Floor Joist Table: Joists needed by Floor width (across joists)
Reference table of joists needed for Floor Joist across a range of floor width (across joists) values — exact, engine-computed figures you can read off at a glance.
Floor Joist Calculator
Verified formula Updated Jun 2026 Private — runs on your device
Verified formula
Private
Joists needed
14
For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.
How to use the Floor Joist Calculator
The Floor Joist Calculator works out your joists needed in an instant. Enter floor width (across joists) and joist spacing and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.
- Enter the floor width (across joists).
- Enter the joist spacing.
- Read off your joists needed — the calculator updates automatically, with no button to press.
Worked example
For example, with floor width (across joists) of 5 m and joist spacing of 0.4 m, the joists needed is 14.
| Floor width (across joists) | 5 m |
|---|---|
| Joist spacing | 0.4 m |
| Joists needed | 14 |
|---|
Results are estimates for educational use, not professional advice.
Frequently asked questions
Divide the floor width by the joist spacing and add one. A 5 m wide floor at 0.4 m spacing needs 14 joists.
0.4 m (16 inches) on centre is common, with 0.3 m or 0.6 m used depending on load and joist size.
Yes. Closer spacing supports more load and reduces bounce, while wider spacing needs larger joists.
No. Bridging or blocking between joists adds stiffness and is counted separately from the joist run.
Enter the floor width (across joists). Enter the joist spacing. Read off your joists needed — the calculator updates automatically, with no button to press.
Related guides
How to Calculate Floor Joist: Formula, Steps & Examples
Learn how to calculate Floor Joist — the formula explained step by step, with worked examples and a free calculator to check your answer.