Percolation Test for a Soak Pit: Step by Step (IS 2470)

Quick Answer

Bore a hole 100 to 300 mm across, straight-sided, down to the depth where the effluent will soak in. Scratch the sides, put 50 mm of coarse sand or fine gravel in the bottom and fill it with at least 300 mm of water. Come back after 24 hours. If water is still there, set it to 150 mm above the gravel and measure the drop over 30 minutes. If the hole is dry, fill to 150 mm and measure every 30 minutes for 4 hours, refilling as needed, and use the last reading. In sandy soil where 150 mm is gone in under 30 minutes, read every 10 minutes for an hour. Then percolation rate = reading time × 25 ÷ drop in mm, in minutes per 25 mm. A 60 mm drop in 30 minutes is 12.5 minutes. Over 30 minutes the soil won't take a soak pit, and over 60 minutes it won't take a trench either (IS 2470 Part 2, Appendix A and Table 2).

Source: IS 2470 (Part 2):1985 Code of practice for installation of septic tanks, Part 2 Secondary treatment and disposal of septic tank effluent: Appendix A (percolation test), Fig. 1, Table 2, checked Oct 2026.

100 to 300 mmwidth of the test hole

24 hourssoak before you take readings

min per 25 mmthe unit of the percolation rate

over 30 mintoo slow for a soak pit

Before you dig: where, how deep and how many holes

The test tells you how fast your soil takes water at the depth where the effluent will go. IS 2470 asks for the hole to go down to the depth of the proposed trench, or more generally to "any depth at which it is intended to dispose of the effluent" (A-1.1, A-1.2). For a soak pit, that's the part of the pit below the inlet pipe. A hole tested at 300 mm in loose topsoil can give a much faster rate than the clay a metre down.

Dig more than one hole. Clause 3.1 of the code asks for tests "in trial pits at more than one place" so you can take an average. On a house plot, two or three holes spread across the area where the pit could go are enough.

Season matters too. The code warns that the time of year affects the result (cl. 3.2 c), which is why the hole is soaked for a full day first: it brings the soil close to how it behaves in the wettest part of the year (A-1.3). While you dig, note whether water seeps into the hole. If ground water comes within 1.8 m of the surface in the monsoon, Table 1 rules out a soak pit whatever the test says.

The test, step by step

You need a post-hole auger or a crowbar and a narrow spade, a steel tape or a ruler marked in millimetres, a straight stick or a peg as a fixed reference, a bucket of coarse sand or fine gravel, a few buckets of water and a watch.

Dig the hole: square or round, 100 to 300 mm across, with vertical sides, down to the soaking depth (A-1.2).
Scratch the bottom and sides with a nail or a knife to break the smeared, polished soil the tool leaves, and lift out the loose crumbs (A-1.2).
Put about 50 mm of coarse sand or fine gravel on the bottom so the water you pour doesn't scour it (A-1.2).
Pour water to at least 300 mm above the gravel and leave it for 24 hours (A-1.3). Keep topping it up through the day in fast soil so the sides stay wet.
Lay a stick across the top or drive a peg beside the hole and measure every water level down from the same point.
Next day, take the readings. The table below shows which routine to follow.
Work out the rate from the final reading only: rate = reading time in minutes × 25 ÷ drop in mm (A-1.6).

Which readings to take after the 24-hour soak (IS 2470 Part 2, A-1.3 to A-1.5)
What you find	Set the water to	Read	Use
Water still in the hole	150 mm above the gravel	Once, after 30 minutes	That 30-minute drop (A-1.3)
Hole is dry	150 mm above the gravel, refill when needed	Every 30 minutes for 4 hours	The last 30-minute drop (A-1.4)
Sandy soil: the first 150 mm goes in under 30 minutes	150 mm above the gravel, refill when needed	Every 10 minutes for 1 hour	The last 10-minute drop (A-1.5)

The percolation test from digging to the rate. IS 2470 (Part 2):1985, Appendix A. The numbers in the last two steps are the first worked example.

Why only the last reading counts

When the hole was dry after the soak, the first readings are fast and then slow down as the soil fills up. The code uses only the drop in the final 30-minute period for that reason (A-1.4). The earlier readings tell you whether the soil had settled. If the last two or three are close, it had. If the drop is still falling sharply at the four-hour mark, the soil hadn't settled yet, and the true rate is slower than your last reading.

Write every reading down anyway. The code says the earlier drops can help adjust the method to local conditions, and they're the record you hand to whoever designs the pit.

Eight 30-minute readings in a hole that was dry after the soak. Second worked example. The drop settles at 20 mm; that last reading gives 30 × 25 ÷ 20 = 37.5 minutes per 25 mm.

From the rate to a soak pit, a trench or neither

The percolation rate is the time in minutes the water takes to fall 25 mm (A-1.6). A small number means fast soil. Fig. 1 of the code turns it into the most effluent each square metre of soaking surface can take in a day: Q = 204 ÷ √t litres per m² per day, where t is the rate. Our soak pit calculator uses that formula.

