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How the decking calculator works
Decking is a grid of two materials — boards running one way, joists running the other — and the count of each comes from a different calculation.
For the boards, the calculator works on the module: the board width plus the gap. With 140 mm boards and a 5 mm gap, each board effectively occupies 145 mm across the deck. Divide the deck width by that module and round up, and you get the number of rows. Then divide the deck length by the board length, round up, and you have how many boards go end-to-end in each row. For a 5 m × 4 m deck in 3.6 m boards that is 28 rows of 2 — 56 boards.
For the joists, centres do the work. Joists run across the width at 400 mm centres, and there must be one at the start and one at the end of every run, so the count is the length divided by the centres, rounded up, plus one. A 5 m deck at 400 mm centres needs 14 joists, and the last bay is only 200 mm because the spacing is never allowed to exceed the centres.
Fixing and hangers
Boards are fixed with two screws at every point where they cross a joist — one near each edge. That is 14 crossings per board row on a 5 m deck, so a 56-board deck takes about 1,568 screws, or seven boxes of 250. Always use stainless or coated decking screws: ordinary steel rusts, stains the timber and eventually snaps.
Joist hangers are counted two per joist — one at each end where the joist meets a rim board or a wall ledger. If you are building a raised deck on posts instead, you will use post bases and carriage bolts rather than hangers, so treat that line as the ledger-and-rim case.
Board direction, spacing and airflow
This calculator assumes boards run along the length and joists across the width. Running boards the other way is perfectly valid — just swap the length and width you type in — but keep the joists perpendicular to the boards and the centres at 400 mm for 25–28 mm boards. Thinner or composite boards want 300 mm centres to stop them feeling springy underfoot.
Leave a 10–15 mm gap at walls and a 5 mm gap between boards. Air needs to move under the deck, so keep the underside clear and put a weed membrane on the ground, not tight against the joists. Use treated timber, cut the board ends so water runs off, and paint the cut ends with preservative before they go down — it is the single cheapest thing you can do to make a deck last.
Decking FAQs
How many boards do I need for a 5 m × 4 m deck?
With 140 mm boards, a 5 mm gap and 3.6 m board lengths: 56 boards. That is 28 rows across the width, each made of two boards end-to-end. Choose a board length that divides your run if you can — a 5 m run in 5.1 m boards would need only one board per row, halving the count on paper.
How many joists do I need?
A 5 m deck at 400 mm centres needs 14 joists: one at the start, one at the end, and one at each 400 mm interval along the run. The last bay works out at 200 mm, which is fine — the rule is that the spacing must never exceed the centres, so a shorter end bay is always the right answer.
Should joists be at 400 mm or 600 mm centres?
Use 400 mm for standard 25–28 mm softwood decking boards. 600 mm is too springy and boards will cup and flex. For thinner boards, or for composite decking that gets hot and soft in the sun, go to 300 mm centres.
How many screws do I need for a deck?
Two screws per board per joist crossing. A 5 m × 4 m deck works out at about 1,568 screws — roughly seven boxes of 250. Buy boxes rather than loose, keep a handful spare, and use stainless or coated decking screws so they do not rust and stain the boards.
Do I need joist hangers?
Only where a joist meets a rim board, ledger or frame — that is two per joist, one at each end. On a low deck sitting on bearers, hangers are not used at all. On a raised deck on posts you need post bases and fixings to the frame, and the structure below the deck usually needs building control sign-off.
How much waste should I allow for decking?
The board count is already rounded up to whole boards, and the end-of-row offcuts can nearly always be reused to stagger the joints on the next row, so a separate waste allowance rarely adds anything. What saves real money is board length: pick a stock length close to a multiple of your run and the offcuts shrink.