Decking Calculator
How many deck boards a surface needs, with the gap between boards included. This covers the decking itself, not the frame underneath.
Switching units keeps your measurements. Nothing is reset.
Areas
Your boards
Added across the width only, not to the board length.
Waste allowance
10% covers cuts and the occasional bowed board you set aside.
Offcuts, breakages, pattern matching and the occasional damaged piece all mean you use more material than the bare area. Keeping a few spare pieces for future repairs is also normal.
This is a planning allowance, not a rule. Adjust it for your layout, your material and your own tolerance for a second trip to the store.
Estimated cost
Cost uses the price you enter. This product never shows invented market prices.
Your result appears here
Enter your measurements to see how much material to buy.
How to use this calculator
- 1
Measure the deck surface. An L-shaped deck is two rectangles; add a second area.
- 2
Measure the actual board width. A board sold as 6 inch is usually about 5.5 inch of real coverage.
- 3
Enter the board length you are buying and the gap you are leaving between boards. The gap is added across the width only.
- 4
Adjust the waste allowance for cuts and for boards you set aside because they are bowed.
A worked example
A 12 ft × 16 ft deck in 5.5 in × 12 ft boards with a 1/4 in gap and a 10% waste allowance:
Measured area
192 ft²192 ft²
Add the waste allowance
192 ft² × 1.1211.2 ft²
Divide by the space one piece occupies
211.2 ft² ÷ 5.75 ft²36.73
Round up to whole pieces
round up 36.7337
You will need
37 boards
This example is produced by running the calculator above, so it always matches what the tool does.
The formula
Each board covers its width plus the gap, multiplied by its length. The deck area with the waste allowance applied is divided by that and rounded up. The gap is added across the width only, because boards run full length rather than being gapped end to end.
Decking notes
This is the surface, not the structure
Joist spacing, beam sizing and footing depth are structural decisions that depend on span, load and local requirements. This calculator deliberately does not touch them: it counts the boards that go on top.
Board length changes the waste
Boards that divide neatly into your deck length waste very little. Boards that leave a short offcut on every run waste a lot. If you have a choice of lengths, the one that fits your deck cleanly is usually cheaper overall.
Gaps are for drainage and movement
The right gap depends on the material and how wet it was when laid. Timber that arrives soaking will shrink; composite moves with temperature. Use the manufacturer figure for the product you are buying.
Common questions
- How many deck boards do I need?
- Divide the deck area by the area one board covers, which is its width plus the gap times its length, then round up and add an allowance for cuts. A 12 ft by 16 ft deck in 5.5 in by 12 ft boards needs about 37 boards at 10% waste.
- Does this include the joists and frame?
- No, deliberately. Framing is a structural question about spans and loads, not a material count, and getting it wrong is a safety issue rather than a shopping one.
- Should I use the nominal or actual board width?
- Always the actual width. A nominal 6 inch board typically covers about 5.5 inch, and using the nominal figure would leave you roughly 9% short.