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Soak Pit Size Calculator

Enter the people, what drains in and how fast your soil soaks. Get the soak pit depth or the trench length from IS 2470 Part 2.

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Quick Answer

IS 2470 (Part 2) sizes a soak pit from two numbers: the water going in each day and how fast your soil soaks. A family of 5 sending only the septic tank outflow puts in about 225 litres a day. In soil that takes 10 minutes to drop 25 mm, each m² of pit wall can take 64.5 litres a day, so the pit needs 3.49 m² of wall. A round pit 1.2 m across gets that at 1.08 m deep below the inlet pipe. If bath and kitchen water go into the same pit, the flow triples and the same pit has to go down about 2.9 m. Above 30 minutes per 25 mm, use a dispersion trench instead.

Figures checked Oct 2026. Source: IS 2470 (Part 2):1985 (reaffirmed 2001), Code of practice for installation of septic tanks, Part 2 Secondary treatment and disposal of septic tank effluent: Table 1 method by ground water and soil, Fig. 1 and Table 2 allowable loading with Notes 1 to 3, cl. 5.2.1 seepage pit, cl. 5.2.2 dispersion trench, cl. 5.3 distances, Appendix A percolation test.

204 ÷ √tlitres per m² per day the soil can take (t = minutes per 25 mm)
0.90 mleast width of a soak pit
18 mleast distance from any well or borewell
30 minslower soil than this needs a trench, not a pit

Guides: Percolation test, Septic tank location.

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The Formula

Soaking area and size = Flow (L/day) = people × litres per person; loading Q = 204 ÷ √t L/m²/day (t = minutes per 25 mm, 1 or less counts as 1); area = flow ÷ Q; pit depth below the inlet = area ÷ wall length (π × D round, 4 × side square) + 0.15 m, at least 1.0 m; trench length = area ÷ width, in runs of 30 m or less

Q = 204/√t is the curve printed under Fig. 1 of IS 2470 (Part 2):1985, and Table 2 lists selected values. A pit counts only its side wall, from 150 mm below the inlet to the bottom (Table 2 Note 2). A trench counts only its bottom (Note 1). Pit minimums are in cl. 5.2.1, trench limits in cl. 5.2.2 and distances in cl. 5.3. The 45 L a person of flushing water is from IS 1172:1993 cl. 4.1.1.

How a family of five gets its soak pit (worked example 1)Five people times 45 litres is 225 litres a day. Soil at 10 minutes per 25 mm takes 204 divided by root 10, 64.5 litres per square metre a day. 225 divided by 64.5 is 3.49 square metres. A round pit 1.2 metres across has 3.77 metres of wall, so it needs 0.93 metres of wet wall plus 0.15 metres: 1.08 metres below the inlet.1Flow5 × 45 L = 225 L a day2Soil at 10 min204 ÷ √10 = 64.5 L/m²/day3Area225 ÷ 64.5 = 3.49 m²4Round pit 1.2 m3.49 ÷ 3.77 m = 0.93 m + 0.15 m = 1.08 m below the inlet
How a family of five gets its soak pit (worked example 1). The calculator's steps for the default inputs.

Assumptions

  • Septic tank outflow is taken as the toilets' flushing water, 45 L a person a day (IS 1172:1993 cl. 4.1.1). That matches IS 2470 Part 1, which sizes small home tanks for toilet waste only.
  • Bath and kitchen water is 90 L a person: the 135 L a day IS 1172 allows for LIG and EWS city homes, less the 45 L of flushing. A big-city home on 150 to 200 L should pick "My own figure".
  • We count all of that water as reaching the pit, so the size errs on the large side.
  • The loading uses the Fig. 1 formula, not the rounded Table 2 numbers. They agree within 2 L except at 45 minutes, where the table prints 33 and the formula gives 30.4; the formula gives the bigger pit.
  • Your percolation rate should come from the Appendix A test done at the depth of the pit, after soaking the hole overnight. A rate guessed from the soil's look is not good enough to dig by.
  • We quote IS 2470 (Part 2):1985 (reaffirmed 2001) and IS 1172:1993, the editions on the free public copies, read on 12 October 2026. Check the current editions and your local body's rules before you build.

