Decking calculator
Enter deck length and width, board width and joist spacing — we work out boards, joists and screws with a safe wastage allowance.
Your job
Results
- Area
- 20 m²
- Boards
- 60
- Joists
- 14
- Screws
- 832
Assumes 5 mm gap between boards and 2 screws per board/joist crossing. Add framing timber separately.
Frequently asked questions
How many decking boards per m²?
For 145 mm boards with a 5 mm gap ≈ 6.7 lin.m per m². A 4 × 3 m deck (12 m²) needs ~80 lin.m — buy 10 % extra for end cuts.
What joist spacing should I use?
400 mm centres for softwood and standard composite; 300 mm centres for hardwood or if boards run diagonally. Wider than 450 mm and boards flex underfoot.
Do I need a gap between boards?
Yes — 5–6 mm for kiln-dried softwood and composite, 3 mm for hardwood. The gap lets water drain and boards move seasonally without cupping.
How high should the deck sit above the ground?
Leave at least 150 mm of airflow beneath and a fall of 1:80 away from the house. Boards laid flush to the ground rot within two winters.
Estimates only. Check the paint tin / adhesive bag for the exact coverage of your product.