Joist calculator: how many joists and how long
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
Worked example
| You enter | Value |
|---|---|
| Clear span | 3.6 m |
| Room length the other way | 4 m |
| Joist centres | 400 mm |
| Bearing at each end | 100 mm |
| Joists on hangers | No |
| Timber length you can buy | 4.8 m |
| Allowance per piece | 100 mm |
You need 11 joists at 3.80 m
- Limit not yet confirmed Clear span against the largest span you entered: 3.6 m Enter the largest span for your joists from TRADA, the I-joist maker or your engineer to check this. UK-wide: TRADA span tables, I-joist maker's tables or engineer's figure (as you entered). Check with the source
- Out of scope Joist size Sizing a joist is out of scope: use TRADA's span tables or an I-joist maker's tables, or ask a structural engineer or building control. UK-wide: TRADA span tables, a structural engineer or building control. Check with the source
| Result | Value |
|---|---|
| Joists | 11 |
| Length of each joist (with bearings) | 3.8 m |
| Timber lengths to buy | 11 |
| Length | Cut into | Offcut |
|---|---|---|
| 1 (4.8 m) | Joist 1: 3.9 m | 0.9 m |
| 2 (4.8 m) | Joist 2: 3.9 m | 0.9 m |
| 3 (4.8 m) | Joist 3: 3.9 m | 0.9 m |
| 4 (4.8 m) | Joist 4: 3.9 m | 0.9 m |
| 5 (4.8 m) | Joist 5: 3.9 m | 0.9 m |
| 6 (4.8 m) | Joist 6: 3.9 m | 0.9 m |
| 7 (4.8 m) | Joist 7: 3.9 m | 0.9 m |
| 8 (4.8 m) | Joist 8: 3.9 m | 0.9 m |
| 9 (4.8 m) | Joist 9: 3.9 m | 0.9 m |
| 10 (4.8 m) | Joist 10: 3.9 m | 0.9 m |
| 11 (4.8 m) | Joist 11: 3.9 m | 0.9 m |
Show the working
- Joists: 4 m ÷ 400 mm, rounded up, + 1 = 11
- Each joist: 3.6 m + 2 × 100 mm bearing = 3.8 m
- Timber: 11 × 3.8 m + 100 mm each, from 4.8 m lengths = 11 lengths
How it works
- Joists = room length ÷ centres, rounded up, + 1
- Length of each joist = clear span + 2 × bearing
- Hangers = 2 per joist; strutting = rows × (joists − 1)
- Check: the clear span against the largest span you entered for your joists
Choosing a joist size is out of scope here: use TRADA's span tables, an I-joist maker's tables, or ask a structural engineer or building control.
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 |
|---|---|---|---|
| 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). |
Regulations and standards
- TRADA span tables (not reproduced: use TRADA's figures, an I-joist maker's tables or a structural engineer)
Explained: Approved Document A (structure).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.