Decking calculator
Enter the deck size, board width and gap. The calculator counts rows, boards, joists and the screws to fix them.
Measuring in feet and inches? Use the US version
Decking boards
26 3.6 m boards
- Rows of boards
- 21 rows
- Linear metres
- 84 m before waste
- Joists
- 11 at 400 mm centres
- Decking screws
- 462 screws (2 per joist)
How it works
Each row of decking takes up one board width plus one gap. The calculator divides the deck width by that to count rows, multiplies by the deck length to get linear metres, adds waste and divides by your board length. Joists are counted across the deck length at your spacing, and screws assume two per board at every joist.
Example
With deck length (direction boards run) 4 m, deck width (across the boards) 3 m, board width: 144 mm, gap between boards 5 mm, board length: 3.6 m, joist spacing: 400 mm centres, waste 10%, the answer is 26 3.6 m boards.
Tips before you buy
- Choose a board length that lets joins land on joists, and stagger joins from row to row.
- Leave a gap of around 5 mm between boards for drainage and movement. Follow the manufacturer for composite boards.
- Composite decking often needs joists at closer centres than timber. Check the product instructions.
Common questions
How many decking boards for a 4 × 3 m deck?
With 144 mm boards, 5 mm gaps and 3.6 m lengths, 26 boards including 10% waste.
What gap should decking boards have?
Around 5 mm is common for timber. Composite boards have their own required gap.
How far apart should deck joists be?
400 mm centres is common for timber decking. Some boards allow up to 450–600 mm, so check the manufacturer.