Septic Tank Size Calculation: Capacity per User Under IS 2470

How is septic tank size calculated?

A septic tank is a watertight underground tank that holds sewage long enough for the solids to settle and rot down. Its liquid volume has to cover three jobs at once, and IS 2470 gives a number for each in clause 3.4.

1. Settling space. The tank needs enough surface area for solids to drop out of the flowing sewage. The code asks for 0.92 square metres of surface for every 10 litres per minute of peak flow (at 25°C), with a settling depth of 250 to 300 mm.

2. Digestion space. Settled solids are broken down by bacteria that work without air. The code allows 0.033 cubic metres (33 litres) per person for this.

3. Sludge storage. What is left after digestion is sludge, and it piles up until the tank is emptied. The code takes 0.00021 cubic metres (0.21 litres) per person per day. Multiply by the number of days between cleanings.

Add the three, then fit a box around the total that obeys the minimum rules: at least 750 mm wide, at least 1 metre of liquid depth, at least 1,000 litres, and a length two to four times the width. On top of the liquid depth sits 300 mm of empty space, called the freeboard, for scum and gas.

The peak flow in step 1 is read from the code’s tables, not measured. For small homes it assumes one water closet (WC) per five people. For 5 users the peak is 9 litres per minute, for 10 users 18, for 15 users 18 and for 20 users 27 (IS 2470 Table 2).

One assumption decides everything: the code’s small-tank sizes treat only WC flush water. Bath, kitchen and washing water (called sullage) normally goes to a separate soak pit or drain. Send it all through the septic tank and the tank is undersized from day one.

What size septic tank do you need for 5, 10, 15 or 20 users?

For a single house or a small block of up to 20 people, IS 2470 Appendix A gives ready sizes. The liquid depth changes with how often you will empty the tank; the length and width do not.

Table 1: IS 2470 recommended septic tank sizes for up to 20 users (WC flow only)

Source: IS 2470 (Part 1):1985, Table 5. The volume and total-depth columns are worked out from the code’s dimensions.

The code adds a warning under its own table: these sizes rest on assumptions, and an exact calculation should be made when choosing a tank. The next section does that calculation, and it lands close to the table.

Worked example: septic tank size for 10 persons

Take a house with ten people, a tank emptied every two years, and only WC water going in.

Table 2: Septic tank capacity calculation for 10 users, cleaned every 2 years

Now fit a box to it. A width of 0.9 m and a length of 2.0 m give 1.8 m² of surface, more than the 1.66 m² needed, and the length is 2.2 times the width, inside the two-to-four rule. Liquid depth = 2.36 / 1.8 = 1.31 m. The code’s table rounds this up to 1.4 m, which gives 2,520 litres, so the hand calculation and the table agree within about 7 percent.

Change one thing, cleaning every year, and the sludge figure halves to 0.77 m³. The total drops to 1.59 m³, a depth of only 0.89 m. That is below the 1 metre minimum, so the depth goes up to 1.0 m, which is exactly what the table shows.

Because the two-year tank holds more than 2,000 litres, IS 2470 clause 3.4.6 allows a partition. The first chamber should hold twice the second, so on a 2.0 m tank the wall sits about 1.33 m from the inlet end, with openings of 100 to 150 mm about 300 mm below the top water level.

The same steps work for 5 users (about 1.18 m³ for two-yearly cleaning, matching the table exactly) and 20 users (about 4.47 m³ against the table’s 4.55 m³). If your count falls between rows, run the steps rather than guess.

1. Inlet: a T-shaped dip pipe on tanks up to 1,200 mm wide, its bottom about 300 mm below the top water level.
2. Freeboard: at least 300 mm of space above the top water level.
3. First chamber: two-thirds of the liquid volume when the tank holds over 2,000 litres.
4. Second chamber: one-third, joined to the first by 100 to 150 mm openings about 300 mm below the top water level.
5. Sludge: settles on a floor sloped 1 in 10 toward the desludging point.
6. Outlet: a 100 mm dip pipe reaching about one-third of the liquid depth, its invert 50 mm below the inlet invert.

Septic tank size for housing colonies, hostels and schools

Above 20 users, flows come from baths and basins as well as WCs, and the peak is spread over more fixtures. IS 2470 gives separate tables for residential colonies (households of five with one WC, one bath and one basin or sink each) and for hostels and boarding schools.

Table 3: IS 2470 recommended sizes for 50 to 300 users

Source: IS 2470 (Part 1):1985, Tables 6 and 7. Add 300 mm freeboard to every depth.

Two rules change at this scale. For more than 100 users, the code asks for two tanks side by side, each with half the total capacity, so one keeps working while the other is emptied (clause 3.4.7). And tanks wider than 1,200 mm need two or more submerged inlets and a full-width outlet weir with a scum board, instead of the single dip pipes used on small tanks. The code covers installations up to 300 users; beyond that, a sewage treatment plant designed by an engineer is the normal route.

What are the minimum dimensions of a septic tank?

Whatever the calculation says, the tank has to meet these floors. They are where most site-built tanks in villages and small towns fall short, usually on depth and on the vent.

Table 4: Minimum dimensions and fittings under IS 2470 (Part 1)

The code also asks for the tank to be open to the sky, as far from the building wall as possible, out of swampy or flood-prone ground, and reachable for cleaning (clause 3.3.1). Rainwater and groundwater must not get in. A tank that floods in the monsoon washes its sludge straight out into the soak pit.

