How the deck calculation works
Deck area = length × width. Board count across width = ceiling[width ÷ (board width + gap)]. Board linear metres = board count × deck length × (1 + waste%). Joist estimate = ceiling(width ÷ joist spacing) + 1.
Worked example
A 6 m × 4 m deck has 24 m² of area. With 140 mm boards and 5 mm gaps, the calculator estimates the number of board runs across the width and adds the chosen waste allowance.
Guide: using this calculator accurately
For the full explanation, worked examples, measurement tips and FAQs, read the Deck Guide →.
Measure a deck area
Measure the finished outside dimensions of the rectangular deck. If the deck has steps, curves or multiple sections, split it into simple shapes.
Why board gaps matter
Deck boards do not normally form a completely solid surface. The gap between boards affects how many boards fit across the deck width, so it belongs in the material calculation.
Estimating linear metres
Once the number of board runs is known, multiply by the board run length. Add a waste allowance for cutting and unusable off-cuts.
Joist spacing is not a universal number
The example spacing is only an input. The correct joist spacing depends on board type, thickness, span tables, loads and the framing system. Follow the manufacturer's specifications and applicable building requirements.
Common deck-calculation mistakes
A material estimate is not the same as a structural design. Bearers, posts, footings, blocking, stairs, ledger details and fixings may all require separate planning.
Frequently asked questions
Does this calculate the full deck structure?
No. It estimates decking boards and a simple joist count for a rectangular deck. It does not design footings, beams, bearers, connections or stairs.
Should I include board waste?
Yes. Cutting and end trimming create off-cuts. The appropriate allowance depends on the deck shape and board layout.
Can I change the joist spacing?
Yes, but use the spacing specified for your decking product and structural design rather than choosing a smaller or larger value solely to change the estimate.