Offcut WorksDecking calculator
Enter the size of your deck and the boards you plan to buy. You get a plan view and a materials list you can take to the timber merchant.
How the quantities are worked out
Rows of boards = (width across the boards + gap) ÷ (board width + gap), rounded up. For a 3 m span with 140 mm boards and 5 mm gaps: (3000 + 5) ÷ 145 = 20.7, so 21 rows.
Boards = rows × deck length × (1 + waste) ÷ board length, rounded up. This assumes you reuse offcuts, which only works where the joints fall over a joist. If the boards you buy are shorter than the deck, the calculator also shows the larger number you would need if you could not reuse offcuts.
Joists = deck length ÷ joist spacing, rounded up, plus one for the end. Face screws = two per board at every joist, plus 10% spare. Hidden clips = one per joist in each gap between boards, plus a starter clip per joist along the first board and face screws for the last board.
Joist spacing, gaps and fixings: why the defaults are only a starting point
Board makers do not agree. In the installation guides we checked, one composite maker specified joists at 300 mm centres, another at 400 mm, and a joist maker recommended 300 mm, or 250 mm for diagonal boards and for decks above ground. Gaps between boards ranged from about 3 mm to 8 mm depending on the clip, the board and the temperature when fitting. The numbers on this page are therefore starting values, and the guide for your boards always wins.
If you choose hidden clips, the clip sets the gap. One retailer describes a T-clip that gives a 6 mm gap and an S-type clip that gives about 3 mm, so pick the clip first and then enter that gap here.
Board length, joints and offcuts
Every board end needs support and room to move. Where boards join end to end, one joist maker recommends doubling the joist under the joint, with a clip each side and an 8 mm gap between the ends for drainage and expansion. If your deck is longer than the boards you are buying, tick the double joists option so the joist count includes them.
Buying the board length that matches your deck run avoids joints altogether. For example, a 3.6 m deck run with 3.6 m boards needs one board per row. A 4 m run needs two boards in every row, and that is where the without-offcuts figure above becomes important.
Common ordering mistakes
- Dividing by board width and ignoring the gap. Each row takes up the board width plus the gap, so leaving the gap out under-counts the rows.
- Ordering exactly the area. Cuts, split ends and mistakes mean you need a waste allowance. If the deck has steps, cut-outs or a picture-frame border, increase it.
- Forgetting that stock lengths are fixed. The boards come in set lengths, so the offcut from one row only helps another row if the joint lands on a joist.
- Using the wrong fixing for the board. Hidden clips, screws and starter clips are specific to the board profile, so buy them from the same range.
- Not checking planning rules. Most garden decks are permitted development in England if they stay low enough, but raised decks are different. Use the planning permission checker.
Questions
How many deck boards do I need?
Work out how many rows fit across the deck by dividing the width by one board plus one gap. Multiply the rows by the length of the deck, add a waste allowance, then divide by the length of the boards you are buying. The calculator does this for you.
How much waste should I allow?
10% is a common allowance for boards laid straight. Diagonal layouts, steps and picture-frame borders need more. See the diagonal decking calculator.
How far apart should the joists be?
It depends on the boards. For timber, BSW and Cladco give 400 mm centres, and the TDCA's guidance caps 22 mm softwood boards at a 400 mm span, although some suppliers allow 450 mm for thicker boards. For composite the guides we checked ranged from 300 mm to 400 mm, with 250 mm suggested for diagonal boards. Follow the installation guide for the product you buy.
Does this include posts, bearers and foundations?
No. It covers boards, joists and fixings. Use the decking subframe calculator for the frame and supports, and check spans against the timber supplier's tables.
What width are decking boards?
It varies by product and supplier. Enter the actual face width of the boards you plan to buy, not a nominal size.
Does it work for composite decking?
Yes for quantities. Choose the composite starting point, then enter the board width, gap and joist spacing from the maker's guide, because composite boards often need different values from timber.
Where the defaults come from
- Timbmet, choosing and using timber decking (summarises TDCA guidance, PDF). Timber Decking and Cladding Association guidance: a gap of 5 mm minimum to 8 mm maximum between boards, two fixings at each joist crossing, C16 minimum timber, and a 400 mm maximum span for 22 mm softwood boards.
- BSW Timber, timber decking installation guidance (PDF). Joists at 400 mm centres, noggins staggered at about 1.2 m centres, a 6 to 9 mm gap between boards, two screws per joist, and board joints over a joist.
- Cladco timber decking installation guide (PDF). Joists at a maximum of 400 mm centres.
- Wickes decking calculator. A UK retailer's calculator that adds 10% wastage to the area.
- eDecks composite decking installation guide (PDF). Joists at 300 mm centres, hidden clips, and a 5 mm gap against solid structures.
- Composite decking joist listing (PBSL Group). Joist centres no greater than 300 mm, 250 mm for diagonal boards or above ground, and double joists under butt joints with an 8 mm end gap.
- BM Steel, how to fit composite decking. Joists at 400 mm centres and one T-clip per joist.
- HD Deck Pro fitting guide (PDF). Joists no more than 300 mm apart, a clip and screw on every joist, and a 5 to 7 mm gap that depends on temperature.
- SAiGE composite decking installation guide. Maximum 400 mm between joist centres, and a 4 to 5 mm gap at 16 °C (6 to 8 mm in cold weather).
- Deckorum, composite decking clips. T-clips giving a 6 mm gap, 3 mm S-type clips, and a table of clips per square metre.
- Timber starting values are now sourced: 400 mm joist centres (BSW, Cladco), a 5 to 8 mm gap (TDCA via Timbmet; BSW says 6 to 9 mm) and 10% waste (Wickes). 400 mm is cautious. Some suppliers allow 450 mm for thicker boards, so follow the guide for your boards.
- The 140 mm board width is a typical size, not a standard. Widths vary by product.
- The 10% spare on screws and 5% on clips are our own allowances.