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 calculator
- 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
- Assumes a standard deck board size (roughly 3.6m × 145mm, ~0.52m² coverage) — check your chosen product's exact dimensions, as these vary by manufacturer.
- 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 coverage per board — based on a standard deck board roughly 3.6m long and 145mm wide.
Worked example
4m × 3m deck, 10% waste
Inputs: length=4, width=3, wastePercent=10
Result: 26 deck boards required
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% waste allowance, about 26 boards, assuming a standard board size.
Does this include the frame underneath?
No — this calculates deck boards only. Joists, bearers, posts and fixings are separate costs.