Septic Tank Size Calculator
Find the right septic tank size for a home from the number of users and desludging interval, with suggested dimensions.
Inputs
About 70–90 litres for a home
Formula
- Sewage volume = users × flow × detention time ÷ 24
- Sludge volume = users × 0.033 m³ per year × years
- Tank capacity = sewage + sludge volume (IS 2470)
IS 2470 sizes a septic tank as the daily sewage held for one day, plus space for sludge that accumulates between cleanings at about 30 litres per person per year. The liquid depth should be at least 1.0–1.8 m and the length about 2–3 times the width, which is what the suggested dimensions follow.
Step-by-step calculation
- 1
Sewage
users × flow × detention/24
0.45 m³
- 2
Sludge
users × 0.033 × years
0.33 m³
- 3
Total
sewage + sludge
0.78 m³
Worked example
- 15 users × 90 litres = 450 litres per day held for 24 h = 0.45 m³.
- 2Sludge for 2 years = 5 × 0.033 × 2 = 0.33 m³.
- 3Total 0.78 m³ → a tank of about 2.0 × 0.9 × 1.3 m liquid depth.
Tips
- Always add a soak pit or dispersion trench — a septic tank alone does not dispose of the effluent.
- Keep the tank at least 2 m from the building and 15 m from any drinking-water well.
Frequently asked questions
What size septic tank for 5 people?
About 1,000 litres of working volume — a common size is 2.0 m × 0.9 m × 1.3 m liquid depth.
How often should it be cleaned?
Every 2–3 years for a household tank; more often if you notice slow drainage or odour.
Can I use a ready-made plastic tank?
Yes for small homes, provided the capacity matches and it is bedded properly in sand.
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