How much water each soil can take (IS 2470 Part 2, Table 2)

Maximum rate of effluent per m² of soaking area a day, against the percolation rate from the test.

Percolation rate (min per 25 mm)Table 2 (L/m²/day)Formula 204 ÷ √tWhat the code allows
1 or less204204.0Pit or trench
2143144.2Pit or trench
3118117.8Pit or trench
4102102.0Pit or trench
59091.2Pit or trench
106564.5Pit or trench
155252.7Pit or trench
303737.2Pit or trench
453330.4Trench only
602626.3Trench only
Over 60--Neither: biological or upflow filter

Table 2 and Notes 1 to 3 of IS 2470 (Part 2):1985, read from the page image on 12 October 2026. The formula column is our arithmetic from the curve printed under Fig. 1; the calculator uses it.

Litres per m² a day the soil can take1 minute 204, 5 minutes 91.2, 10 minutes 64.5, 15 minutes 52.7, 30 minutes 37.2, 45 minutes 30.4, 60 minutes 26.3 litres per square metre per day.1 min2045 min91.210 min64.515 min52.730 min37.245 min30.460 min26.3
Litres per m² a day the soil can take. Fast soil takes eight times as much water per m² as soil at the 60-minute limit.

Worked Examples

Depth below the inlet, round pit 1.2 m across, 675 L a daySoil at 2 minutes 1.39 metres, 5 minutes 2.11 metres, 10 minutes 2.93 metres, 20 minutes 4.08 metres, 30 minutes 4.96 metres.2 min1.39 m5 min2.11 m10 min2.93 m20 min4.08 m30 min4.96 m
Depth below the inlet, round pit 1.2 m across, 675 L a day. Five people sending all the house water into one pit. Past about 10 minutes per 25 mm, a single pit gets very deep; that is where a second pit or a trench makes sense.

Family of 5, septic tank outflow, soil at 10 minutes

  1. Flow: 5 × 45 L = 225 L a day
  2. Loading: 204 ÷ √10 = 64.5 L per m² per day
  3. Area: 225 ÷ 64.5 = 3.49 m²
  4. Round pit 1.2 m across: wall length π × 1.2 = 3.77 m
  5. Wet wall: 3.49 ÷ 3.77 = 0.93 m; plus 0.15 m = 1.08 m below the inlet pipe
  6. 1.08 m is above the 1.0 m minimum, so dig to 1.08 m below the inlet

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All the house water from 5 people, test reading 150 mm in 30 minutes

  1. Percolation rate: 30 min × 25 mm ÷ 150 mm = 5.0 minutes per 25 mm
  2. Flow: 5 × 135 L = 675 L a day
  3. Loading: 204 ÷ √5 = 91.2 L per m² per day
  4. Area: 675 ÷ 91.2 = 7.40 m²
  5. Square pit 1.2 m a side: wall length 4 × 1.2 = 4.80 m
  6. Wet wall: 7.40 ÷ 4.80 = 1.54 m; plus 0.15 m = 1.69 m below the inlet pipe

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10 people, all the house water, clayey soil at 40 minutes, high ground water

  1. 40 minutes is over the 30-minute limit for a pit, so a dispersion trench
  2. Flow: 10 × 135 L = 1,350 L a day
  3. Loading: 204 ÷ √40 = 32.3 L per m² per day
  4. Area: 1,350 ÷ 32.3 = 41.85 m² of trench bottom
  5. Trench 0.60 m wide: 41.85 ÷ 0.60 = 69.8 m, so 3 runs of 23.3 m, at least 2.0 m apart
  6. Ground water within 1.8 m: build the trenches partly or fully above ground in a mound

