Septic tank size calculator for India, the UK, the US and Australia
This septic tank size calculator works out the capacity and dimensions of a septic tank from the number of people using it (India and the UK) or the number of bedrooms (the US and Australia), using the published rules for each place. For India it reads the recommended sizes straight from the IS 2470 tables and adds the soak pit or dispersion trench needed for your soil; for the UK it applies the Approved Document H capacity and drainage-field formulas.
Sizes found online often disagree because they mix rules from different countries. Here each result names the standard or code it comes from and shows every step, in metres or feet.
It is a planning aid. Septic tanks and soakaways need approval from the local authority, and the final design must be done by a qualified engineer, plumber or installer who has seen the site and the soil.
How to use the Septic Tank Size Calculator
- Choose the rules for your country and whether you want dimensions in metres or feet.
- India: enter the number of users, the type of building and how often the tank will be desludged. UK: enter the number of users and the percolation value Vp from your test.
- US: enter the bedrooms and pick a state rule, or enter your county’s minimum and per-bedroom figures. Australia: enter the bedrooms (Western Australia rule).
- For India, add the effluent per person and the soil percolation rate to size the soak pit or dispersion trench.
- Read the capacity, dimensions and disposal area, and copy the summary to discuss with your engineer.
Septic Tank Size Calculator features
IS 2470 tables
Recommended tank sizes for houses (up to 20 users), residential colonies and hostels up to 300 users, for 1- or 2-year desludging.
Soak pit and trench
IS 2470 Part 2 application rates from the percolation test, soak pit depth for your diameter, or dispersion trench length.
UK Approved Document H
2,700 litres for up to four users plus 180 litres per extra user, and the drainage field area At = p × Vp × 0.25.
US bedroom rules
Washington and North Carolina rules built in, plus a custom rule for your county.
Suggested dimensions
Length, width and total depth with free board for any capacity, in metres or feet.
IS 2470 septic tank sizes (India)
From IS 2470 (Part 1):1985, Appendix A, Tables 5 and 6. The sizes assume only WC waste goes to the tank and need 300 mm of free board above the liquid. The standard also sets a minimum width of 750 mm, a minimum liquid depth of 1 m, a minimum capacity of 1,000 litres, a length of 2–4 times the width, and two chambers (the first twice the second) when the capacity exceeds 2,000 litres.
| Users | Length × breadth | Liquid depth (1 yr / 2 yr) | Capacity (1 yr) |
|---|---|---|---|
| 5 | 1.5 × 0.75 m | 1.0 / 1.05 m | 1,125 L |
| 10 | 2.0 × 0.90 m | 1.0 / 1.40 m | 1,800 L |
| 15 | 2.0 × 0.90 m | 1.3 / 2.00 m | 2,340 L |
| 20 | 2.3 × 1.10 m | 1.3 / 1.80 m | 3,289 L |
| 50 (colony) | 5.0 × 2.0 m | 1.0 / 1.24 m | 10,000 L |
| 100 (colony) | 7.5 × 2.65 m | 1.0 / 1.24 m | 19,875 L |
Soak pit design calculation
IS 2470 (Part 2):1985 limits how much effluent each square metre of soil can take, depending on the percolation rate — the time for water in a test hole to fall 25 mm. The calculator interpolates between the table values; for a seepage pit the absorption area is the side wall from 150 mm below the inlet to the bottom.
- Example: 5 users × 45 L = 225 L/day. At 5 minutes per 25 mm the soil takes 90 L/m²/day, so 2.5 m² is needed — a 1.5 m soak pit easily covers that with the minimum 1 m depth below the inlet, or a 0.6 m wide trench 4.2 m long.
- Slower than 30 minutes: soak pits are unsuitable, use dispersion trenches. Slower than 60 minutes: no soil absorption system will work.
| Percolation (min per 25 mm) | 1 | 2 | 3 | 5 | 10 | 15 | 30 | 45 | 60 |
|---|---|---|---|---|---|---|---|---|---|
| Max. application (L/m²/day) | 204 | 143 | 118 | 90 | 65 | 52 | 37 | 33 | 26 |
UK, US and Australian rules
- UK (England and Wales): Approved Document H (2015) para 1.18 — at least 2,700 litres below the inlet for up to 4 users, plus 180 litres for each additional user. Drainage field floor area At = p × Vp × 0.25 m², usable only when Vp is 12–100 s/mm.
- US: sizes are set by state and county codes. Washington: 1,000 gallons for up to four bedrooms plus 250 gallons for each additional bedroom (WAC 246-272A-0232, effective 1 April 2025). North Carolina: 1,000 gallons up to four bedrooms, 1,250 for five, and flow-based sizing beyond that.
- Australia: Western Australia’s Department of Health requires at least 3,180 litres for an all-waste system serving up to five bedrooms. Other states follow AS/NZS 1547 and their own plumbing codes.
- Digging the hole? Estimate the spoil with the excavation calculator and the walls or base with the concrete calculator and brick calculator.
Septic Tank Size Calculator FAQ
What size septic tank do I need for 5 people in India?
IS 2470 recommends 1.5 m long × 0.75 m wide with 1.0 m liquid depth for yearly cleaning (1,125 litres), or 1.05 m liquid depth for cleaning every two years, plus 300 mm free board. The septic tank size calculator shows the size for any number of users up to 300.
What is the septic tank size for 10 users?
From IS 2470 Table 5: 2.0 × 0.9 m with 1.0 m liquid depth (1,800 litres) for yearly desludging or 1.4 m (2,520 litres) for two-yearly desludging, with 300 mm of free board on top.
How do I calculate septic tank size in feet?
Choose feet in the calculator. For example, the IS 2470 size for 5 users is about 4.9 × 2.5 ft with 3.3 ft liquid depth and 4.3 ft total depth.
How big should a septic tank be in the UK?
At least 2,700 litres below the inlet for up to four people, plus 180 litres for each extra person — 3,060 litres for six people (Approved Document H, para 1.18).
What size septic tank for a 3-bedroom house in the US?
In Washington and North Carolina it is 1,000 gallons (3,785 litres). Many other states also use 1,000 gallons as the minimum for three bedrooms, but check your county code.