| Primary value | 2.0 m³ (5 users) (10 users → 2.8 · 20 users → 5.4 · 50 users → 13 m³) |
| Applies to | Domestic septic tanks for buildings without sewerage connection · Single dwellings, group housing and small institutions · Sites with adequate setback to dispersion trench / soak pit |
| Exceptions | 5 users — minimum tank → 2.0 × 0.9 × 1.3 m (≈ 2000 L) |
| 10 users → 2.0 × 0.9 × 1.7 m (≈ 2800 L) | |
| 20 users → 2.3 × 1.1 × 1.8 m (≈ 5400 L) — 3 chambers | |
| 50 users → 4.0 × 1.4 × 1.8 m (≈ 13000 L) | |
| Liquid retention period → 24 h minimum | |
| Sludge accumulation per user / year → 30 – 35 L | |
| Measured as | Total internal liquid capacity of the tank — sum of working volume + sludge storage volume. Free space of 0.3 m above water level for gases. Inlet 50 mm above outlet level. |
| Source | IS 2470 (Part 1) — Clause 5 ✓ Verified |
3 related items across IS codes, knowledge articles, design rules, maps and tools
Septic tank size is the single most-asked sanitation question on Indian residential sites. The IS 2470 chart maps directly to 24-hour retention + 1-year sludge accumulation per user — undersized tanks fill in 6 months and overflow, oversized tanks become anaerobic dead zones. Hitting the chart is non-negotiable for the mandatory NOC most municipalities now demand.
Plot owners typically build 2 m × 1 m × 1.8 m (≈ 3600 L) for a 4-bedroom home — about 8-user equivalent — and connect to a 1.5 m diameter soak pit downstream. Three-chamber tanks are standard above 15 users; two-chamber suffice up to 10.