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Septic Tank Size Calculator

Enter the users and how often the tank will be emptied. Get the length, breadth, liquid depth and litres from IS 2470.

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Quick Answer

IS 2470 (Part 1) gives the size straight from the number of users. For a family of 5 whose tank is emptied every 2 years, it is 1.5 m long, 0.75 m wide and 1.05 m of liquid: about 1,181 litres. Add 300 mm of free board, so the inside depth is 1.35 m. For 10 users the tank grows to 2.0 × 0.90 × 1.40 m, or 2,520 litres, and anything above 2,000 litres gets two chambers. These sizes assume only the toilets drain into the tank; bath and kitchen water go to a soak pit.

Figures checked Oct 2026. Source: IS 2470 (Part 1):1985, Code of practice for installation of septic tanks, Part 1 Design criteria and construction: cl. 3.1 to 3.4 design, cl. 3.4.3 sludge, cl. 3.4.5 to 3.4.10 dimensions, chambers, free board, covers and vent, cl. 5 sludge withdrawal, Appendix A Tables 5, 6 and 7.

1,181 Ltank for 5 users, emptied every 2 years
300 mmfree board above the water line
2,000 Labove this, split the tank into two chambers
2 yearslongest gap between emptying for a small home tank

Part of the system: Water tank and sump size. Steps, checks and a free printable page.

Guides: Percolation test, Septic tank location.

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The Formula

Tank size = Row = the first IS 2470 table row with users ≥ your users (Table 5 for a house up to 20 users, Table 6 for a colony, Table 7 for a hostel); liquid volume = length × breadth × liquid depth for your cleaning interval; inside depth = liquid depth + 0.30 m; sludge check = users × (0.033 + 0.00021 × 365 × years) m³

Sizes are the recommended rows in IS 2470 (Part 1):1985, Appendix A, Tables 5, 6 and 7. Free board is cl. 3.4.8. The sludge check uses cl. 3.4.3.2 (0.033 m³ per person for digestion) and cl. 3.4.3.3 (0.000 21 m³ of digested sludge per person per day). Chambers follow cl. 3.4.6 and twin tanks cl. 3.4.7.

How a family of five gets its tank (worked example 1)Five users in a house read Table 5, row 5. Emptied every 2 years gives a liquid depth of 1.05 m. 1.5 m by 0.75 m by 1.05 m is 1,181 litres. Adding 0.30 m of free board makes the tank 1.35 m deep inside.15 users, a houseIS 2470 Table 5, row for 5 users2Emptied every 2 yearsliquid depth 1.05 m3Size1.5 × 0.75 × 1.05 m = 1,181 L4Dig depth1.05 m + 0.30 m free board = 1.35 m inside
How a family of five gets its tank (worked example 1). The calculator's steps for the default inputs.

Assumptions

  • For a house up to 20 users, only the WC goes into the tank. That is the condition under Table 5 (Note 1). If the bath and kitchen water go in too, the tank needs a bigger, exact design by an engineer.
  • A count between two rows takes the next row up, so 7 users get the 10-user tank. Below 5 users the 5-user row applies; it already meets the code minimum of 1,000 litres (cl. 3.4.5.1).
  • A house above 20 users is read from Table 6 (colonies), whose first row is 50 users. A hostel below 50 users is read from the 50-user row of Table 7.
  • Rectangular tank only. IS 2470 also allows a round tank at least 1.35 m across and 1.0 m deep (cl. 3.4.5.3), but gives no size table for it.
  • The sludge check is our addition. It shows how much of the tank the code's own sludge figures fill, so you can see why a 2-year tank is deeper.
  • We quote IS 2470 (Part 1):1985, the edition on the free public copy, read on 11 October 2026. Check the current edition and your local body's rules before you build.

Septic tank sizes for a house (IS 2470 Table 5)

Recommended sizes for up to 20 users, when only the WC drains into the tank. Add 300 mm of free board above the liquid depth.

UsersLengthBreadthLiquid depth, emptied yearlyLiquid depth, emptied every 2 yearsLitres (2 years)
51.5 m0.75 m1.0 m1.05 m1,181
102.0 m0.90 m1.0 m1.40 m2,520
152.0 m0.90 m1.3 m2.00 m3,600
202.3 m1.10 m1.3 m1.80 m4,554

Read from IS 2470 (Part 1):1985, Appendix A, on 11 October 2026. The 15-user tank is printed deeper than the 20-user one for 2-year emptying; that is how the code prints it, and the 20-user tank still holds more because it is longer and wider. Litres are length × breadth × depth, worked out by us.

Liquid capacity by users, emptied every 2 years (Table 5)5 users 1,181 litres, 10 users 2,520 litres, 15 users 3,600 litres, 20 users 4,554 litres.5 users1,181 L10 users2,520 L15 users3,600 L20 users4,554 L
Liquid capacity by users, emptied every 2 years (Table 5). Each step of 5 more users adds about 950 to 1,350 litres.

Worked Examples

Where 2,520 litres go in a 10-user tank (worked example 2)Sludge being digested 330 litres, two years of digested sludge 1,533 litres, settling space 657 litres, total 2,520 litres.330 L1,533 L657 LSludge being digested 330 L2 years of digested sludge 1,533 LSettling space 657 L
Where 2,520 litres go in a 10-user tank (worked example 2). Two years of digested sludge take most of the tank. The rest is room for sewage to settle.