What the rate means (IS 2470 Part 2, Fig. 1, Table 2 Note 3 and Table 1)
Percolation rate	Loading, L/m² a day	What the code allows (ground water below 1.8 m)
2.5 min	129.0	Soak pit or trench
12.5 min	57.7	Soak pit or trench
30 min	37.2	Soak pit or trench (the limit for a pit)
Over 30, up to 60 min	37.2 down to 26.3	Dispersion trench only
Over 60 min	None	No soil absorption: a filter, sized by an engineer

If you dug more than one hole, average the rates before you size anything. If one hole is far slower than the others, find out why (a clay lens, a buried rubble patch) before you average it away.

Allowed loading at five percolation rates. Q = 204 ÷ √t from IS 2470 (Part 2) Fig. 1. Above 30 minutes the code rules out a soak pit; above 60 it rules out any soil absorption.

Worked examples

1. Water still in the hole the next morning

After 24 hours some water is left, so set it to 150 mm above the gravel (A-1.3)
Drop in 30 minutes: 60 mm
Rate: 30 × 25 ÷ 60 = 12.5 minutes per 25 mm
Loading: 204 ÷ √12.5 = 57.7 L per m² a day
Septic outflow for 5 people at 45 L each: 225 L a day, so 225 ÷ 57.7 = 3.90 m² of soaking wall
Round pit 1.2 m across: wall 3.77 m round, 3.90 ÷ 3.77 = 1.03 m, plus 0.15 m = 1.18 m below the inlet

Result: 12.5 minutes per 25 mm. A soak pit is allowed; a 1.2 m round pit needs about 1.18 m of depth below the inlet pipe.

2. Hole dry the next morning, clay-loam soil

Hole is empty, so fill to 150 mm and read every 30 minutes for 4 hours (A-1.4)
Drops: 38, 30, 26, 24, 22, 21, 20, 20 mm; the last one counts
Rate: 30 × 25 ÷ 20 = 37.5 minutes per 25 mm
Over 30 minutes: no soak pit; up to 60 minutes: a trench is allowed (Table 2 Note 3)
Loading: 204 ÷ √37.5 = 33.3 L per m² a day; all house water for 5 people, 675 L a day ÷ 33.3 = 20.3 m² of trench bottom
In a 0.6 m wide trench that's 33.8 m, more than the 30 m limit for one run, so two runs of about 17 m, 2 m apart

Result: 37.5 minutes per 25 mm: this soil takes a dispersion trench, not a pit.

3. Sandy soil that drains in minutes

After the soak, 150 mm of water is gone in 22 minutes, under 30, so read every 10 minutes for an hour (A-1.5)
Last 10-minute drop: 100 mm
Rate: 10 × 25 ÷ 100 = 2.5 minutes per 25 mm
Loading: 204 ÷ √2.5 = 129.0 L per m² a day
675 L a day ÷ 129.0 = 5.23 m²; in a 1.2 m round pit, 5.23 ÷ 3.77 + 0.15 = 1.54 m below the inlet

Result: 2.5 minutes per 25 mm: fast soil and a small pit. Keep it at least 18 m from any well, because effluent moves quickly through sand.

Tips and tricks

Use a scale with millimetre marks and read it at eye level against the same peg every time. On a 20 mm drop, reading 5 mm out either way turns 37.5 minutes into 30 or 50.
Write the clock time next to every reading. It's easy to lose track of which 30-minute period you're in during a four-hour test.
Bore the hole rather than chopping it with a spade if you can. A spade smears clay soil, and a smeared hole drains slowly.
Mark the 150 mm level on a stick held against your peg, so each refill takes seconds and lands at the same height.
If two holes give very different rates, test a third between them before you average.
Keep the reading sheet. Whoever designs or approves the pit will ask for it, and so will a plumber if the pit ever overflows.

What we would do

We'd dig two or three holes where the soak pit could go, soak them overnight, and take every reading the code asks for, then put the average rate into the soak pit calculator. If the rate comes out over 30 minutes, or the monsoon water level is within 1.8 m, we'd stop planning a pit and ask a local engineer about a trench or a filter instead.

Frequently asked questions

What is a percolation test for a soak pit?

It's a simple field test that measures how fast your soil absorbs water. You time the fall of water in a small hole, and the result, in minutes per 25 mm, decides whether a soak pit is allowed and how big it must be (IS 2470 Part 2, Appendix A).

How do I calculate the percolation rate?

Divide the reading time by the drop and multiply by 25. A 60 mm drop in 30 minutes is 30 × 25 ÷ 60 = 12.5 minutes per 25 mm. Use only the final reading.

What percolation rate is good for a soak pit?

Anything up to 30 minutes per 25 mm. Between 30 and 60 minutes IS 2470 allows a dispersion trench but not a pit, and over 60 minutes the soil can't take effluent at all.

How deep should the test hole be?

As deep as the soil that will absorb the effluent: the bottom of a trench, or the part of a soak pit below the inlet pipe. The hole itself is only 100 to 300 mm across.

Why soak the hole for 24 hours first?

Dry soil swallows water fast at first. The overnight soak lets the soil swell to roughly its wettest-season state, so the reading shows how the pit will behave in the monsoon, when it matters most.

Sources and date checked

This guide covers measuring how fast soil takes water, for planning. Where the pit or trench may go, how far it must sit from wells and buildings, and any approval belong to your local body and a licensed engineer. Never climb into a pit, septic tank or manhole; the gases inside can kill.

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