How many decking boards do I need?

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.

Worked example: a 5 m × 4 m deck

A 5 m × 4 m deck in 140 mm boards with 5 mm gaps, using 3.6 m boards on joists at 400 mm centres.

How we got there

1. 28 rows of boards — 140 mm boards with a 5 mm gap need 28 rows to cross the 4 m width.
2. 2 boards per row — A 3.6 m board does not span 5 m, so each row takes 2 boards.
3. 28 × 2 = 56 boards — 56 boards of 3.6 m for the deck surface.
4. Joists: 14 lengths — Joists at 400 mm centres come to 14 lengths, 56 m in total.

What you need to buy

The order for the worked example above, straight from this calculator's own shopping list.

• Decking boards 140 mm × 3.6 m — 56 boards (28 rows of 2 boards end to end. Offcuts can be reused to stagger joints.)
• Joists, treated timber — 14 × 4.00 m (56.0 m) (At 400 mm centres. Buy longer lengths and splice on a joist if your span is not a stock size.)
• Decking screws (box of 250) — 7 boxes (≈ 1,568 screws at two per board per joist crossing.)
• Joist hangers — 28 (Rule of thumb: two per joist where the joist meets a rim board or wall ledger — 28 here. A freestanding deck on posts needs different fixings.)
• Weed control fabric (optional) — 22 m² (Under an elevated deck it keeps weeds and light down and looks tidy from the house.)
• Hanger nails — square twist — 280 nails (2 boxes of 250) (Rule of thumb: about 10 per hanger so every hole in the hanger is filled. The hanger maker’s fixing schedule is the figure that counts, and it does not always call for a nail — some hangers are specified with screws. Square twist, not round wire: round wire pulls through.)
• Coach bolts and washers (ledger to a masonry wall) — as needed (Rule of thumb: only where a ledger board is bolted to a wall instead of being carried on posts — about one M10 or M12 bolt per joist position, with a washer under the head and at the nut. Do not bolt through a damp-proof course.)
• Post shoes / post brackets — as needed (Rule of thumb: one per post where a raised deck sits on timber posts carried on concrete pads rather than dug into the ground. Keeps the end grain out of the water, which is where a post rots first.)
• Weed control fabric pegs — as needed (Rule of thumb: about one peg every 600 mm along the edge and the laps of the fabric above. Loose fabric moves and tears.)

Figures for the worked example only. Change the measurements above and the list recalculates.

Waste and over-ordering

Boards are ordered whole. The cutting waste is the offcut from the last board in each row, and it grows if the deck is not a simple rectangle or if you are laying a diagonal pattern.

Regs & planning for this calculator

A deck is the clearest case where planning and building regulations both have something to say.

• Planning — outbuildings and raised platforms The Class E limits, and why a raised platform is expressly excluded from the class that would otherwise cover it. Read the requirement.
• Approved Document K — guarding Guarding is required where there is a drop of more than 600 mm in a dwelling, at 900 mm for a floor edge and 1100 mm for an external balcony or roof edge. Read the requirement.
• Approved Document A — structure Post foundations and the loads the frame has to carry. Read the requirement.
• Party Wall etc. Act 1996 — boundary excavations Post holes near a neighbour's structure, and the three-metre and six-metre triggers. Read the requirement.