| Primary value | 115 mm (for 4.0 m short span (L / 35)) |
| Applies to | Square or near-square panels (ly / lx ≤ 2) · Slabs supported on beams on all four edges · Typical bedroom/bathroom slabs in residential framing |
| Exceptions | Flat slab with drops → L / 32 to L / 36 |
| Rectangular panel (1 < ly/lx ≤ 2) → Use shorter span × L/35 | |
| Heavy load or deflection-critical → L / 28 to L / 30 | |
| Measured as | Use the shorter effective span lx as L. Effective depth d = overall thickness minus clear cover minus half bar diameter. |
| Source | IS 456 — Clause 24.1, 23.2.1 ✓ Verified |
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Two-way action distributes load along both directions, so a square panel can be ~25% thinner than the same span as a one-way slab. L/35 holds up to a 4 m × 4 m panel — beyond that, deflection creep and partition cracking force you to thicker. Most flats fall in this zone.
115 mm is the smallest you'll see in practice; 125–150 mm is the standard for 3.5–4.5 m square panels. Bathroom slabs often go thicker (150 mm) because of the 50 mm sunken depth for plumbing.