How to measure for composite decking
Measure the outside of the deck frame you plan to build: the length (A) and the width (B). On a composite deck the fascia covers the edges of the frame, so the frame’s outline is what you are covering. For an L-shaped deck, split it into two rectangles and use Add another area; boards, clips, joists and fascia are added up across both.
Choose which way the boards run with Boards run; the joists go the other way, under them. Try both: a deck may fit the boards much better one way round than the other.
Most makers’ guides ask for a gap of 3–7 mm between boards, and the calculator starts in the middle of that range. Composite boards are sold in metric lengths, so if you measure in feet and inches, switch the units at the top and the boards still come out in metres.
Recommended joist spacing, gaps and clips
These are the figures the calculator starts from, taken from the UK installation guides of Trex, Cladco, Millboard and Ultra Decking and from Wickes. Composite boards want their joists at least as close as softwood, and several makers ask for them closer, especially when the boards run at an angle. Your maker’s guide always wins: type its figures into the calculator.
| Boards | Gap between boards | Joist centres | Fixings where a board crosses a joist | Apply |
|---|---|---|---|---|
| Boards laid straightThe calculator's starting point | 3–7 mm | 300–400 mm | 1 hidden clip, plus a start clip on each joist for the first board | |
| Boards laid at an angleDiagonal layouts | 3–7 mm | Closer than for straight boards: see the maker’s guide | 1 hidden clip | |
| Softwood boardsFor comparison | 5–8 mm | 400 mm | 2 screws |
This is a rough guide, not a design. The calculator assumes an ordinary deck on a timber frame close to the ground. It starts at the closer joist spacing, which Wickes and Ultra Decking ask for; if your boards are rated for the wider one, as Trex, Cladco and Millboard allow, type it in. It counts the joists and two rim joists but not the bearers, posts or pads the frame stands on. Raised decks, long spans, heavy planters or a hot tub need the frame designed for them, so check yours with the board maker, your merchant or a professional before you build.
How many composite boards do I need?
Composite boards are sold by the length, so the area alone won’t tell you what to order. The calculator counts the rows across the deck, each one board plus one gap wide, then cuts each row from each stock length in turn, reusing the leftover piece from one row to start another where it is long enough to reach a joist gap.
Leave Board length on Least waste to see the three common lengths side by side, with the one that uses the fewest metres of board marked. Some makers sell lengths in between, and Trex’s boards are a little longer than the round metric sizes. If yours differ, use the nearest length as a guide and ask your supplier to check the count.
Fascia boards
Fascia boards cover the ends of the deck boards and the face of the frame. With Add fascia boards around the edge switched on, the calculator takes the perimeter of each area, adds the extra allowance and divides by the length of a fascia board, rounding up.
That assumes every side is on show. If one side runs against the house or a wall, it needs no fascia, so take that length off. The same goes for the edge where two areas meet if you have added a second one: the calculator counts both sides of that join.
How much extra to allow
We suggest an extra 5–10% for cuts at the edges and the odd damaged board. It is our own rule of thumb, not a maker’s figure, because none of the UK guides we checked gives one; change it with the Extra for cuts and split boards slider. Offcuts are already reused in the count, so a simple rectangle needs less than an awkward shape. The same allowance goes on the clips and the fascia; the joists are counted without it, each one cut from its own stock length.
Worked example: a composite deck
Here is the full calculation for a rectangular deck in 150 mm composite boards running along its length, with joists at the starting spacing, hidden and start clips, fascia on every side and the extra allowance we suggest. Every line comes from the same code as the calculator above.
- Area 4.2 m × 3 m = 12.60 m²
- Rows of boards (3 m + 5 mm gap) ÷ (150 mm board + 5 mm gap), rounded up = 20 rows
- Boards per row each row is 4.2 m long, cut from 3.6 m boards; offcuts reused if they span a joist gap = 24 boards
- Plus extra 24 boards + 10%, rounded up = 27 boards
- Joists 4.2 m ÷ 300 mm centres, rounded up, + 1 = 15 joists, each 3 m
- Joist lengths each joist cut from 3.6 m stock, plus two rim joists 4.2 m long = 18 lengths
- Hidden clips 20 rows × 15 joists × 1 + 10% + start clips = 345 → 4 boxes
On the list:
- 273.6 m boards of composite decking20 rows of boards + 10%
- 153.6 m joists (47 × 100 or 47 × 150 mm)15 joists at 300 mm centres, 45.0 m in total
- 33.6 m joists of for the rim (47 × 100 or 47 × 150 mm)two rim joists, 8.4 m in total
- 4boxes of 100 clips345 clips in total
- 53.6 m fascia boards14.4 m of edge
Notice that the clips come to more than rows × joists: the extra allowance goes on the clips too, and a start clip is added for every joist. Notice as well how the deck’s length compares with a board. When each row is only a little longer than one board, the short end pieces can be cut several to a board, which keeps the waste down.
UK composite decking sizes
Composite boards vary more between makers than softwood does. Widths of about 140 mm, 150 mm and 176 mm are common, and the three lengths in the calculator cover most of what UK merchants sell. The calculator lists treated softwood joists for the frame; clips and fascia should come from your board maker.
| Product | Size | Good for |
|---|---|---|
| 140 mm composite | 140 mm wide | For example 25 × 140 mm boards. |
| 150 mm composite | 150 mm wide | For example 22–25 × 150 mm boards. |
| 176 mm composite | 176 mm wide | A wide 32 mm composite board. |
| 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. |
| Hidden clips, box of 100 | 100 in a box | One clip where each board crosses a joist. |
| Composite fascia, 3.6 m | 3.6 m long | Finishes the edge of the deck. |
Thickness doesn’t change the count, but width does. If your board is a few millimetres off one of the widths here, choose the nearest and check the row count in the steps: across a wide deck, a few millimetres a board can add up to a row.
Common mistakes with composite decking
- Using timber joist spacing. Many composite boards need joists closer than a softwood deck. Check the maker’s guide before you build the frame, not after.
- Forgetting start clips. The first board needs a start clip on every joist. The calculator adds them; a count done by hand often misses them.
- Mixing clip systems. Clips are made for one maker’s groove. Buy the ones your board maker specifies.
- Butting boards tight. Composite moves as it warms up and cools down. Keep the gap the maker asks for, at the sides and at the ends of boards.
- Ordering fascia for every side. A side against the house or a wall needs none. Take that length off the count.
- Assuming every board is clipped. Some, such as Millboard, are screwed down. Check how yours are fixed before you order.
- Keeping the straight-board spacing for an angled layout. Boards at an angle span further between joists, so makers ask for the joists closer together.
What else you'll need
- Comparing with timber? The decking calculator does the same sums for softwood boards and screws.
- Post-fixing concrete if the frame stands on posts: the postcrete calculator counts the bags.
- Weed membrane and gravel under the deck to stop weeds growing up through the gaps: the gravel calculator works out both.
- A patio or path leading to the deck: the patio calculator works out the slabs.
- Fixings for the frame, and the screws for the fascia that your board maker’s guide lists.
Last checked 28 October 2026