How this calculator works
This runs our decking calculator. Decking boards run across the joists. The rows across the width are the width divided by a board plus its gap; the boards are the rows times the length with waste divided by the board length; and the joists are spaced along the length plus one. Each board is screwed down with two screws at every joist.
Worked example: a 4 × 3 m deck in 144 mm boards with 5 mm gaps and 3.0 m boards: 21 rows, 31 boards with 10% waste, 11 joists at 400 mm centres and about 462 screws, with 13 m² of membrane underneath. With 4.8 m boards it is 24 boards and no butt joints.
Assumptions and sources
- Decking more than 300 mm above the ground generally needs planning permission, and so do decks in conservation areas or on listed buildings. Posts, bearers and footings depend on the height and the ground, and are not included. Leave a gap of about 10 mm at the house and the end joints, and ventilate the space underneath.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Boards and joists by deck size (144 mm boards, 5 mm gaps, 3.0 m boards, 400 mm centres)
With 10% waste.
| Deck | Rows | Boards | Joists | Screws |
|---|---|---|---|---|
| 3 × 2.4 m | 17 | 19 | 9 | 306 |
| 3 × 3 m | 21 | 24 | 9 | 378 |
| 4 × 3 m | 21 | 24 | 11 | 462 |
| 4.8 × 3.6 m | 25 | 28 | 13 | 650 |
Tips before you buy
- Use UC4 pressure-treated timber for joists and posts that touch the ground, and treat all cut ends.
- Boards as long as the deck mean no butt joints, which must fall over a joist.
- Leave a gap of about 5 mm between boards so that water drains and the boards can move.
- Check the planning rules before you build: height above the ground is the main limit.
- Check for buried services before you dig, using the free LSBUD service.
Tools & materials for this project
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Frequently asked questions
How many decking boards do I need for a 4m x 3m deck?
31 boards of 3.0 m, using 144 mm boards with 5 mm gaps and 10% waste. That is 21 rows across the 3 m width.
How far apart should decking joists be?
400 mm centre to centre is standard for most decking, and it gives 11 joists along a 4 m deck. Some thicker boards allow wider spacing, and diagonal boards need closer joists.
Do I need planning permission for decking?
Usually not, if the deck is no more than 300 mm above the ground. A higher deck, or one in a conservation area, may need it. Check with your local council.
How many screws do I need for decking?
Two for each board at every joist it crosses. A 4 × 3 m deck uses about 462, so buy a couple of boxes.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.