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How to Use This Calculator

  1. Count the people. Add up everyone in the house now and in the next ten years, plus daily staff.
  2. Choose what drains in. Septic tank outflow alone, bath and kitchen water alone, or both. Most homes send toilets to the septic tank and both outflows to the pit.
  3. Do the percolation test. Dig a 100 to 300 mm hole to the pit depth, fill it, leave it overnight, then time how far the water falls. Enter the rate, or the reading and let the calculator work it out.
  4. Check the ground water. Ask neighbours or look at an open well after the monsoon. If water stands within 1.8 m of the surface, a pit is out.
  5. Pick a pit or a trench and read the size. For a pit, enter its diameter or side and read the depth below the inlet. For a trench, enter the width and read the length and number of runs.

Tips and tricks

  • Keep rain off it. A masonry ring a little above ground stops yard water from flooding the pit in the first storm (cl. 5.2.1).
  • Line it with brick or stone laid dry, with open joints, and pack at least 75 mm of coarse aggregate behind the lining so the wall keeps soaking.
  • Don't plant trees beside a trench. Fine roots grow into the open joints and block the pipe.
  • If the depth comes out past about 3 m, two pits side by side are easier to dig and line than one deep one. Split the flow and size each for half.
  • Run the inlet pipe down to about 0.9 m inside the pit. The code suggests it to keep mosquitoes from breeding at the top.
  • Never connect a roof drain or a rainwater pipe to the pit. One heavy rain can fill it to the top.

What we would do

For a family home we'd do the percolation test in two spots before deciding anything, send the septic tank outflow and the bath and kitchen water to the same pit if the soil is faster than 10 minutes, and dig two shallower pits once one pit would pass about 3 m. In clay over 30 minutes we'd go straight to trenches and ask the engineer to plan them with the house layout.

Frequently Asked Questions

What size soak pit do I need for 5 people?
It depends on the soil. For septic tank outflow alone (225 L a day) in soil at 10 minutes per 25 mm, a round pit 1.2 m across needs to go 1.08 m below the inlet pipe. If bath and kitchen water go in too (675 L a day), the same pit needs about 2.93 m.
How do I do a percolation test for a soak pit?
Dig a hole 100 to 300 mm wide down to the depth of the pit, scratch the sides, put 50 mm of coarse sand in the bottom and fill 300 mm of water. Leave it overnight. Next day, top up to 150 mm and measure the drop over 30 minutes (10 minutes in sandy soil). The rate is the minutes the water takes to fall 25 mm.
What is the minimum size of a soak pit?
IS 2470 Part 2 sets the least width at 0.90 m and the depth at no less than 1.0 m below the inlet pipe (cl. 5.2.1). Bigger pits may narrow at the top to keep the cover slab small.
When should I use a dispersion trench instead of a soak pit?
When the soil takes more than 30 minutes to drop 25 mm, or when ground water rises within 1.8 m of the surface. A trench works up to 60 minutes; above that the soil can't take septic outflow at all.
How far should a soak pit be from a well?
At least 18 m from any well or borewell used for drinking water, and at least 6 m from a building (IS 2470 Part 2, cl. 5.3). Your local body may ask for more.
Can bathroom and kitchen water go into the soak pit?
Yes. IS 2470 Part 1 sends bath and kitchen water to a soak pit or trench instead of the septic tank. Size the pit for the total flow, and add a grease trap on the kitchen line so fat doesn't seal the soil.

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Sources & Limitations

What this calculator does not model

  • It sizes the soaking area only. Lining, cover slab, the inlet and vent, and the pit's distance from your own and your neighbours' foundations are the engineer's job.
  • It does not design biological filters or upflow anaerobic filters, which IS 2470 uses for tight soil or high ground water.
  • It trusts your percolation rate. One test in the dry season can overstate how fast the soil soaks.
  • It does not check local rules on soak pits, which some city bodies restrict where a sewer line is available.