How this calculator works
Decking boards run across the joists. The number of rows across the width is the deck width divided by the board width plus the gap, rounded up. Multiply by the length for the total length of board, add waste, and divide by the board length. If your boards are at least as long as the deck, you need one board for each row (or one for every couple of rows, if they are twice as long), plus the waste. Joists are spaced at your chosen centres along the length, plus one for the end, and 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: 3,000 ÷ 149 = 20.1, so 21 rows. That is 84 m of board, or 92.4 m with 10% waste, which is 31 boards of 3.0 m. At 400 mm centres you need 11 joists and about 462 screws.
Tips before you buy
- Decking that is more than 300 mm above the ground generally needs planning permission, and so does a deck in a conservation area or on a listed building. Check with your council before you build.
- Use treated timber (UC4 for ground contact) for joists and posts, and stainless or coated decking screws, which will not stain the boards.
- Leave a gap of 5 mm or so between boards, and a 10 mm gap at the house, so that water drains and the boards can move.
- Ventilate the space under the deck, and lay a weed-control membrane and gravel beneath it.
- Grooved boards are slip-resistant, but hold more leaves and moss. Brush the deck regularly, and treat it once a year.
- Buy the boards at the length of the deck if you can, to avoid butt joints, which are weak points and must fall over a joist.
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. Some thicker boards allow 450 or 600 mm, and diagonal boarding needs closer joists. Check the maker's instructions.
Do I need planning permission for decking?
Usually not if it is no more than 300 mm above the ground, but 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 460.
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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.