How often should a septic tank be cleaned?

IS 2470 clause 5.1 says cleaning every six months or every year is desirable, and that small household tanks may be cleaned at least once in two years, provided no more people use the tank than it was designed for. The practical trigger in the code’s note: clean when the scum on top and the sludge at the bottom together take up more than half the tank’s depth.

Two rules for the cleaning itself. Leave at least 25 mm of sludge at the bottom, because it seeds the bacteria for the next cycle (clause 5.2). And avoid handling sludge by hand; the code calls for a pump or a sludge pipe, and for the sludge to go into a covered pit or a vehicle, never spread on open ground nearby.

This is where the size choice pays back. A tank built to the 2-year depth gives a household slack if the cleaning truck is late. A tank built to the 1-year depth and then left for three years overflows solids into the soak pit, and a choked soak pit is a far bigger job than a cleaning.

How big should the soak pit be?

A septic tank only treats sewage partly. The liquid leaving it, called effluent, still needs the soil to finish the job, which IS 2470 (Part 2):1985 covers. The usual answer for a house is a soak pit, which the code calls a seepage pit: a lined pit with open joints that lets effluent soak into the ground.

Its size depends on how fast your soil drinks water. That is found with a percolation test: dig a test hole, soak it overnight, and time how many minutes the water takes to fall 25 mm. The code then limits how much effluent each square metre of pit wall may take.

Table 5: Allowable effluent per square metre of soak pit wall, by percolation rate

Source: IS 2470 (Part 2):1985, Table 2 (selected rows).

For a soak pit, only the side wall counts, measured from 150 mm below the inlet pipe down to the bottom. As an example, say a tank sends out 600 litres a day and the test shows 10 minutes. You need 600 / 65 = 9.2 m² of wall. A round pit 1.5 m across has 4.7 m² of wall for each metre of depth, so it needs about 2 m of working depth below the inlet.

The code’s limits on placement are firm. Keep the pit at least 18 m from any well or other drinking water source, and at least 6 m from the nearest house. The pit must be at least 0.9 m across and at least 1.0 m deep below the inlet pipe. If the percolation rate is over 30 minutes the soil is unsuitable for soak pits, and over 60 minutes it cannot take any soil absorption system; in that case Part 2 points to a dispersion trench in a raised mound, a biological filter or an upflow anaerobic filter.

From tank size to quantities: digging, concrete and brickwork

Once the inside size is fixed, the build quantities follow by simple geometry. Here is the 10-user, two-yearly tank: inside 2.0 m long, 0.9 m wide and 1.7 m deep (1.4 m liquid plus 300 mm freeboard), with 200 mm brick walls.

IS 2470 sets the floor concrete grade but not its thickness, and it says nothing about working space in the pit. The figures below use 150 mm for the floor and 300 mm of working space on every side only as an example; take both from your engineer’s drawing.

Table 6: Example quantities for a 10-user septic tank (inside 2.0 x 0.9 x 1.7 m)

From here, the building material calculation guide turns the concrete and brickwork into bags of cement, sand, aggregate and bricks, and the plaster quantity calculation does the same for the 12 mm coat in 1:3 mortar. If you would rather buy the floor concrete than mix it, RMC vs site mix concrete sets out when each makes sense. To put the tank into a whole-house budget, it is one line in the house construction cost calculator, and the construction estimate format shows how such items sit in a bill of quantities.

Ten cubic metres of soil is slow work by hand, and the walls of a 1.85 m pit in loose soil need shoring if people work inside it. A machine digs it faster and leaves straighter sides. A mini excavator such as the Komatsu PC35MR-3 reaches 3,110 mm deep and cuts a straight wall to 2,690 mm, as per its current Oct 2026 spec, enough for this pit with room to spare and small enough for a house plot. A backhoe loader such as the JCB 3DX digs to 4,770 mm with a 790 mm standard bucket and can also backfill and carry away the spoil with its front shovel. How deep each class digs is set out in mini excavator digging depth by size and backhoe loader dig depth and reach.

Need a machine for the pit, the soak pit and the drain trench? Compare mini excavator models and prices and connect with a dealer near you.

Mistakes that make a septic tank fail early

Most failed tanks were sized right on paper and built wrong. The common ones:

Bath and kitchen water piped in, so a tank sized for WC flow runs at two or three times its design load. Depth cut short to save digging, which shrinks the sludge store and halves the time between cleanings. No partition or no dip pipes, so scum and solids ride straight out of the outlet. No vent, or a vent ending at head height beside a window. And a soak pit placed close to a hand pump to save pipe, which the 18 m rule exists to stop.

Each of these is cheaper to avoid at the drawing stage than to fix once the tank is buried. For more on planning a house within budget, see low-cost house construction and the other construction cost guides. Planning the digging for a tank, soak pit and drains? Compare backhoe loader models and prices and connect with a dealer.

The sizing rules on this page are from IS 2470 (Part 1):1985, Code of Practice for Installation of Septic Tanks: Design Criteria and Construction, and the soak pit rules from IS 2470 (Part 2):1985, Secondary Treatment and Disposal of Septic Tank Effluent, read in October 2026.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication. Sizes and clauses on this page are as published in IS 2470 (Parts 1 and 2):1985, read in October 2026; local building bye-laws and pollution control rules may add requirements, and a septic tank for a large building should be designed by a qualified engineer.