The short version
Dig a trial pit 0.3 to 1 m wide and 1 to 3 m long, with vertical sides, where the soakaway will go and to the depth it will reach. Fill it quickly to its effective depth, record the level as it falls and time the fall from 75% to 25% of that depth. Let it drain, then fill it again — three fills, on the same day or consecutive days. Work out the infiltration rate for each fill and design on the lowest.
What you need
- A digger for the pit, and a way to trim its sides vertical.
- Water, and plenty of it. One pit-full per fill: a 2.4 × 0.6 m pit filled to 1.5 m holds 2.16 m³, about 2,160 litres, so three fills take about 6,480 litres. A bowser is the usual way — the pit has to be filled quickly.
- A tape and a staff or batten marked at the 75%, 50% and 25% levels of the effective depth.
- A stopwatch and something to write on. SoakMonkey runs on a phone at the pit, works offline, and its stopwatch writes each time straight into the test.
- A camera. Photos of the pit with the tape in it, dated, are part of the record.
Step by step
- Pick the spot. Test where the soakaway will go and at the depth it will reach. Keep the soakaway position at least 5 m from any building or road, out of made ground, and know where the services run before you dig.
- Dig the trial pit. A pit 0.3 to 1 m wide and 1 to 3 m long, with vertical sides trimmed square, dug down to the base of the proposed soakaway. If the sides will not stand, fill the pit with clean granular material around a perforated standpipe you can read the level in, and say so on the record.
- Measure it and mark the levels. Measure the length, width and effective depth: from the base up to where the inlet pipe will enter. Mark the 75%, 50% and 25% levels on a staff or batten in the pit.
- Fill it quickly to the effective depth. A bowser is the usual way: the fill has to be fast, so the clock starts on a full pit rather than one still being topped up.
- Record the level as it falls. Note the water level against time at regular intervals, and time the fall from the 75% mark to the 25% mark. Let it drain below 25% before the next fill.
- Repeat: three fills. Three fills, in quick succession on the same day or on consecutive days. The first fill wets the ground and flatters the rate; the rate usually falls with each fill.
- Work out the rate for each fill. f = V(75–25) ÷ (a(p50) × t(75–25)): the water drained between the 75% and 25% levels, divided by the pit’s sides up to half depth plus its base, and by the time in seconds.
- Design on the lowest rate. Use the slowest fill’s rate, never the average of the three, and write the test up: where, when, how deep, what soil, whether groundwater showed, and every reading.
The pit is the test, so its geometry matters as much as the times: the calculation uses the length, width and effective depth, not just the minutes. Write them down before the first fill.
Working out the infiltration rate
For each fill:
f = Vp75–25 ÷ ( ap50 × tp75–25 )
- Vp75–25 is the water drained between the 75% and 25% levels: length × width × half the effective depth.
- ap50 is the pit’s internal surface area up to half the effective depth including the base: (length + width) × effective depth + length × width. (The soakaway’s own design leaves its base out, because it silts up; the trial pit’s base is new ground and counts.)
- tp75–25 is the time for the fall from 75% to 25%, in seconds.
Worked example. A pit 2.4 m long and 0.6 m wide, effective depth 1.5 m. The water drained between 75% and 25% is 2.4 × 0.6 × 0.75 = 1.080 m³. The area to half depth, base included, is (2.4 + 0.6) × 1.5 + 2.4 × 0.6 = 5.94 m².
| Fill | 75% → 25% time | f = V ÷ (ap50 × t) |
|---|---|---|
| 1 | 62 min (3,720 s) | 4.89 × 10⁻⁵ m/s |
| 2 | 75.5 min (4,530 s) | 4.01 × 10⁻⁵ m/s |
| 3 | 96 min (5,760 s) | 3.16 × 10⁻⁵ m/s — lowest, used for design |
The design rate is the third fill’s, 3.16 × 10⁻⁵ m/s — the rate fell with each fill as the ground wetted up, which is normal. Open this example in SoakMonkey to see the test record it makes, or use the percolation test calculator with your own times.
What the number means
| Infiltration rate f | What to expect |
|---|---|
| Above about 1 × 10⁻³ m/s | Unusually fast for a soakaway test — check the times and the pit size. |
| 1 × 10⁻⁵ to 1 × 10⁻³ m/s | Workable: a sensibly sized soakaway half-empties within the 24 hours BRE 365 allows. |
| 1 × 10⁻⁶ to 1 × 10⁻⁵ m/s | A soakaway can work but grows quickly, and the 24-hour half-empty check starts to govern. In Wales, the SuDS standards say soakaways usually need about 1 × 10⁻⁵ m/s or more. |
| Below about 1 × 10⁻⁶ m/s | Too slow for a soakaway on its own: in England the National Standards for SuDS say infiltration should not be relied on to drain all the run-off below this rate. Look at another destination. |
A soil type is not a substitute: one lead local flood authority lists an infiltration rate assumed from the soil type among the common mistakes it sees. It has to be measured, on the site, at depth.
If the pit will not drain
A fill that never falls to 25% has not given an infiltration rate. Some authorities suggest letting it drain further still — below 10% — and many will not accept a rate extrapolated from a partial fall, because extrapolating flatters slow ground. Leave it, record that it did not drain, and take it as an answer: the ground is probably too tight for a soakaway, and the surface water needs another route.
When to test — the season matters
Ground that has been dry for months drinks the first fills and gives a rate the soakaway will not see in a wet winter. Several lead local flood authorities ask for infiltration tests in the winter months (one names January and February), and the National Standards for SuDS want groundwater levels confirmed during a high-groundwater period. Council guidance also warns against testing in abnormal weather: heavy rain, a hard frost or a drought.
