Planning drainage for a new house? Enter how many people will use the toilets, how often the tank will be cleaned and your soil type, and get the septic tank size from the IS 2470 table, its liquid capacity, and the soak pit diameter and depth, with a section drawing for your contractor. Part of our free home planning tools.
How to use the septic tank calculator
- Set the number of households and people in each. Size for the most people who will live there, including future rented floors.
- Choose how often the tank will be cleaned: every year, or every two years (a deeper tank).
- Pick the soil where the soak pit will go, or enter your percolation test result under More settings.
- Read the tank size, depth and capacity, the soak pit size, and the section and top view.
- Optionally mark the tank position for a Vastu check, then download a PNG, print or share the result.
Septic Tank & Soak Pit Calculator
Tank size from IS 2470 and soak pit size for your soil, for Indian homes
More settings
How we got these numbers
Things to check
Vastu
Traditional guidance; schools differ. Health rules and distances from water sources come first.
Educational estimate. Tank sizes follow the IS 2470 (Part 1) table for residential use as commonly published; the soak pit uses the absorption rule commonly cited from IS 2470 (Part 2). Your local authority's rules, the soil test and an engineer's design decide the final size. Never enter a septic tank: the gases inside can kill.
Septic tank size for homes (IS 2470 table)
IS 2470 (Part 1) gives standard tank sizes for residential use. The calculator picks the row for your number of users, rounding up, and adds about 300 mm (1 ft) of free space above the liquid.
| Users | Length × breadth (inside) | Liquid depth, cleaned every year | Liquid depth, cleaned every 2 years |
|---|---|---|---|
| Up to 5 | 1.5 m × 0.75 m (4′11″ × 2′5½″) | 1.0 m | 1.05 m |
| 6–10 | 2.0 m × 0.9 m (6′6½″ × 2′11½″) | 1.0 m | 1.4 m |
| 11–15 | 2.0 m × 0.9 m (6′6½″ × 2′11½″) | 1.3 m | 2.0 m |
| 16–20 | 2.3 m × 1.1 m (7′6½″ × 3′7½″) | 1.3 m | 1.8 m |
Sources reproduce this table under different titles and editions of IS 2470; check the edition your local authority follows. Above 20 users (several rented floors, apartments, hostels) the tank needs an engineer’s design, and many cities require a sewage treatment plant or a sewer connection instead.
How the soak pit is sized
The effluent leaving the tank is taken as about 80% of the water the users consume: 5 people at 135 L each produce about 540 L a day. The soil can absorb about 130 ÷ √t litres per m² of pit wall a day, where t is the percolation time (minutes for water to drop 25 mm in a soaked test hole), a rule commonly cited from IS 2470 (Part 2). The wall area below the inlet pipe must take the whole day’s effluent.
Example: in loamy soil (about 5 minutes per 25 mm) the soil takes about 58 L/m² a day, so 540 L needs about 9.3 m² of wall: a pit about 1.2 m across and 2.5 m deep below the inlet. In sandy soil the same house needs only about 1.0 m × 1.9 m. Where water takes more than about 30 minutes to drop 25 mm (clay, black cotton soil) or the groundwater is high, a soak pit is not suitable.
Practical points before you build
- Use two chambers: the first about two-thirds of the length, so cleaner effluent reaches the soak pit.
- Keep the tank and soak pit well away from wells, borewells and your water sump, and downhill of them if you can; many guides cite 15 m or more from a well or borewell. Follow your local rules.
- Fit a 100 mm (4 in) vent pipe carried above the roof. Keep rainwater and kitchen grease out of the tank.
- Desludge through a licensed service every one or two years, as designed. Never enter the tank.
Septic tank position as per Vastu
Commonly cited Vastu guidance places the septic tank in the north-west of the plot, with west as an alternative, and avoids the north-east, the south-west and the centre. Schools differ, and distance from wells and water tanks matters more. Plan the drinking water sump on the opposite side, in the north-east: see the water tank and sump calculator.
Frequently asked questions
What size septic tank do I need for 5 people?
The table in IS 2470 (Part 1) for homes gives a tank 1.5 m long and 0.75 m wide with 1.0 m of liquid for 5 users if it is cleaned every year, or 1.05 m of liquid for cleaning every two years. Add about 300 mm above the liquid, so the inside depth is about 1.3 to 1.35 m.
What size septic tank do I need for 10 people?
For 10 users the IS 2470 table gives 2.0 m by 0.9 m, with 1.0 m of liquid for yearly cleaning or 1.4 m for cleaning every two years, plus about 300 mm free space above the liquid.
How is soak pit size calculated?
Work out the daily effluent (about 80% of the water the users consume), then divide it by how much the soil can absorb. A commonly cited rule from IS 2470 (Part 2) allows 130 divided by the square root of the percolation time (minutes for water to drop 25 mm) litres per square metre per day. The result is the wall area the pit needs below the inlet pipe.
How often should a septic tank be cleaned?
Home septic tanks are usually desludged every one to three years, depending on their size and use. A tank sized for a two-year interval is deeper than one sized for yearly cleaning. Use a licensed desludging service; never enter a septic tank, because the gases inside can kill.
Why does a septic tank have two chambers?
The first chamber, about two-thirds of the length, lets most of the solids settle and scum float. The second chamber settles finer solids, so cleaner effluent goes to the soak pit and the pit clogs less.
Can I build a soak pit in clay soil?
Soak pits work poorly in clay or black cotton soil, where water takes more than about 30 minutes to drop 25 mm in a percolation test, and where the groundwater is high. Ask an engineer about a dispersion trench, a raised filter bed, or a sewer connection.
Where should the septic tank be as per Vastu?
Commonly cited Vastu guidance places the septic tank in the north-west of the plot, with west as an alternative, and avoids the north-east, south-west and the centre. Schools differ, and health and distance rules from wells and water tanks come first.
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Sources and limitations
Tank sizes are the residential table of IS 2470 (Part 1) as reproduced in published sources; the soak pit uses the rate of 130 ÷ √t L/m² a day commonly cited from IS 2470 (Part 2). Clause numbers and editions differ between sources, and state rules take precedence. The tool does not design the tank’s structure, and it does not replace a percolation test. Vastu notes are traditional guidance, not guaranteed outcomes. Report errors via our Corrections Policy; see our Editorial Policy and Methodology.
Disclaimer: Online educational publication only. Not a substitute for licensed architectural, structural, plumbing or legal services. Full disclaimer · contact@speakarch.com
Last updated: 8 November 2026