Decking Board Calculator
Assumes 144 mm boards with a 5 mm gap. Joists and fixings are separate.
How many decking boards do I need?
Enter the length and width of your deck and the calculator works out the boards, based on a standard 3.6 m board 144 mm wide laid with a 5 mm expansion gap, plus 10% for cuts and waste. It also gives the total length of board in metres, which is handy when boards are priced per metre.
This covers the deck boards themselves. The supporting joists, posts and fixings are separate, and joists are usually spaced 400 mm apart for domestic decking, closer if you lay the boards diagonally. Leave the expansion gaps so the timber can move without cupping.
Frequently asked questions
Is the decking boards calculator free?
Yes. It is completely free, with no sign-up and nothing to install. It runs entirely in your browser.
How accurate is it?
It gives a reliable estimate for ordering based on standard UK figures, but real quantities vary with ground conditions, wastage and how materials settle. Always add a margin and confirm with your supplier before buying.