Septic tank size calculator
Look up the septic tank size for toilet waste from the number of users and the cleaning interval, using the dimensions in IS 2470 (Part 1).
Read this first. These sizes are for a tank that receives only toilet (WC) waste. Water from the kitchen, bathroom and washing needs a separate seepage pit or trench. The soak pit or trench is sized by a percolation test, so this page does not size it. This is a planning estimate, not a design.
What size septic tank does a house of 5 need?
For 5 users and cleaning once a year, the table gives an inside length of 1.50 m, a width of 0.75 m and a liquid depth of 1.00 m, plus 0.30 m of free board above the liquid. That holds 1,125 litres of liquid, and the total inside depth is 1.30 m.
What does the table say?
| Users | Inside length × width | Liquid depth, cleaned every 1 year | Liquid depth, cleaned every 2 years |
|---|---|---|---|
| 5 | 1.50 m × 0.75 m | 1.00 m (1,125 litres) | 1.05 m (1,181 litres) |
| 10 | 2.00 m × 0.90 m | 1.00 m (1,800 litres) | 1.40 m (2,520 litres) |
| 15 | 2.00 m × 0.90 m | 1.30 m (2,340 litres) | 2.00 m (3,600 litres) |
| 20 | 2.30 m × 1.10 m | 1.30 m (3,289 litres) | 1.80 m (4,554 litres) |
Add 0.30 m of free board above each liquid depth. The calculator rounds your number of users up to the next row and never down.
What rules does the standard set for the tank?
- Width of at least 750 mm and liquid depth of at least 1 m.
- Liquid capacity of at least 1,000 litres.
- Length between 2 and 4 times the width.
- Free board of 300 mm above the liquid.
- A tank larger than 2,000 litres may be built in two chambers, with the first chamber twice the size of the second.
What does the calculator leave out?
- Kitchen, bathroom and washing water. Table 5 is for toilet waste only, and the other water needs its own seepage pit or trench, or an engineer's calculation.
- The soak pit or trench. The soil's percolation test decides its size and whether it can be used at all, and it must be kept away from drinking-water sources.
- More than 20 users, shared tanks for flats, bio-digesters and twin-pit systems. Ask an engineer.
- Local bye-laws, which can ask for more. A septic tank only treats waste partly, so the liquid must go to a soak pit, a trench or a sewer.
Sources
- IS 2470 (Part 1):1985, Code of Practice for Installation of Septic Tanks: Design Criteria and Construction (Table 5).
- IS 2470 (Part 2):1985, Code of Practice for Installation of Septic Tanks: Secondary Treatment and Disposal of Septic Tank Effluent.
More calculators
General information only. It is not a structural design or professional advice. Ask a licensed structural engineer or architect to check your plans and site.