Family of 5 in a house, emptied every 2 years

  1. House, 5 users: IS 2470 Table 5, row for 5 users
  2. Emptied every 2 years: liquid depth 1.05 m
  3. 1.50 m × 0.75 m × 1.05 m = 1.18 m³ = 1,181 L
  4. Inside depth: 1.05 m + 0.30 m free board = 1.35 m
  5. Sludge check: 5 × (0.033 + 0.00021 × 365 × 2) = 0.93 m³ = 932 L of the 1,181 L
  6. 1,181 L is under 2,000 L, so one chamber is enough

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Joint family of 10, emptied every 2 years

  1. House, 10 users: Table 5, row for 10 users
  2. Emptied every 2 years: liquid depth 1.40 m
  3. 2.00 m × 0.90 m × 1.40 m = 2.52 m³ = 2,520 L
  4. Inside depth: 1.40 m + 0.30 m = 1.70 m
  5. 2,520 L is over 2,000 L, so a partition at two-thirds of the length: first chamber 1.33 m, second 0.67 m
  6. Sludge check: 10 × 0.033 = 0.33 m³ being digested, plus 10 × 0.00021 × 730 = 1.53 m³ stored, 1,863 L in all

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Hostel of 60 students, emptied every year

  1. Hostel, 60 users: Table 7, next row up is 100 users
  2. Emptied every year: liquid depth 1.40 m
  3. 5.70 m × 2.10 m × 1.40 m = 16.76 m³ = 16,758 L
  4. Inside depth: 1.40 m + 0.30 m = 1.70 m
  5. Over 2,000 L: two chambers, 3.80 m and 1.90 m long
  6. Sludge check: 60 × (0.033 + 0.00021 × 365) = 6.58 m³ = 6,579 L

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How to Use This Calculator

  1. Count the users. Add up everyone who lives in the building, and any staff who use its toilets every day. Plan for the family you expect in ten years, not today's.
  2. Pick the building type. Choose a house or colony, or a hostel. Hostels use their own IS 2470 table.
  3. Choose how often it will be emptied. Every 2 years is the longest gap IS 2470 allows a small home tank. Yearly gives a shallower tank but more tanker visits.
  4. Read the size. The result gives length, breadth and liquid depth, the litres, the inside depth with free board, and whether to split it into two chambers.
  5. Hand it to the engineer. Take the size to your engineer or mason, together with where the soak pit will go.

Tips and tricks

  • Send only the toilets to the tank and run bath and kitchen water to a soak pit. The Table 5 sizes assume exactly that, and soap and grease slow down the bacteria that break the sludge down.
  • Put the tank where a suction tanker can reach it. A tanker's hose reaches only so far, and a tank behind the house with no gate wide enough costs extra every time it is emptied.
  • Lay the last 12 m of the inlet pipe no steeper than 1 in 50, and use a pipe of at least 100 mm. A steep drop stirs up the settled sludge.
  • Fit a vent pipe of at least 50 mm with a mosquito mesh on top. If the tank sits within 20 m of a building, take the vent 2 m above that building's roof.
  • Empty it when the scum and sludge together fill half the depth, and leave about 25 mm of sludge at the bottom to restart the tank.
  • Don't pour phenyl, acid or bleach down the toilet by the bottle. Strong disinfectants kill the bacteria the tank depends on.

What we would do

For a family home, we'd count the people you expect in ten years, pick 2-year emptying, and take the size from Table 5. We'd send only the toilets to the tank, plan the soak pit with the engineer from the start, and leave a clear path for the tanker.

Frequently Asked Questions

What size septic tank do I need for 5 people?
IS 2470 recommends 1.5 m long, 0.75 m wide and 1.05 m of liquid if it is emptied every 2 years, about 1,181 litres. If emptied every year, the liquid depth drops to 1.0 m (1,125 litres). Add 300 mm of free board above the liquid.
What size septic tank for 10 people?
2.0 m long and 0.90 m wide. Liquid depth is 1.0 m for yearly emptying (1,800 litres) or 1.40 m for every 2 years (2,520 litres). The 2-year tank is over 2,000 litres, so it gets two chambers.
How deep should a septic tank be?
Liquid depth comes from the table, at least 1 m. Add 300 mm of free board on top. A 5-user tank emptied every 2 years is 1.35 m deep inside; the slab and the inlet depth add to the digging.
When does a septic tank need two chambers?
When it holds more than 2,000 litres. IS 2470 puts a partition so the first chamber holds twice the second, with openings of 100 to 150 mm about 300 mm below the water line. Above 100 users, it asks for two separate tanks side by side.
How often should a septic tank be emptied?
IS 2470 calls emptying every 6 months or every year desirable, and allows small home tanks every 2 years if they are not overloaded. Empty it when scum and sludge fill half the depth.
Should bathroom and kitchen water go into the septic tank?
Not into a tank sized from Table 5. Those sizes assume only the WC drains in. IS 2470 sends bath and kitchen water to a soak pit or trench instead.

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Sources & Limitations

What this calculator does not model

  • Sizes come from the code's recommended tables, not an exact design. IS 2470 itself says exact calculations should be made when choosing the size; your engineer does that.
  • Rectangular tanks only, up to 300 users.
  • It does not size the soak pit or dispersion trench, the walls, the floor slab or the cover. Those depend on soil and loads and are the engineer's job.
  • It does not check how far the tank must be from a well, a borewell or the boundary. Ask your local body.