How we calculate: decking and fencing
Every formula and default figure our decking and fencing calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Decking calculator
- Rows = deck width ÷ (board width + gap), rounded up
- Boards = rows × deck length × (1 + waste) ÷ board length, rounded up
- Joists = deck length ÷ joist spacing, rounded up, + 1; joist lengths = joists × deck width × (1 + waste) ÷ joist length, rounded up
- Screws = rows × joists × screws per crossing
Bearers, posts, footings and fascia boards aren't included.
Joist spacing: check the board maker's maximum span.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Board width | 144 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job A common board width; enter yours. |
| Gap between boards | 5 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job Gap between boards. |
| Board length | 3.6 m | Common UK size Not yet checked against the source | From your product Lengths of 2.4, 3.6 and 4.8 m are common. |
| Joist spacing | 400 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job Check the board maker's maximum span. |
| Joist length | 4.8 m | Common UK size Not yet checked against the source | From your product Lengths of 2.4, 3.6 and 4.8 m are common. |
| Waste allowance | 10 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Cuts and offcuts. |
| Screws per board crossing | 2 screws | Our assumption Not yet checked against the source | Our choice: change it to suit your job Two screws where each board crosses a joist. |
Fence calculator
- Panels per run = (run − post width) ÷ (panel width + post width), rounded up
- Posts = panels + 1 per run, less 1 for each corner where runs share a post
- Gravel boards = panels; trim = panels × panel width + posts × post width − run
Post concrete comes from the maker's instructions, so enter it from the bag.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Panel width | 1.83 m | Common UK size Not yet checked against the source | From your product 6 ft panel. |
| Post width | 0.1 m | Common UK size Not yet checked against the source | From your product 100 mm post. |
| Post-fix bags per post | Enter from the bag, pack or tin | Typical value Not yet checked against the source | From your product Enter from the post-fix maker's instructions. |
Postcrete calculator
- Fill per post (m³) = (hole area − post area) × fill depth, where fill depth = hole depth − the gap left at the top
- Round area = π × (diameter ÷ 2)²; square area = width²
- Bags per post = fill per post (L) ÷ yield per bag, rounded up; total whole bags = posts × bags per post
- Sharing bags between holes: total fill (L) ÷ yield, rounded up
Hole size and depth come from the post-fix maker's instructions and the post height: follow them for your fence.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Post width | 0.1 m | Common UK size Not yet checked against the source | From your product 100 mm post. |
Deck substructure calculator
Open the deck substructure calculator
- Joists = deck length ÷ joist centres, rounded up, + 1, each as long as the deck width
- Bearer lines = deck width ÷ bearer spacing, rounded up, + 1 (bearer spacing = your figure, or the largest joist span you entered)
- Posts per line = deck length ÷ post spacing, rounded up, + 1 (post spacing = your figure, or the largest bearer span you entered)
- Hangers = 2 per joist; noggins = rows × (joists − 1); 2 rim boards along the deck length
- Checks: the distance joists and bearers actually run between supports against the largest spans you entered
Sizing joists, bearers and posts is out of scope: use your timber supplier's or TRADA's span tables, or ask a structural engineer.
Use timber treated for ground contact where it touches the ground (BS 8417 treatment classes).
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Joist centres | 400 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job Check the board maker's maximum span. |
| Timber length you can buy | 4.8 m | Common UK size Not yet checked against the source | From your product Lengths of 2.4, 3.6 and 4.8 m are common. |
| Allowance per piece | 0.1 m | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance per piece for squaring ends and mitres (brief v2 FL2). |
| Coach bolts per post | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job From the designer's or timber supplier's fixing detail. |
| Height above which decking needs planning permission | 300 mm | Regulation or HSE figure Planning Portal: decking more than 300 mm above the ground Not yet checked against the source | planningportal.co.uk (checked 19 October 2026) brief v2 F2 (raised deck > 300 mm → planning note). Researcher to confirm and check Wales. |