Working out how many decking boards you need comes down to three numbers: the width of your deck, the width of each board, and the gap you leave between boards. Get those right and the rest is simple arithmetic. This guide walks through the formula, gives you a worked example, and covers the joists and fixings most people forget until they are standing in the timber yard.
The quick formula
Standard UK decking boards are 32mm x 125mm. You always leave a 5mm expansion gap between boards so the timber can move and drain, which means each board effectively covers 130mm of width. From there:
- Boards per row across the deck = deck width (mm) divided by 130
- Number of rows = the figure above, rounded up to a whole board
- Total length of decking = number of rows multiplied by deck length
Always round up, then add roughly 10% for cutting, waste and the odd split board. Decking is far cheaper to over-order slightly than to stop halfway through a job waiting on one more length.
Step 1: measure your deck
Measure the area the boards will cover, not the whole garden. Work in millimetres to keep the maths clean. A deck that is 4 metres long and 3 metres wide is 4000mm by 3000mm. Decide which way the boards will run, as the boards always span the length and the joists run underneath at right angles to them.
Step 2: work out the boards per row
Take the width the boards have to cover and divide by 130mm. For a 3000mm wide deck that is 3000 divided by 130, which gives 23.1, so you round up to 24 rows of boards. Each of those rows then needs enough board length to cover the run of the deck.
Step 3: factor in board length and waste
Decking boards come in 2.4m, 3.0m, 3.6m and 4.8m lengths. Pick the length that covers your run with the least offcut. A 4 metre run is most efficiently covered by a single 4.8m board, leaving a short offcut you can reuse for noggins. Multiply your number of rows by one board each, then add 10% on top.
How many joists do I need?
The boards are only half the order. Underneath, the joists carry the load, and the spacing you choose directly changes how many you need.
Joist spacing: 400mm or 600mm centres
For a solid, bounce-free residential deck we recommend joists at 400mm centres. You can stretch to 600mm centres for lighter use, but the deck will feel softer underfoot and the boards can cup over time. To count joists, divide the deck length by the spacing and add one for the final joist. A 4000mm deck at 400mm centres needs (4000 divided by 400) plus 1, which is 11 joists, each spanning the 3 metre width.
Joist size: 100mm or 150mm
Joists come in 47mm x 100mm and 47mm x 150mm. The 100mm section is fine for low, ground-level decks with closely spaced support; the deeper 150mm joists are the safer choice for raised decks or longer spans between supports. If in doubt, go deeper. Whatever you choose, every external joist must be pressure treated, or it will rot within a few seasons.
Worked example: a 4m x 3m deck
- Boards: 24 rows, each covered by a 4.8m board = 24 boards, plus 10% = order 27 boards
- Joists: 11 joists at 400mm centres, each 3m long (cut from 3.6m lengths)
- Screws: two fixings per board at every joist crossing, so budget one 200-box of decking screws per 5 to 6 square metres
That is the manual method. If you would rather skip the arithmetic, Try our free decking calculator and it will work out the exact number of boards, joists and screws for your deck size and add them straight to your basket.
Don't forget the extras
- Expansion gaps: 5mm between boards, and a few millimetres at the ends, so water drains and the timber can move
- Noggins: short cross-pieces between joists to stop them twisting, usually cut from board or joist offcuts
- Treatment: seal any cut ends with end-grain preserver, as the saw exposes untreated timber
Once you know your quantities, you can see the full range in one place. Browse our complete decking range to choose your boards, joists, screws and treatment, all available to collect from Bridgend.
Frequently asked questions
How many decking boards are in a square metre?
With standard 125mm boards at a 5mm gap, you get roughly 7.7 linear metres of board per square metre of deck. For a 32mm x 125mm board, that works out at just under 8 boards per square metre for a one metre run.
What gap should I leave between decking boards?
Leave a 5mm expansion gap. It lets rainwater drain through and gives the timber room to swell in wet weather without buckling.
How far apart should decking joists be?
400mm centres for a firm residential deck. You can go to 600mm for lighter loads, but expect more flex and a higher risk of the boards cupping.
Should I add extra for waste?
Yes. Add about 10% to your board count to cover cuts, the occasional split board and trimming around the edges. It is cheaper than a second trip to the yard.