How to measure for decking
Measure the finished deck from edge to edge: the length (A) and the width (B), to the outside of the boards. A deck that steps round a corner is easier as two rectangles: use Add another area for the second part and the calculator adds everything together.
Then decide which way the boards will run. Boards run tells the calculator whether they follow the length (A) or the width (B); the joists go the other way, underneath. Boards often run along the longer side so each row has fewer joins, but either way works, and switching it can change the count more than you would expect, so try both.
Softwood boards are laid with a 5–8 mm gap so rain drains through and the timber has room to swell. The calculator starts at the narrow end of that range. If you measure in feet and inches, switch the units at the top: the boards still come out in metre lengths, because that is how UK merchants sell them.
Recommended board gaps, joist spacing and screws
These are the figures the calculator starts from, from the Timber Decking and Cladding Association’s builder’s guide and the laying guides of Wickes and Champion Timber. Composite boards are shown for comparison, because they are fixed differently.
| Boards | Gap between boards | Joist centres | Fixings where a board crosses a joist | Apply |
|---|---|---|---|---|
| Softwood deck boards120 mm or 144 mm wide | 5–8 mm | 400 mm | 2 screws | |
| Composite deck boardsFor comparison | 3–7 mm | 300–400 mm | 1 hidden clip |
Joist sizes and spacings are a rough guide, not a design. They suit an ordinary deck on a frame close to the ground. A raised deck, long spans, a hot tub or anything fixed to the house needs the frame designed for it, so check yours with your timber merchant or a professional. Guides also differ a little: Howarth Timber allows wider joist centres for straight boards than for herringbone. Whatever your merchant or board maker says wins; type it into Joist spacing.
How much decking do I need?
Start with the area, but don’t stop there. A 4 m × 3 m deck is 12 m², yet boards are sold by the length, so the real question is how many boards of each length. The calculator divides the distance across the boards by one board plus one gap and rounds up, because a part row still needs a full board. Then it works along each row: a row longer than a board needs a join over a joist, and the piece left over is used to start another row where it fits. An end piece has to reach at least one joist gap, so where the leftover would be shorter, a full board is cut down for it instead.
How many decking boards do I need?
The length you choose is where most of the waste comes from, and the part that is hardest to work out without a decking calculator. UK merchants sell softwood boards in 2.4 m, 3.6 m and 4.8 m lengths, and a deck rarely divides neatly into any of them.
Leave Board length on Least waste and the calculator counts the boards for all three lengths and picks the one with the fewest metres of board. The table under the result shows them side by side, so it doubles as a decking board calculator: if the longest boards won’t fit in your car, you can see at once how many of another length you would need.
Joists and screws
Joists run under the boards, at right angles to them. The calculator spaces them at the centres you set, rounds up and adds one, so there is a joist at both ends. Each joist is as long as the deck is wide across the boards and is cut from its own stock length of treated softwood. Two rim joists along the sides close the frame, and the list counts them separately. It does not calculate what the frame stands on, such as bearers, posts or pads, nor noggins or the bolts that hold it together: those depend on how the deck is built, so list them from your own plan.
Decking screws calculator
Every time a board crosses a joist it takes 2 screws, as the TDCA’s guide and Champion Timber’s both say. So the screws are rows × joists × 2, plus the extra allowance, which makes this a quick decking screws calculator too. The list rounds up to boxes of 1,000; use decking screws of the size shown in the table below.
How much extra to allow
The board count already reuses offcuts, so the extra allowance covers what you can’t plan: a board that splits near the end, one with a bad knot or a bow, and cuts at the edges. We suggest 5–10%. That is our own rule of thumb rather than a published figure, because none of the UK guides we checked gives one, so treat it as a starting point and change it with the Extra for cuts and split boards slider. For a simple rectangle and straight boards the low end is usually plenty; for an awkward shape, stay at the top.
Worked example: a rectangular softwood deck
Here is the full calculation for a rectangular deck in 144 mm softwood boards running along its length, with joists at the standard spacing and the extra allowance we suggest. Board length is left on Least waste, so the calculator chooses the length. Every line comes from the same code that runs the calculator above.
- Area 5 m × 3.5 m = 17.50 m²
- Rows of boards (3.5 m + 5 mm gap) ÷ (144 mm board + 5 mm gap), rounded up = 24 rows
- Boards per row each row is 5 m long, cut from 2.4 m boards; offcuts reused if they span a joist gap = 52 boards
- Plus extra 52 boards + 10%, rounded up = 58 boards
- Joists 5 m ÷ 400 mm centres, rounded up, + 1 = 14 joists, each 3.5 m
- Joist lengths each joist cut from 3.6 m stock, plus two rim joists 5 m long = 17 lengths
- Screws 24 rows × 14 joists × 2 + 10% = 740 → 1 box
On the list:
- 582.4 m boards of softwood decking24 rows of boards + 10%
- 143.6 m joists (47 × 100 or 47 × 150 mm)14 joists at 400 mm centres, 49.0 m in total
- 33.6 m joists of for the rim (47 × 100 or 47 × 150 mm)two rim joists, 10.0 m in total
- 1box of 1,000 screws740 screws in total
Notice how close the lengths come in the calculator’s table. Every row needs an end piece at least one joist gap long, and those pieces are cut several to a board, so the longest boards are not always the best buy and two lengths can come out level. When they do, pick whichever is easier to carry or to get.
UK decking board sizes
Softwood deck boards in the UK are pressure-treated and sold by the length, usually about 120 mm or 144 mm wide. Joists are treated softwood too, and decking screws come in large boxes.
| Product | Size | Good for |
|---|---|---|
| 144 mm softwood | 144 mm wide | The common wide board, about 27–28 mm thick. |
| 120 mm softwood | 120 mm wide | Narrower board, about 25–28 mm thick. |
| 2.4 m boards | 2.4 m long | Standard board length. |
| 3.6 m boards | 3.6 m long | Standard board length. |
| 4.8 m boards | 4.8 m long | Standard board length. |
| Joist, 3.6 m | 47 × 100 or 47 × 150 mm, 3.6 m | Treated softwood joist; longer lengths are sold too. |
| Decking screws, box of 1,000 | 4.5 × 60–65 mm | Two screws where each board crosses a joist. |
Widths vary by a millimetre or two between merchants, and the longest boards are not sold in every width, so check what yours stocks. If your boards are a little wider or narrower than the options here, choose the nearer width and check the row count in the steps.
Common mistakes when ordering decking
- Ordering by the square metre. Boards are sold by the length. Dividing the area by the size of a board ignores the gaps, joins and offcuts, and gives a number you can’t lay.
- Forgetting the gaps. Boards butted tight trap water and have nowhere to swell. Leave the 5–8 mm the guides give.
- Joins that miss a joist. Where two boards meet end to end, both ends need a joist under them. Plan where the joins fall before you cut.
- Picking a board length out of habit. The longest boards are not always the least waste. Check the comparison in the result.
- Forgetting what the frame stands on. The calculator counts the joists and rim joists, not the bearers, posts or pads underneath.
- One screw per joist. It’s 2 at every joist, which on a big deck is more screws than most people expect.
What else you'll need
- Thinking of composite instead? It takes hidden clips and often closer joists; the composite decking calculator works those out.
- Decking oil to protect the boards: the fence paint calculator has a deck option and works out the litres and tins.
- Post-fixing concrete if the frame stands on posts: the postcrete calculator counts the bags for each one.
- Weed membrane and gravel under the deck to stop weeds coming up through the gaps: the gravel calculator works out both.
- Fixings for the frame, such as coach bolts, frame screws or joist hangers, depending on how you build it.
Last checked 28 October 2026