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Decking Calculator (UK)
Planning a timber deck? Enter the size and your board dimensions and this works out roughly how many decking boards, joists and screws you'll need — including a wastage allowance. A two-minute estimate before you order.
All figures include your 10% wastage allowance and are rounded up. Board coverage width is 0 mm. Screws assume 2 per board at every joist crossing — that's roughly 0 box(es) of 200.
How this is worked out
Boards are estimated by area and coverage width, not by laying out every board — so a real cutting plan may shift the count by one or two. Joists are assumed to run across the width of the deck (boards run along the length). Your actual frame design, board direction, picture-frame borders and any diagonal boards will change these numbers.
How many decking boards do I need?
Divide your deck area by the coverage width of one board (the board width plus the expansion gap), then divide by the board length to get the number of boards — and add a wastage allowance for offcuts and mistakes. This tool does that for you: it takes your deck area, multiplies by 1 plus your wastage percentage, and divides by the coverage area of a single board. It rounds up, because you can't order a fraction of a board. Treat the result as an ordering estimate, not an exact cutting list — a real layout with staggered joints can move the count by a board or two.
What gap should I leave between decking boards?
Leave roughly 5–8mm between boards. Timber decking expands and contracts with moisture, so a gap lets boards swell in the wet without buckling, and it lets rainwater and debris fall through instead of pooling on the surface. Very dry, freshly-kiln-dried boards can be laid a touch tighter because they'll swell; damp or reclaimed boards want a slightly wider gap because they'll shrink as they dry. The default here is 5mm — adjust it to match your boards and the manufacturer's guidance.
How far apart should deck joists be?
400mm centres is the typical spacing for standard timber decking laid straight across the joists. Drop to 300mm centres for heavier use, for thinner or softer boards, or where boards are laid at an angle (diagonal decking spans a greater distance between joists, so it needs more support). Always check the board manufacturer's span tables — composite and hardwood boards often specify their own maximum joist spacing, which can be tighter than 400mm.
Fencing the boundary or setting posts in concrete?
Building a fence around the new deck? Our Fencing Calculator works out panels, posts, gravel boards and postcrete for a straight run. Setting deck support posts in a full concrete footing rather than postcrete? Our Concrete Calculator has a post-hole mode built in.
Does this tool store my details?
No. This calculator runs entirely in your browser — every figure you type is worked out on your own device and nothing is uploaded, saved or sent anywhere. There's no sign-up and no email required to see your results.
Please note: this tool gives a guide or estimate only. It may not be 100% accurate and is not professional advice — always check the figures before you rely on them. These are rough material estimates to help you plan and order — joist layout, board direction and frame design vary, so confirm quantities against a proper cutting plan before you buy. Everything you type stays in your browser.
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