BRE 365 Infiltration Rate Calculator
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The BRE 365 infiltration rate calculator below works out the soil infiltration rate (f) of a soakaway trial pit to BRE Digest 365, the standard UK method for surface water soakaway design. Enter the pit dimensions and the time the water took to fall from 75% to 25% of the test depth, for up to three soakage tests, and the tool returns the design infiltration rate, which BRE 365 takes as the lowest f of the tests completed, in both m/s and mm/hr, ready to carry into a drainage strategy.
It is free to use, nothing you type is stored, and you can download a PDF report of the result. Results are indicative and should always be confirmed by a qualified engineer against current guidance.
BRE 365 Infiltration Rate Calculator
Calculate the soil infiltration rate (f) of a soakage trial pit to BRE Digest 365 — from the time the water level takes to fall from 75% to 25% of the effective test depth. Enter up to three tests; the design rate is the lowest of the three, as the Digest requires.
1 · Trial pit dimensions
Measure the pit carefully before testing. Sides should be vertical and trimmed square.
2 · Report details — optional, used in the PDF
These appear in the header of the downloadable PDF report. Leave blank to omit.
3 · Soakage test timings
Fill the pit and let it drain three times. For each test, record the elapsed time (from the moment filling stops) at which the water level passes the 75% and 25% marks.
| Test | Time to 75% level (min) | Time to 25% level (min) |
|---|---|---|
| Test 1 | ||
| Test 2 | ||
| Test 3 |
Enter at least one test. The design infiltration rate is taken as the lowest f of the tests completed (slowest drainage = worst case).
One of our experienced Flood Risk Consultants will get back to you within 60 minutes
What the BRE 365 infiltration rate calculator does
It turns raw soakage test timings into a design infiltration rate (f), the single figure that drives how big a soakaway needs to be. You enter each trial pit's length, width and effective water depth, then the elapsed time at which the level passed the 75% and 25% marks. The calculator works out f for every test, takes the lowest (slowest-draining) value as the design rate, and gives you a results table plus a downloadable PDF report.
Because the worst-performing test governs the design, you can enter one, two or three tests and the tool will always report the lowest f of those completed. That matches how BRE Digest 365 expects the result to be read.
How the infiltration rate (f) is calculated
BRE Digest 365 defines the soil infiltration rate as the volume of water lost through the pit, divided by the soakage surface area and the time taken. The calculator applies the Digest's equation exactly:
f = Vp ÷ (ap50 × tp 75–25)
- Vp is the volume of water stored between the 75% and 25% levels, which equals half the effective depth multiplied by the plan area: ½ × length × width × effective depth.
- ap50 is the internal surface area through which water escapes, taken to 50% of the effective depth and including the base: (length + width) × effective depth + (length × width).
- tp 75–25 is the time, in seconds, for the water level to fall from 75% to 25% of the effective depth.
The result is an infiltration rate in metres per second, typically a small number such as 2.1 × 10⁻⁵ m/s. The calculator also converts it to mm/hr, which is often easier to sense-check.
How to carry out a BRE 365 soakage test
Excavate a trial pit at the proposed soakaway location, usually between 1 and 2.5 metres deep, with vertical, square-trimmed sides. Fill it rapidly with water and record the time, from the moment filling stops, at which the level passes the 75% and 25% points of the effective test depth. Then refill and repeat, so that three separate fill-and-drain runs are timed in the same pit.
Run the tests at the position and depth the soakaway will actually occupy, keep the structure at least five metres from buildings, and make sure its base sits at least one metre above the highest expected groundwater level. If water enters the excavation during the works, record the resting groundwater level; a high water table can rule out infiltration altogether.
Why the design rate is the lowest of the three tests
BRE Digest 365 asks for at least three tests because a single reading can flatter the ground. The first fill often drains faster than later ones as fines wash into the soil pores and any swelling clays take up water, so the slowest run usually represents the long-term, in-service condition. Taking the lowest f as the design rate is the conservative choice, and it is the value an engineer and a Lead Local Flood Authority will expect to see used in the soakaway calculation.
What counts as a workable infiltration rate
As a rough guide, soakaways become impractical once the infiltration rate falls below about 1 × 10⁻⁶ m/s; below that the ground drains too slowly to empty a soakaway between storms. Faster rates, in the order of 10⁻⁴ to 10⁻⁵ m/s, generally support a sensibly sized soakaway. These are guides, not pass/fail thresholds: a low rate does not automatically rule out infiltration, but it does push the design towards a larger structure, shallower spread systems, or an alternative discharge route. Whether infiltration is viable at all also depends on groundwater levels and the separation distances above.
BRE 365 or BS 6297: which test do you need?
BRE Digest 365 is the method for surface water soakaways, such as roof and hardstanding runoff. If you are designing a drainage field for foul effluent from a septic tank or package treatment plant, the relevant standard is BS 6297, which uses a different percolation test and an acceptable range of 15–100 seconds per millimetre. The two are not interchangeable. For the foul drainage case, see Unda's explainer on BS 6297 infiltration testing for foul drainage fields.
From infiltration rate to soakaway design
The infiltration rate is an input, not the finished design. Once you have a defensible f value, it feeds the soakaway sizing alongside the rainfall design event, the impermeable area being drained and the discharge hierarchy set out in the 2025 National Standards for SuDS. If you want the background to the test itself, our explainer on BRE365 infiltration testing and soakaway design covers why it matters for planning, and do you need infiltration testing? helps you decide whether your site requires it at all.
The method itself comes from BRE Digest 365, Soakaway design (2016), and soakaway construction also has to satisfy Approved Document H of the Building Regulations.
At least three runs in the trial pit. The calculator accepts up to three and uses the lowest infiltration rate as the design value.
The BRE 365 infiltration rate (f) is a speed, in m/s, for surface water soakaways. A percolation rate, used in BS 6297 for foul drainage fields, is a time in seconds per millimetre. They measure related things but are not the same number and are not interchangeable.
Most sites relying on a soakaway or other infiltration drainage will need it to satisfy the Lead Local Flood Authority. Our guide to whether you need infiltration testing sets out when it is required.
A very low infiltration rate or a high water table usually means a conventional soakaway will not work, and you will need a different surface water solution. Record the groundwater level in the test and speak to a drainage engineer about the alternatives.
Turn your test result into an approved drainage strategy
A defensible infiltration rate is the start of the conversation with a Lead Local Flood Authority, not the end of it. Unda prepares surface water drainage strategies and SuDS designs that take your BRE 365 result through to a scheme planners will accept. Get in touch to discuss your site.