Decking calculator: boards, joists and screws
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Work out how many deck boards, joist lengths and screws to buy. Enter your measurements below; every figure we start from is shown and you can change it.
Worked example
| You enter | Value |
|---|---|
| Deck length, along the boards | 4 m |
| Deck width, across the boards | 3 m |
| Board width | 144 mm |
| Gap between boards | 5 mm |
| Board length | 3.6 m |
| Joist spacing | 400 mm |
| Joist length | 4.8 m |
| Waste allowance | 10% |
| Screws per board crossing | 2 |
Order: 26 boards and 8 joists
| Result | Value |
|---|---|
| Deck boards | 26 |
| Joist lengths | 8 |
| Deck screws | 462 |
| Rows of boards | 21 |
| Joists | 11 |
| Board metres (before waste) | 84 m |
Show the working
- Rows: 3,000 mm ÷ (144 + 5) mm = 20.13, rounded up to 21
- Board metres: 21 × 4 m = 84 m
- Boards: 84 × 1.1 (10% waste) ÷ 3.6 m = 25.67, rounded up to 26
- Joists: 4,000 mm ÷ 400 mm, rounded up, + 1 = 11
- Joist pieces: 11 × 3 m × 1.1 ÷ 4.8 m = 7.56, rounded up to 8
- Screws: 21 rows × 11 joists × 2 = 462
How it works
- 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.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| 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. |
FAQs
Guides
Decking and fence rules in the UK
Fence heights, raised decks and boundaries: where to find the official rules for England, Wales, Scotland and Northern Ireland.
How many decking boards do I need?
Work out rows of boards, board lengths, joists and screws for a deck, and why joist spacing comes from the board maker.