| Primary value | 450 mm depth (for 4.5 m span (L / 12 to L / 15)) |
| Applies to | Simply-supported RCC beams (use L/12) · Continuous RCC beams (use L/15) · Plinth beams and tie beams |
| Exceptions | Cantilever beam → L / 7 |
| Pre-stressed beam → L / 20 to L / 25 | |
| Heavily loaded transfer beam → L / 8 to L / 10 | |
| Measured as | L is the centre-to-centre span. Depth is overall, including slab thickness if monolithic. Round up to the nearest 50 mm and ensure depth is at least 1.5× width for stability. |
| Source | IS 456 — Clause 23.2.1 ✓ Verified |
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Beam depth is the single biggest driver of deflection and steel quantity. Designers default to L/12 for simply-supported and L/15 for continuous because the IS 456 deflection check (Cl. 23.2) is almost always governing. Going shallower forces heavier reinforcement and tighter detailing — usually not worth the saving in headroom.
For a typical 4.5 m flat slab beam, contractors size 230 × 450 mm or 230 × 500 mm. Architects who push for shallow beams (L/18 to L/20) end up with wider sections (350+ mm) and chunky reinforcement.