What size septic tank does a house need for 5, 10 or 20 people? This guide gives the standard septic tank sizes used in India (IS 2470), explains the two-chamber layout, freeboard and soak pit, and includes a free septic tank size calculator.
| 5 users | 1.5 × 0.75 m, liquid depth 1.0 – 1.05 m |
|---|---|
| 10 users | 2.0 × 0.9 m, liquid depth 1.0 – 1.4 m |
| 15 users | 2.0 × 0.9 m, liquid depth 1.3 – 2.0 m |
| 20 users | 2.3 × 1.1 m, liquid depth 1.3 – 1.8 m |
Septic tank size chart for houses
| Users | Length (m) | Width (m) | Liquid depth, minimum (m) | Liquid depth, longer cleaning interval (m) | Liquid capacity (litres) |
|---|---|---|---|---|---|
| 5 | 1.50 | 0.75 | 1.00 | 1.05 | 1,125 – 1,180 |
| 10 | 2.00 | 0.90 | 1.00 | 1.40 | 1,800 – 2,520 |
| 15 | 2.00 | 0.90 | 1.30 | 2.00 | 2,340 – 3,600 |
| 20 | 2.30 | 1.10 | 1.30 | 1.80 | 3,290 – 4,550 |
How a septic tank is sized
A septic tank needs enough volume for two jobs: holding the incoming sewage long enough for solids to settle (usually about a day), and storing the sludge and scum that build up between cleanings. That is why the size grows with the number of users and with the cleaning interval. IS 2470 also recommends:
- Length 2 to 4 times the width, so flow is slow and even
- Minimum width 750 mm and minimum liquid depth 1.0 m
- Freeboard of about 300 mm above the liquid level for scum and gases
- Two compartments, the first about two-thirds of the total length, separated by a partition wall with openings at mid-depth
- Inlet and outlet through T-pipes or baffles, with the outlet slightly lower than the inlet
- A vent pipe taken above roof level and covered with mesh
Free septic tank size calculator
Worked example: family of 6
- 6 users is more than the 5-user size, so use the 10-user tank: 2.0 m × 0.9 m inside
- Choose the deeper option for longer cleaning interval: 1.4 m liquid depth + 0.3 m freeboard = 1.7 m total depth
- Liquid capacity = 2.0 × 0.9 × 1.4 = 2.52 m³ = 2,520 litres
- Partition at about 1.33 m from the inlet end (two-thirds of 2.0 m)
Soak pit and effluent disposal
The liquid leaving a septic tank is still polluted, so it must go to a soak pit or dispersion trench (covered in IS 2470 Part 2), or to a sewer where one is available. Soak pits work only in permeable soil and where the groundwater table is low; in clay or high-water-table areas, use dispersion trenches or a different system. Keep the tank and soak pit well away from any well or borewell (a distance of at least 15 m is commonly required) and follow your local authority’s rules.
Safety and maintenance
- Never enter a septic tank. Gases such as hydrogen sulphide and methane can kill within minutes. Manual cleaning of septic tanks without protective gear is prohibited under the Prohibition of Employment as Manual Scavengers and their Rehabilitation Act, 2013. Use a mechanised desludging (suction) service.
- Desludge regularly, typically every 2–3 years for a household tank, and leave a little sludge behind to restart the bacterial action.
- Do not send rainwater, kitchen grease, chemicals or plastics into the tank.
Frequently asked questions
What is the septic tank size for 5 persons?
1.5 m long × 0.75 m wide inside, with a liquid depth of 1.0–1.05 m and about 300 mm freeboard, giving roughly 1,125–1,180 litres of liquid capacity.
What is the septic tank size for 10 persons?
2.0 m × 0.9 m inside, with a liquid depth of 1.0–1.4 m plus freeboard, giving about 1,800–2,520 litres.
How deep should a septic tank be?
The liquid depth should be at least 1.0 m, plus about 300 mm freeboard. Larger tanks and longer cleaning intervals need more depth.
How often should a septic tank be cleaned?
Typically every 2–3 years for a household tank, or when sludge and scum fill about a third of the tank. Always use a mechanised desludging service.
Related guides
References: IS 2470 (Part 1):1985 Septic Tanks — Design and Construction; IS 2470 (Part 2):1985 Secondary Treatment and Disposal of Septic Tank Effluent; CPHEEO Manual on Sewerage and Sewage Treatment. Sizes are compiled from commonly published tables based on IS 2470; check the code and local rules before construction.
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