Decking calculator
Enter the size of your deck and the board lengths your merchant sells. The calculator lays the deck out the way you’d build it, with joins on the joists and staggered row to row, and tells you how many boards to buy, the frame and the fixings.
Decking calculator
A 4m × 3m deck in 144mm timber boards is loaded as an example. Change it to your own.
How it is worked out
- Rows. The width the boards cover, divided by one board plus one gap. A 3m deck in 144mm boards with 5mm gaps is 21 rows. Whatever is left over is the last row, ripped down to fit.
- Joins. Where a row is longer than your longest board, it is split so every join lands on a joist, and the joins move one joist along on every other row so they don’t line up across the deck.
- Boards. Every piece goes through the same cutting engine as the CutNest app, which finds the fewest boards in the lengths you can buy and uses the offcut from one row in another where it fits. When it can prove no plan uses fewer boards, it says so.
- Frame and fixings. Joists at your spacing across the whole deck, two rim joists, and two screws per board on every joist (or one hidden clip for composite), plus 5% spare.
- Sides. Tick the sides you’re cladding and enter their height. The boards for the sides are laid in rows up from the ground and go in the same cutting plan as the deck, so an offcut from the top can clad a side.
Posts, bearers and footings depend on how high the deck is and what it sits on, so they are left to you.
Decking sizes in the UK
| Timber (treated softwood) | Composite | |
|---|---|---|
| Board width | 120mm or 144mm | Usually 135–150mm |
| Thickness | 28mm or 32mm | Usually 21–25mm |
| Lengths | 2.4m, 3.6m, 4.8m | Often 3.6m or 4.8m |
| Gap between boards | About 5mm | Set by the clips, often 5–8mm |
| Joist spacing | Usually 400mm centres | Often 300–400mm: check the maker’s instructions |
| Fixing | Two deck screws per joist | Hidden clips |
These are typical figures, not rules. Composite especially varies from one brand to the next, and the maker’s spacing is what their guarantee depends on. Boards laid on the diagonal need closer joists.
Doing decks for customers?
Your own deck, start to finish
The timber cutting calculator works out the fewest lengths for the joists and bearers too.
A cutting list for the job
Open the result in the app for a cutting plan for every board, and keep it on your phone on site.
The Pro deck planner
For L-shapes, several areas, borders, steps, fascia, bearers and posts. It lays out the whole deck, gives you the fewest lengths of every timber, a saw list and labels, and a customer quote priced per m².
Straight answers
Try it on your own job
Free, in your browser, no account. Your cut list stays on your computer.