Decking calculator
With 144 mm boards and 5 mm gaps, every metre of deck width needs about 7 rows of boards.
- Joist lengths
- 10
- Deck screws
- 420
- Rows of boards
- 21
- Deck area
- 10.80 m²
3.6 × 3 m deck, 144 mm boards with 5 mm gaps, joists at 400 mm centres.
Joists include one at each end. Add two lengths for the outer frame along the sides, plus posts or bearers for raised decks.
Materials for common deck sizes
144 mm boards, 5 mm gaps, joists at 400 mm, 10% extra boards. Boards in the nearest standard length.
| Deck size | Boards | Joist lengths | Screws |
|---|---|---|---|
| 2.4 × 2.4 m (2.4 m boards) | 19 | 4 | 238 |
| 3 × 3 m (3.6 m boards) | 24 | 9 | 378 |
| 3.6 × 3 m (3.6 m boards) | 24 | 10 | 420 |
| 3.6 × 3.6 m (3.6 m boards) | 28 | 10 | 500 |
| 4.8 × 3.6 m (4.8 m boards) | 28 | 13 | 650 |
| 4.8 × 4.8 m (4.8 m boards) | 37 | 13 | 858 |
How we calculate this
Boards run along the length of the deck. We divide the width by one board plus its gap to get the number of rows, then work out how many boards of your length that takes, allowing for offcuts.
Joists run across the boards, spaced at your joist spacing, with one at each end. Each board is fixed with two screws at every joist it crosses.
Questions people ask
What spacing should decking joists be?
Usually 400 mm between centres. Some boards, especially thinner or composite ones, need 300 mm. The board maker’s instructions always come first.
How big a gap should I leave between deck boards?
About 5 mm is typical for softwood boards, so water drains and the timber can swell. Composite boards often need a specific gap, so check the instructions.
What length of deck board should I buy?
Match the board length to your deck if you can, so each row is one board with no joins. If your deck is longer than the boards, joins must land on a joist, and staggering them looks better.
How many screws do I need for decking?
Two screws for every place a board crosses a joist. The total is shown in your result. Buy a box or two extra for fixing the frame.
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