Decking Calculator: How Many Boards and Joists Do I Need?
Work out how many deck boards you need for a timber or composite deck, which board length wastes least, and the joists, screws or hidden clips to go with them.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Work out your decking
Decking Calculator
Work out how many deck boards a timber or composite deck needs, which board length wastes least, and the joists and screws or clips to go with them.
1 Your deck
Deck size: length (along the boards) × widthMeasure the length in the direction the boards will run. Split an L-shaped deck into two rectangles.
2 The boards
3 Frame and fixings
Stops weeds growing up through the gaps. We add 10% for the overlaps.
4 Prices (optional)
Add the prices you've seen to get a total. Leave any box empty to skip it.
Deck boards you need
0 boards
Deck area
0 m²
Joists
0
Fixings
–
What you'll need
Breakdown
| Area | Size | Rows | Joists |
|---|
How it's worked out: rows of boards = width ÷ (board width + gap), rounded up. Boards: when the deck is shorter than a board, each board gives as many rows as fit in its length; when it's longer, boards = rows × length ÷ board length, with the joints staggered over joists. Then the extra % is added. Joists = length ÷ spacing + 1, each as long as the width, plus two side beams along the length, with 10% extra. Fixings: 2 screws (timber) or 1 hidden clip (composite) per board per joist, plus 5%.
Estimates only. Follow the board maker's instructions for the joist spacing, gaps and fixings. The supports under the frame (pads, blocks or posts) depend on the joist size, the span and the ground, so they aren't counted here.
How it works
Rows, board lengths and joists
Deck boards are sold by the length, not by the m², so the calculator counts rows of boards and works out which length leaves the least waste. The sources and assumptions behind it are on our How we calculate page.
Count the rows
Measure the deck’s length in the direction the boards will run, and its width across them. Rows = width ÷ (board width + gap), rounded up: a 3 m wide deck takes 21 rows of 144 mm boards with 5 mm gaps. The last row is cut down to fit.
Pick the board length
If the deck is shorter than a board, each board covers one row (or two, if it’s twice as long). If it’s longer, boards are joined end to end over a joist, with the joints staggered. The calculator tries 2.4 m to 4.8 m boards and picks the length that buys the fewest metres.
Add the frame and fixings
Joists run across the boards, 400 mm apart for timber and usually 350 mm for composite, with a side beam along each edge. Each board is fixed to every joist: 2 screws for timber, or 1 hidden clip for composite.
Examples
How many deck boards do I need?
The same calculator, a few common decks. As a rule of thumb, 144 mm boards give about 7 rows per metre of width, and you need a little over 2 boards of 3.6 m per m² once offcuts and spares are counted.
Illustrative only, from the calculator above, with the board length that wastes least. 5 mm gaps; timber joists 400 mm apart, composite 350 mm; joist timber in 4.8 m lengths including two side beams; 10% extra on boards and timber, 5% on screws and clips.
Tips
Before you build
The boards are the part you see; the frame underneath and where the deck sits decide how long it lasts.
Check the height and planning
In England and Wales, decking is permitted development only if it’s no more than 30 cm above the ground and, with other extensions and outbuildings, covers no more than half the garden. Higher decks, flats and listed buildings need planning permission. In Scotland, Ireland and elsewhere, check your local rules.
Support the frame
Use treated C24 joists (47 × 150 mm, or 47 × 100 mm close to the ground) on concrete pads, deck blocks or adjustable pedestals. How far apart the supports go depends on the joist size and the ground, so follow a span table or the supplier’s guide; this calculator doesn’t count them.
Leave gaps and let it drain
Leave about 5 mm between boards and a gap at walls so rain drains away and the boards can move. Slope the deck about 1 in 100 away from the house and keep it at least 150 mm below the damp-proof course. Seal the cut ends of timber boards with end-grain preserver.
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Common questions
Frequently asked questions about how much decking you need
Common questions about boards, lengths, joists, fixings and planning.
How many deck boards do I need?
Work out the rows (the deck’s width ÷ the board width plus the gap), then how many boards each row needs at the length you buy, and add about 10% for cuts. A 4 × 3 m deck with 144 mm boards needs 21 rows and 26 boards of 3.6 m. The calculator works it out for your own deck, including L-shapes.
How many decking boards per m²?
144 mm boards with 5 mm gaps give about 7 rows per metre of width, which works out at a little over 2 boards of 3.6 m per m² once offcuts and 10% extra are counted. The exact number depends on the deck’s shape and the board length, because offcuts can’t always be used.
What length of decking board should I buy?
The length that leaves the least waste for your deck. Boards just longer than the deck waste the most: on a 4 m deck, 4.8 m boards leave a 0.8 m offcut on every row, while 3.6 m boards joined over a joist waste far less. The calculator compares 2.4 m to 4.8 m boards and picks the best.
How far apart should deck joists be?
400 mm centre to centre for timber boards 28–32 mm thick, and usually 350 mm for composite boards. Diagonal boards need joists closer together, often 300 mm. Always check the board maker’s instructions.
How many screws do I need for decking?
Two screws per board at every joist. A 4 × 3 m timber deck with joists 400 mm apart has 21 rows crossing 11 joists, so about 460 screws plus a few spare: one box of 500. Decking screws about 4.5 × 60 mm suit 28 mm boards.
How many hidden clips for composite decking?
One clip per board at every joist, plus start clips along the first and last board. A 4 × 3 m composite deck with joists 350 mm apart needs about 290 clips (three packs of 100) and 26 start clips. Clip packs usually say how many m² they cover.
Do I need planning permission for decking?
In England and Wales you don’t, as long as the decking is no more than 30 cm above the ground, isn’t in front of the house, and with other extensions and outbuildings covers no more than half the garden. Listed buildings and flats need permission. Rules differ in Scotland, Northern Ireland, Ireland and the rest of Europe, so check locally.
Are the results exact?
No. They are estimates: how many boards you use depends on the cuts, the joints and the boards themselves. The calculator adds 10% to boards and timber and rounds everything up. It doesn’t count the supports under the frame, steps, fascia or rails.