United Kingdom · Materials & DIY
Decking Calculator
Change the pack size
Enter the deck area and your board width and gap. The calculator returns linear metres and the number of boards, and the guidance covers joist spacing, which is where most decks are actually built wrong.
How many decking boards do I need?
Divide one metre by the board width plus the gap to get linear metres of board per square metre, then multiply by the deck area. A 144 mm board with a 5 mm gap covers 149 mm, so you need 6.71 linear metres per square metre. A 20 square metre deck needs about 134 linear metres, plus 10% for cuts.
How to use this calculator
- Measure the deck, not the frameThe area you are actually covering with board. If the deck is L-shaped, split it into rectangles and add them.
- Work out your linear metres per m²One divided by the board width plus the gap, both in metres. The gap matters — leaving it out understates the board count by three to four per cent.
- Set the board length you can buy3.6 m is common, and 2.4, 3.0 and 4.8 m are all sold. Choosing a length that divides into your deck run reduces waste substantially.
- Keep the 10% waste allowanceBoards get cut, ends get trimmed square, and there is usually one in a pack that has cupped enough to reject.
How the calculation works
A decking board covers its own width plus the gap beside it. That combined figure is what determines coverage, and forgetting the gap is the most common estimating error on decks.
A nominal 150 mm board is usually 144 mm actual, and with a 5 mm gap it occupies 149 mm of deck. One divided by 0.149 gives 6.71 linear metres of board for every square metre of deck. On a 20 m² deck that is 134 linear metres. Using the nominal 150 mm figure instead would suggest 133 metres — close enough to seem fine, and short once you add the gaps back in.
Converting linear metres into boards is where the length choice pays. 134 linear metres in 3.6 m boards is 38 boards. But if the deck runs 5 m in the board direction, every 3.6 m board leaves a 1.4 m offcut that may or may not be usable, and the real number climbs. Running boards along the shorter dimension, or choosing a length that divides evenly, is worth thinking about before ordering.
linear m per m² = 1 ÷ (board width + gap) · total = deck area × that figure × (1 + waste)Joist spacing — the part that matters more than the boards
| Board thickness | Maximum joist spacing | Notes |
|---|---|---|
| 19 – 21 mm softwood | 400 mm centres | Thin boards flex noticeably at wider spacing |
| 28 mm softwood | 450 – 500 mm centres | The common UK domestic board |
| 32 mm+ softwood | 600 mm centres | Heavier and stiffer |
| Composite board | 300 – 400 mm centres | Composites are less stiff than timber — always follow the manufacturer |
| Any board, laid diagonally | Reduce spacing by about 30% | The diagonal span between joists is longer |
Composite decking catches people out here. It looks and handles like a heavier, better material, and it is considerably less stiff in bending than the equivalent softwood board. A composite deck built on a frame designed for timber will feel bouncy and will sag between joists over time. The manufacturer's spacing is not a suggestion.
Worked example: a 5 by 4 metre deck
A deck 5 m by 4 m is 20 m². Using 144 mm boards with 5 mm gaps, that is 6.71 linear metres per square metre, so 134 linear metres.
Add 10% waste and you need 148 linear metres. In 3.6 m boards, that is 41 boards. Running the boards across the 4 m dimension means each board needs cutting from a 3.6 m length plus a joint, so it is worth running them along the 5 m dimension and accepting a 1.4 m offcut per board — or buying 4.8 m boards and cutting one 4 m length from each with a 0.8 m offcut.
The frame is a separate calculation. At 450 mm centres, a 5 m by 4 m deck needs about ten joists at 5 m plus the perimeter, which is another 60-odd linear metres of timber that this calculator does not cover.
Gaps, drainage and the things that ruin decks
The gap between boards is not decorative. It lets water drain and lets the boards move as they take up and lose moisture. Butt them tight and the deck will cup, trap water and rot from the top face down.
5 mm is the usual figure for pressure-treated softwood installed dry. If the boards are wet when you fit them — and yard-stored treated timber often is — they will shrink as they dry, so a smaller gap is appropriate. Fit them dry and they will swell instead. Composite boards have their own expansion requirements, usually larger and specified by length as well as width.
The other consistent failure point is ventilation underneath. A deck built tight to the ground with no airflow beneath it will hold moisture against the joists indefinitely, and the frame will fail long before the boards do. Leave a genuine air gap and keep the underside clear of leaf litter.
Grooved boards are widely believed to improve grip. In practice the grooves collect debris and hold water, and the smooth face is usually the better surface to lay uppermost. Most manufacturers now state that either face can be used.
Assumptions and limitations
- Calculates board only — joists, bearers, posts, fixings and edge trim are separate quantities
- Assumes boards run in one direction across a rectangular deck; diagonal or herringbone layouts waste substantially more
- The waste allowance assumes reasonably efficient cutting and board lengths that suit the deck
- Board widths are actual, not nominal — a 150 mm board is usually 144 mm
- Joist spacings quoted are conventional maximums, not a structural design
- Raised decks, decks over 300 mm high and decks attached to a dwelling may have building control implications
Questions people ask about this in United Kingdom
How many decking boards per square metre?
For a 144 mm board with a 5 mm gap, 6.71 linear metres per square metre. Divide that by your board length to get whole boards — in 3.6 m lengths it is about 1.9 boards per square metre.
Always include the gap in the calculation. Using the board width alone understates the requirement by three to four per cent.
What gap should I leave between decking boards?
Around 5 mm for pressure-treated softwood fitted dry. If the boards are wet from the yard they will shrink as they dry, so a smaller gap is appropriate. Composite boards have their own specified expansion gaps, usually larger, and set by length as well as width.
How far apart should decking joists be?
450 to 500 mm centres for a standard 28 mm softwood board, and 400 mm for thinner boards. Composite decking generally needs 300 to 400 mm, because composites are less stiff in bending than timber despite feeling more substantial. Follow the manufacturer's figure.
Should decking boards be laid groove up or down?
Smooth side up is generally the better choice. The grooves collect grit and hold water rather than improving grip meaningfully, and a smooth surface is easier to clean. Most manufacturers now confirm either face may be used.
Do I need building regulations approval for a deck?
Usually not for a low, freestanding deck. Approval becomes relevant where the deck is raised significantly, forms part of a means of escape, or is structurally attached to the house. Planning permission can also apply to raised platforms — check with the local authority before building something high.
How long does a softwood deck last?
Fifteen to twenty years is realistic for pressure-treated softwood that is properly ventilated underneath and maintained. Decks that fail early almost always fail at the frame rather than the boards, because moisture was trapped against the joists with no airflow beneath.
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Method, source and review date
- How it is worked out
- Coverage by effective board width including the expansion gap
- Formula used
linear metres per m² = 1 ÷ (board width + gap, in metres)- Basis
- Standard sheet, board and roll sizes, editable on the page
- Last reviewed
- Confidence
- High
Spotted a figure that has moved, or a formula you can show is wrong? Tell us — send a correction. Fixes to rates and formulas are treated as urgent.
Disclaimer
This is a planning estimate, not a substitute for a design signed off by a qualified engineer. Anything structural, electrical or gas-related must be checked and certified by a competent person before it is built or connected.