Span to Depth Ratio Calculator
The structural depth a span needs, from the span-to-depth ratios each system typically achieves — a sketch-design check before an engineer sizes anything.
Every structural system has a span-to-depth ratio it works at: about 20 for a reinforced concrete beam, 25 for a steel beam, 12 for a glulam beam, 30 or more for a truss.
How the span to depth ratio calculator works
Every structural system has a span-to-depth ratio it works at: about 20 for a reinforced concrete beam, 25 for a steel beam, 12 for a glulam beam, 30 or more for a truss. Divide the span by the ratio and you have the depth the structure will take out of your floor-to-floor height.
It is a rule of thumb for early design, and it is remarkably reliable for it. What it does not do is check strength, deflection or fire — that is the engineer's job.
Formula: depth = span / typical ratio
Worked examples
| Inputs | Structural depth (mm) | Note |
|---|---|---|
| A 9 m concrete beam | 450 | 450 mm |
| Post-tensioned slab | 225 | half the depth |
| A long-span truss | 2,500 | 2.5 m deep |
FAQFrequently asked questions
What is a span-to-depth ratio?
The span divided by the structural depth it needs. Each system has a typical value — 20 for a concrete beam, 25 for steel, 40 for post-tensioned slabs.
Is this a structural design?
No — a sketch-design estimate for setting floor-to-floor heights. An engineer sizes the member for strength, deflection, vibration and fire.
Why does depth matter so much?
Because it comes out of every floor. Two hundred millimetres saved per floor over twenty storeys is four metres of building — or an extra storey.
What is the services zone?
The space below the structure for ducts, pipes and lights. Three to five hundred millimetres for an office; it can be integrated into the structural depth with care.
Which system spans furthest for its depth?
Post-tensioned concrete and CLT for floors; trusses and cable structures for roofs, where depth is less costly.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- Australian Building Codes Board — National Construction Code — the body that writes Australia's building code
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.