Decking calculator
Work out how many deck boards you need for a deck area, from its dimensions and a waste allowance for cutting.
Result
Deck boards needed
How to use this tool
- Measure the length and width of the deck area.
- Leave the waste allowance at the 10% default, or increase it for a deck with lots of cuts around obstacles.
- Read the number of deck boards to order.
Assumptions
- The estimate uses illustrative coverage of about 0.52m² (roughly 3.6m × 145mm); board dimensions and installed coverage vary by product, spacing and layout. Account for joints and cuts when planning.
- Does not include the substructure (joists, bearers) or fixings — this is boards only.
Technical details: what the fields mean
- Length
length - The longest side of the deck area.
- Width
width - The other side of the deck area.
- Waste allowance
wastePercent - Extra to cover cut boards at edges, obstacles and end joints.
Methodology
Boards required = area × (1 + waste%) ÷ 0.52m² assumed coverage per board. This is based on an example board roughly 3.6m long and 145mm wide; verify coverage for your chosen product and layout.
Worked example
4m × 3m deck, 10% waste
Inputs: Length: 4 m, Width: 3 m, Waste allowance: 10%
Result: 26 deck boards using the calculator's illustrative 0.52m² coverage per board; check the selected product and layout.
Practical guidance
- Leave small gaps between boards for drainage and seasonal movement — check the manufacturer's recommended gap.
- Order a few extra boards beyond the calculated figure to allow for offcuts and any damaged boards.
Common mistakes
- Forgetting the substructure (joists and bearers) and fixings, which this calculator doesn't include.
- Not accounting for board length when planning joints — boards rarely span a deck's full length without a joint.
FAQs
How many deck boards do I need for a 4m x 3m deck?
With a 10% allowance and illustrative 0.52m² coverage per board, the estimate is 26 boards. Actual coverage depends on board dimensions, spacing and layout.
Does this include the frame underneath?
No — this calculates deck boards only. Joists, bearers, posts and fixings are separate costs.
Last reviewed: 2026-10-11
Assumes a standard board size — always confirm your chosen product's exact coverage before ordering. Boards only, not the substructure.
This tool provides an indicative estimate, based on the assumptions stated above.
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