| Primary value | 250 mm |
| Applies to | Single-family houses · Apartment internal stairs |
| Exceptions | Assembly / educational / commercial → 300 mm min |
| Measured as | Horizontal depth of the tread from riser face to nosing, excluding any nosing overhang. |
| Source | NBC 2016 — Part 4, Clause 4.4.2 ✓ Verified |
250 mm is the smallest depth on which an adult foot can land securely while descending. Below this, descent becomes hazardous because the heel doesn't have a stable platform.
Comfortable residential stairs use 275–300 mm treads. Below 250 mm feels tight; above 300 mm requires more horizontal space than tight Indian apartments allow.