If you have to test in a dry spell, say so on the record; some authorities accept a test outside the winter months only by prior agreement. We have written about BRE 365 testing in drought conditions and testing while groundwater is low.
What goes in the test record
The times on their own are not a test record. What councils and LLFAs ask to see:
- Where: the pit’s position on a site plan, at the proposed soakaway.
- The pit: length, width, total depth and the effective depth tested.
- The ground: what it was dug in, layer by layer, and whether groundwater was met, and at what depth.
- When: the date and time of each fill, and the weather.
- The readings: water level against time for every fill, the 75%–25% time, and the calculation.
- Photos of the pit with a tape or staff in it, dated.
SoakMonkey’s free test record takes the pit, the fills and those details and lays them out as a one-page PDF with the rate worked out, checks on the fills, and a link that reopens the test to size the soakaway. Attach your level readings and photos to it. The paid soakaway report then carries the same record.
Mistakes that get a test rejected
- Testing away from the soakaway, or at a different depth from its base.
- A pit not filled to the effective depth, or filled so slowly the timing starts on a half-empty pit.
- One fill instead of three, or the average of three instead of the lowest.
- A rate extrapolated from a fill that never reached 25%.
- A rate assumed from the soil type rather than measured.
- Testing in made ground — fill and rubble do not behave like the ground beneath.
- A small auger hole or the Part H 300 mm hole where the request was for BRE 365.
- No record of the pit size: without it the times cannot be turned into a rate.
BRE 365 test or the Part H percolation test?
They are different tests. Approved Document H’s percolation test, for a septic-tank drainage field, uses a 300 mm square hole and a 150 mm fall timed in seconds, giving a percolation value Vp in seconds per millimetre. The BRE 365 soakage test uses a full-size trial pit and gives an infiltration rate in metres per second. Approved Document H does let the small-hole test stand in for a small soakaway, but where a planning condition, an LLFA or Building Control asks for BRE 365, it is the trial pit. The percolation test page sets out the difference.
From the test to the soakaway
The rate is the one number the design cannot do without. BRE Digest 365 then sizes the soakaway against the design storm across a range of durations and checks it half-empties within 24 hours — SoakMonkey does that in one go, free on screen, and the report for Building Control is £25 +VAT. If you are not sure what size you are heading for, start with what size soakaway do I need.
Frequently asked questions
How do you do a BRE 365 soakaway test?
Dig a trial pit 0.3 to 1 m wide and 1 to 3 m long, with vertical sides, where the soakaway will go and to the depth it will reach. Fill it quickly to its effective depth, record the level as it falls and time the fall from 75% to 25% of that depth. Let it drain, then repeat: three fills, on the same day or consecutive days. Work out the infiltration rate f = V(75–25) ÷ (a(p50) × t(75–25)) for each fill and design on the lowest.
Is BRE365 the same as BRE 365?
Yes. BRE365, BRE 365 and BRE Digest 365 are the same document: BRE Digest 365, Soakaway design, first published in 1991 and revised in 2016. Its soakage test is the trial-pit test on this page.
How much water do I need for a soakaway test?
One pit full per fill: length × width × effective depth. A 2.4 × 0.6 m pit filled to 1.5 m holds 2.16 m³ — about 2,160 litres — so three fills need about 6,480 litres. That is why a bowser is the usual way to do it: a garden hose cannot fill a pit quickly enough.
How long does a BRE 365 soakaway test take?
As long as the ground takes. Each fill has to drain below 25% before the next, and there are three of them, on the same day or consecutive days. In free-draining gravel that can be a morning; in slower ground it runs over consecutive days.
Can I average the three results?
No. Design on the lowest infiltration rate, which is normally the last fill. Guidance from lead local flood authorities says this in terms and tells you not to average.
What if the water does not drain away?
If the level never falls to 25%, the fill has not given an infiltration rate, and many authorities will not accept a rate extrapolated from a partial fall. It usually means the ground is too tight for a soakaway — below about 1 × 10⁻⁶ m/s a soakaway is rarely practical — and the drainage has to go somewhere else, such as attenuation with a controlled discharge.
What time of year should I do the test?
Winter if you can. Several lead local flood authorities ask for infiltration tests in the winter months, when groundwater is high, and the National Standards for SuDS want groundwater confirmed in a high-groundwater period. A test done in a drought, a hard frost or straight after heavy rain is not representative.
Is the BRE 365 test the same as a percolation test?
It is often called one, but it is not the Approved Document H percolation test for a septic-tank drainage field. That one uses a 300 mm square hole and gives a percolation value Vp in seconds per millimetre. The BRE 365 test uses a full-size trial pit and gives an infiltration rate in metres per second, which is what a soakaway is sized from.
Do I need a professional to do it?
On a house extension the builder or groundworker often digs and times it; on larger developments it is usually part of the site investigation. Either way, what Building Control or the lead local flood authority checks is the record: where and how deep, the pit size, the soil, groundwater, every reading, and the calculation.
Sources. BRE Digest 365, Soakaway design (BRE, 2016 edition), described here in our own words; Approved Document H (drainage and waste disposal); Defra, National Standards for Sustainable Drainage Systems (2025), Standard 3; Welsh Government, Statutory standards for sustainable drainage systems (2018), G1.15; and infiltration-testing guidance published by lead local flood authorities and councils, among them Hampshire, West Sussex, Arun, North Yorkshire, Windsor and Maidenhead, and West Devon.