Decking Calculator
Work out how many decking boards you need for your deck.
Decking boards
42boards
- Rows of boards
- 21 rows
- Deck area
- 12 m²
Boards are counted running along the deck length, allowing for the gap between each board.
v1.0 · Reviewed September 2026
How this is worked out
Each board occupies its own width plus the gap. The deck width is divided by that pitch and rounded up to give the number of rows, and the deck length is divided by the board length and rounded up to give the boards per row. The two are multiplied.
Worked example
- Deck 4 m × 3 m
- Board 144 mm wide, 3.6 m long
- 5 mm gap
pitch 149 mm, so 21 rows; 4 ÷ 3.6 rounds up to 2 boards per row; 21 × 2 = 42
42 boards
Assumptions and limits
- Boards run along the deck length and each row is counted as whole boards, so offcuts are not carried over into the next row.
- Joists, bearers, fixings and any picture frame edging are not included.