| Primary value | 150 mm (for 4.0 m span (L / 26)) |
| Applies to | One-way slabs simply supported on two parallel walls/beams · Aspect ratio ly / lx > 2 · Mild exposure RCC slabs in residences |
| Exceptions | Continuous slab → L / 30 |
| Cantilever slab → L / 7 | |
| Two-way slab (square panel) → L / 35 | |
| Modification factor for tension steel → Multiply by Cl. 23.2.1 fig. 4 factor | |
| Measured as | Effective span L is the lesser of (clear span + effective depth) or (centre-to-centre distance between supports). Effective depth d is overall depth minus clear cover minus half bar diameter. Round up to nearest 25 mm in practice. |
| Source | IS 456 — Clause 23.2.1 ✓ Verified |
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Span-to-depth ratio is the first cut for slab thickness — it controls deflection without explicit calculation. For a 4 m span, L/26 gives ~155 mm effective depth, ~175 mm overall — the same number you see in 99% of Indian apartment drawings. Going thinner without a deflection check almost always fails IS 456 serviceability.
Most contractors round to 125 / 150 / 175 / 200 mm overall thickness. Slabs over 4.5 m typically jump to 175–200 mm even when L/26 would permit less, because tile and finishes add load that the basic ratio doesn't account for.