How deep should a beam be? (rule of thumb)
A fast preliminary estimate uses the span-to-depth (L/d) ratio: a wood floor joist runs about L/18–L/20, a glulam/LVL beam L/16–L/20, a steel beam L/20–L/24, and a one-way concrete slab L/24–L/28. For a 20-ft steel beam, L/20 works out to about 12 inches deep. These are rules of thumb for early layout — not span tables and not a code check.
Source: Rule-of-thumb span-to-depth ratios (not a code check) · Verify against your AHJ and the adopted code edition.
Early in a layout you often just need a rough depth to reserve space for a beam. The span-to-depth ratio (L/d) gives one: divide the span by a typical ratio for the member type.
| Member | Typical L/d | 20-ft span ≈ |
|---|---|---|
| Wood floor joist | L/18 – L/20 | 12–13 in |
| Glulam / LVL beam | L/16 – L/20 | 12–15 in |
| Steel beam | L/20 – L/24 | 10–12 in |
| Open-web steel joist | L/18 – L/24 | 10–13 in |
| One-way concrete slab | L/24 – L/28 | 9–10 in |
So a 20-ft steel beam lands around 12 inches deep as a first guess.
This is preliminary only. L/d ratios reserve space and sanity-check a scheme — they are not span tables and not a structural design. The real depth depends on load, spacing, species/grade or steel section, and deflection limits. Confirm with published span tables (AWC/IRC) or a structural engineer before you build.
Work it out: the beam-span quick-reference applies these ratios to your span and member type to return an estimated depth range.