Beam Size Calculator
Enter the clear span, the joist spacing and width, the timber grade and what the beam carries. The calculator checks bending strength and deflection and suggests a standard timber depth.
- Bending and deflection
- C16 and C24
- Floors and roofs
For guidance and early planning only. Building control requires span tables or an engineer's calculation.
A floor joist usually fails the bounce test before it would ever break. Timber is strong, but it bends. A joist that is deep enough not to snap can still sag and bounce underfoot. Floors are designed to deflect no more than span ÷ 333, and that rule often decides the depth.
Joist span chart
47 mm wide, domestic floor. Search for a depth.
| Depth (mm) | C24 at 400 | C24 at 600 | C16 at 400 |
|---|---|---|---|
| 97 | 2.05 | 1.7 | 1.75 |
| 122 | 2.6 | 2.15 | 2.2 |
| 147 | 3.1 | 2.6 | 2.65 |
| 170 | 3.6 | 3 | 3.05 |
| 195 | 4.15 | 3.45 | 3.55 |
| 220 | 4.7 | 3.85 | 4 |
| 245 | 5.2 | 4.3 | 4.45 |
Maximum clear span in metres from this calculator's method. Official span tables include more checks and may be a little shorter.
Span and joist depth
47 mm wide at 400 mm centres. Slide the span.
Deflection grows with the fourth power of span. A 20% longer span needs about 15% more depth.
How a beam is sized
Two checks. Step through it.
Find the load
Floor area each joist carries.
Notches and holes weaken joists. Keep them in the zones the span tables allow.
Solid, engineered or steel
Beam options. Compare them.
C16 or C24 joists. Cheap and easy to cut.
Engineered timber, straight and stable.
For long spans or heavy point loads.
I-joists and glulam span further and stay straighter than sawn timber, but cost more per metre.
Before you build
Approvals and details. Tick each item.
Tick each item as you go.
Joists must bear at least 90 mm on walls or hang in proper joist hangers.
What you get
A first size. Each tile explains one result.
Width × depth.
C16 or C24.
Before rounding.
Bending or deflection.
Per joist.
The governing check tells you what limits the beam. If deflection governs, a deeper section helps more than a wider one.
Frequently Asked Questions
No question matches that search.
What size joist for a 4 m span?
About 47 × 195 mm C24 at 400 mm centres for a domestic floor.
What size joist for a 3 m span?
About 47 × 147 mm C24 at 400 mm centres.
What is the difference between C16 and C24?
C24 is stronger and stiffer, so it spans about 10% further.
How far can a 2x8 joist span?
A 47 × 195 mm (2x8) C24 joist spans about 4 m at 400 mm centres.
Why do floors bounce?
The joists are too shallow for the span, or lack strutting.
Does spacing affect joist size?
Yes. 600 mm centres need deeper joists than 400 mm.
Do I need building control?
Yes for structural work in most places. Use official span tables or an engineer.
A beam that does not bounce
Enter your span, spacing and load to get a first size, checked for both strength and stiffness. Confirm it with span tables or an engineer, and the floor will feel solid underfoot.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.