How many joists do I need?
Enter the width the floor spans across and we'll work out the number of joists and the cost — with the calculation shown. Joists typically sit at 400 mm centres.
1 m per m.
| C24 floor joist (47×170) | 42 m | £273 |
- 1Total joist length: 40 m
- 2Usage rate: 1 m per m
- 3Allowing for waste: 40 × 1 × 1.05 waste = 42 m
- 4Quantity: 42 m @ £6.5 each
- Material cost only — labour is estimated separately in the schedule.
- Indicative trade-counter price; confirm with a current supplier quote.
How to work out floor joists
- 1Measure across the joists
Measure the width the joists run across (perpendicular to their length) — this is what the count is based on.
- 2Divide by the spacing
At 400 mm centres you need about one joist every 0.4 m, plus one at the end — roughly (width ÷ 0.4) + 1. At 600 mm centres it's fewer.
- 3Get the size and span right
Joist depth is set by the span and load (from span tables) — a longer span needs a deeper joist. This is a structural decision, not just a count.
- 4Add noggins and hangers
Allow for noggins between joists and joist hangers or wall plates at the ends — part of the timber order.
Counting joists from the span and spacing is straightforward; the part that isn't is the joist size, which is a structural decision set by span tables or an engineer. Especially in a loft conversion, get the sizing signed off — the count is cheap, an under-sized floor is not.
Floor joist quantities (indicative)
| Spacing | Rule of thumb | Notes |
|---|---|---|
| 400 mm centres | (width ÷ 0.4) + 1 | Common domestic spacing |
| 600 mm centres | (width ÷ 0.6) + 1 | Where the span/load allows |
| Joist depth | from span tables | Longer span = deeper joist |
| Noggins & hangers | extra | Between joists / at ends |
Joist size and span are a structural matter — use span tables or an engineer. This is a quantity/cost guide only.
Frequently asked questions
How many floor joists do I need?+
At 400 mm centres, count about (span width ÷ 0.4) + 1 joists — so a 4 m wide floor needs roughly 11 joists, plus noggins and hangers. At 600 mm centres you need fewer.
What size joists do I need?+
Joist depth depends on the span and the load, read from span tables (or specified by an engineer) — a longer span needs a deeper joist. Don't guess structural timber sizes; the count is the easy part, the sizing isn't.
Related tools & guides
Want to know how these figures are calculated? See our cost methodology.
Cost figures shown are indicative estimates, not quotations. You are responsible for verifying all costs (obtain contractor quotes) and any figures submitted to a lender. ScopeWise is a documentation tool, not financial, tax, structural or planning advice. HMO compliance prompts are guidance only — confirm requirements with your local council, as standards and licensing vary by authority.