Beam Span Quick-Reference
Estimate a rough member depth from a span, using typical span-to-depth ratios for early layout.
- Using a span-to-depth ratio of L/20–L/24 for a steel beam.
- Preliminary depth only — confirm with span tables or an engineer (not a code check).
Span-to-depth, at a glance
Early in design you often need a rough beam or joist depth to draw a section before any analysis. The shortcut is the span-to-depth ratio (L/d): a member's depth is roughly its span divided by a number that depends on the material and system. Deeper members (smaller ratio) are stiffer; shallower ones (larger ratio) save headroom but work harder.
Typical ranges used here: wood joists about L/18–L/20, glulam/LVL beams L/16–L/20, steel beams L/20–L/24, open-web steel joists L/18–L/24, concrete beams L/12–L/16, and one-way concrete slabs L/24–L/28.
These are rules of thumb for preliminary sizing only — they are not span tables and not a code check. Loads, spacing, species/grade, and deflection limits all matter, so confirm the real size with published span tables (AWC/IRC) or a structural engineer.
Frequently asked questions
How deep should a beam be for its span?
As a rough starting point, divide the span by a span-to-depth ratio for the material: wood joists about L/18–L/20, steel beams L/20–L/24, concrete beams L/12–L/16. A 24-ft steel beam works out to roughly 12–14 inches deep. This is preliminary sizing, not a code check.
What is a span-to-depth ratio?
The span divided by the member depth (L/d). A smaller ratio means a deeper, stiffer member; a larger ratio means a shallower one that saves headroom but works harder. Typical ratios depend on the material and framing system.
Can I use this instead of a span table?
No. These ratios give a rough depth for early layout only — they aren't span tables and don't account for loads, spacing, species or grade, or deflection limits. Confirm the real size with published span tables (AWC/IRC) or a structural engineer.