Septic tank size calculator
Work out the required septic tank capacity — in litres and cubic metres — from the number of users, water use, detention time and sludge storage, then get a suggested length × breadth × depth.
Based on IS 2470 design guidance commonly used across Nepal for homes, hostels and institutions. A planning-level tool, computed entirely in your browser.
Tank parameters
Persons contributing to the tank.
Litres per capita per day reaching the tank — 135 is a common design value.
Detention time for the liquid — usually 1 day (24 h).
Settled sludge & scum per person each year — about 30 L.
Years between cleaning. IS 2470 suggests 2–3 years.
Required capacity
975 L
≈ 0.97 m³ effective tank volume
Suggested size (L × B × D)
1.27 × 0.51 × 1.50 m
Liquid volume
675 L
Sludge storage
300 L
Total volume
0.97 m³
Liquid depth
1.50 m
| Sizing equation | V = users × LPCD × days + users × sludge × interval |
| Liquid (detention) | 5 × 135 × 1 = 675 L |
| Sludge storage | 5 × 30 × 2 = 300 L |
| Total capacity | 675 + 300 = 975 L (0.97 m³) |
| Suggested dimensions | 1.27 L × 0.51 B × 1.50 D m (L:B ≈ 2.5:1) |
A planning-level estimate following IS 2470 guidance. The suggested tank uses a liquid depth of about 1.5 m and a length-to-breadth ratio of roughly 2.5:1; add 0.3 m freeboard above the liquid level and a baffle/two-chamber layout in practice. Always confirm sizing, percolation and clearances with a licensed engineer and local code.
From users to tank volume
A septic tank must hold the day's wastewater long enough to settle, plus several years of accumulated sludge between cleanings. Add the two and you have the required capacity.
Liquid volume
Users × water use (LPCD) × retention days. With a 1-day detention, this is roughly one day's wastewater flow.
Sludge storage
Users × sludge per person per year × the desludging interval — space so the tank only needs cleaning every few years.
Dimensions
Pick a liquid depth of ~1.5–1.8 m and an L:B ratio of 2:1 to 3:1, then solve length and breadth for the total volume.
Septic tank sizing, answered
How do you calculate the size of a septic tank?+
Add the liquid (detention) volume to the sludge storage volume. Liquid volume = number of users × water use per person per day (LPCD) × retention days. Sludge volume = number of users × sludge accumulated per person per year × the desludging interval in years. The total, in litres, divided by 1000 gives the required capacity in cubic metres.
What water-use value (LPCD) should I use?+
LPCD is the litres per capita per day reaching the tank. A common design figure is 135 LPCD for piped domestic supply, though smaller systems or institutions may use less. Only the wastewater that actually enters the septic tank should be counted, so check your local supply norms.
What detention time and desludging interval are typical?+
IS 2470 commonly assumes about one day (24 hours) of liquid detention and a desludging interval of 2 to 3 years. Sludge accumulates at roughly 30 litres per person per year. Longer intervals mean a larger tank because more sludge storage must be provided.
What dimensions should a septic tank have?+
Once you know the required volume, pick a liquid depth of about 1.5–1.8 m and a length-to-breadth ratio of roughly 2:1 to 3:1, then solve for length and breadth. Add about 0.3 m of freeboard above the liquid level, and use a two-chamber (baffled) layout for better settling.
Is the tank capacity the same as the excavation size?+
No. The calculated capacity is the effective (liquid + sludge) volume below the outlet. The physical tank is larger because you add freeboard above the liquid, wall thickness and a soak pit or drain field downstream. Treat this tool as a planning estimate, not a construction drawing.
Sources & data note
Sizing follows IS 2470 design guidance: capacity = liquid detention volume + sludge storage volume. Defaults (135 LPCD, 1-day retention, 30 L sludge per person per year, 2-year desludging, ~1.5 m liquid depth, L:B ≈ 2.5:1) are indicative and widely used in Nepal, but vary by code edition and local authority. These values are indicative only — verify the current design requirements with a licensed engineer and your local code before construction.