Frequently Asked Questions
How big should the gap between decking boards be?
It depends on the timber's moisture at installation. Green or wet boards will shrink as they dry, so start with 6-10mm gaps that will narrow over time. Dry or kiln-dried boards are stable and need only a 3-5mm gap for drainage and airflow. Hardwoods generally move less than softwoods.
Why not lay decking boards tight together?
Without gaps, water sits on the deck and cannot drain or evaporate, leading to rot, mould and a slippery surface. Boards also need room to swell in wet weather - laid tight, they push against each other and cup or buckle. The gap is essential for a deck that lasts.
Should gaps run only between boards or also at the ends?
Both. The side gaps handle the main drainage and width movement. End-to-end butt joints should also have a small gap (around 3-5mm) to let the board ends move and to stop water wicking into the end grain, which is the most vulnerable part of the board for rot.
ResourcesView the formulasFormula Library
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