About This Tool This calculator determines the soil infiltration rate from trial pit soakaway test data in accordance with BRE Digest 365 (2016). It is designed for geotechnical engineers, drainage consultants, and anyone carrying out soakaway feasibility assessments as part of a ground investigation.
The BRE 365 method is the UK standard approach for determining whether a site is suitable for soakaway drainage. It uses the fall in water level between 75% and 25% of the effective depth to calculate the infiltration rate, avoiding the initial and final portions of the test where boundary effects may influence results.
How to Use Step 1: Enter the trial pit dimensions — length and width in metres.
Step 2: Enter the pit depth (total depth from ground surface) and the water level at the start of the test (depth to water surface in m bgl). The calculator derives the effective depth automatically.
Step 3: Choose an input mode. In Quick mode, enter the times at 75% and 25% effective depth directly. In Detailed mode, enter your falling-head readings (time and depth to water in m bgl) and the calculator will interpolate the 75% and 25% times for you. Note: BRE 365 requires three fill-drain cycles — calculate f for each and use the lowest value for design.
Step 4: The calculator will automatically compute the infiltration rate (f) in m/s and mm/hr, along with intermediate values and a suitability assessment.
The infiltration rate is calculated as:
f = Vp75-25 / (ap50 × tp75-25)
Where:
Effective depth = pit depth − water level (m). The maximum storage depth of water in the pit.
Vp75-25 = volume of water that drains between 75% and 25% of effective depth (m³), calculated as pit length × pit width × (depth75 − depth25).
ap50 = internal surface area of the pit to 50% of effective depth (m²), calculated as 2(L × d50) + 2(W × d50) + (L × W), where d50 = effective depth × 0.5.
tp75-25 = time for the water level to fall from 75% to 25% of effective depth (seconds).
Suitability Thresholds The calculated infiltration rate determines whether soakaway drainage is feasible at the site:
f > 1 × 10⁻⁵ m/s — Good. Soakaway drainage is feasible. The soil has sufficient permeability for standard soakaway design.
f > 1 × 10⁻⁶ m/s — Acceptable. Soakaway may be feasible but will require enhanced design (e.g. larger soakaway, additional storage). A drainage engineer should review the design.
f ≤ 1 × 10⁻⁶ m/s — Unsuitable. Soakaway drainage is unlikely to be feasible. Alternative drainage solutions (e.g. attenuated discharge to watercourse or sewer) should be considered.
Note: BRE 365 does not define specific suitability tiers. It states that soakaways should discharge from full to half-volume within 24 hours, and that infiltration rates around 10⁻⁷ m/s are likely to be unsuitable. The thresholds above are based on widely adopted industry practice and provide a useful rule of thumb for initial feasibility assessment.
Testing Notes The following guidance is based on BRE Digest 365 and common industry practice. Getting the basics right makes all the difference between results you can trust and ones that are not reliable.
Three fill-drain cycles. BRE 365 requires the pit to be filled and drained three times on the same or consecutive days. The first two cycles pre-wet the soil and allow it to reach a near-steady state. Calculate the infiltration rate for all three cycles and use the lowest f value for design.
Do not extrapolate from partial readings. If your readings do not cover the water level dropping from 75% to 25% of the effective depth, the test is incomplete. Extrapolating from partial data usually gives unrealistic results. This calculator will not produce a result unless the data spans the required range.
Minimum pit dimensions. Pits should be at least 0.3 m wide and 1.0 m long with square, stable sides. Consider gravel infill to maintain side stability if ground conditions are poor. Avoid pits that are too small or irregular in shape.
Desk study first. Carry out a detailed desk study before testing, including a review of geology, potential contamination, groundwater regime, and geotechnical hazards such as unstable ground, slope instability, and risk from dissolution features (especially in chalk).
Single stratum. Keep the test within one stratum, ideally within the layer with the best chance of infiltration. Testing across multiple soil types will give a blended result that does not represent any single layer.
Nearby structures. Consider nearby buildings and structures when positioning the test pit. Soakaways should not be placed where they could affect foundations or other infrastructure.
References BRE Digest 365 — Soakaway Design (2016 edition). Building Research Establishment. The primary reference for soakaway testing methodology in the UK.
BS 8582:2013 — Code of practice for surface water management for development sites. References BRE 365 for infiltration testing.
CIRIA C753 — The SuDS Manual (2015). Provides guidance on sustainable drainage systems including infiltration design using BRE 365 test data.
Revision history 8 September 2026: Initial release
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
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