Soakaway and SuDS sizing calculator.
Get a preliminary storage-volume estimate for a soakaway or SuDS attenuation system, following the BRE Digest 365 principle of storm runoff volume less infiltration during the storm. Enter each catchment area and surface type, pick a design storm and your soil's infiltration rate from a percolation test, and this calculator estimates the storage volume needed. This is a preliminary sizing estimate for early design and budgeting only — a real soakaway design needs a site percolation test to BRE365 and sign-off against your local authority's SuDS approval requirements before construction.
Drop an Excel or CSV file or to replace the table below.
Column A: catchment name. Column B: catchment area. Column C (optional): surface type — rows without a recognised surface type default to roof.
Parsed entirely in your browser — your file is never uploaded to our servers.
| Catchment | Catchment area (m²) | Surface type | Runoff volume (m³) | Row actions |
|---|---|---|---|---|
| — | ||||
| — | ||||
| — |
How we calculate this.
Get a preliminary storage-volume estimate for a soakaway or SuDS attenuation system, following the BRE Digest 365 principle of storm runoff volume less infiltration during the storm. Enter each catchment area and surface type, pick a design storm and your soil's infiltration rate from a percolation test, and this calculator estimates the storage volume needed. This is a preliminary sizing estimate for early design and budgeting only — a real soakaway design needs a site percolation test to BRE365 and sign-off against your local authority's SuDS approval requirements before construction.
For each catchment (row): runoff volume = area × runoff coefficient × design rainfall depth. Runoff volumes are summed across every catchment for the total. Outflow during the storm is estimated as infiltration rate × wetted area × storm duration, and the required storage volume is the total runoff less that outflow (never shown as less than zero).
| Assumption | Value | Source |
|---|---|---|
| Runoff volume | catchment area × runoff coefficient × design rainfall depth | Basic hydrology relationship: volume of runoff generated equals catchment area times rainfall depth, reduced by a runoff coefficient for the proportion of rainfall that actually runs off a given surface type. |
| Runoff coefficients | Roof 0.95, hard paving 0.85, permeable paving 0.30 | Standard published runoff coefficients by surface type, widely used in UK drainage design guidance including SuDS manuals. |
| Design storm rainfall depths | 10-25 mm depending on duration/return period | Illustrative representative UK rainfall depths for the stated storm durations and return periods, broadly consistent with published UK rainfall data ranges (e.g. FEH). A real design must use site-specific rainfall data for the actual location, not these illustrative figures. |
| Soil infiltration rate (f) | Sand/gravel 1×10⁻³, sandy loam 1×10⁻⁴, silty sand 1×10⁻⁵, clay <1×10⁻⁶ m/s | Representative infiltration-rate ranges by soil type, consistent with BRE Digest 365's stated minimum viable infiltration rate of 1×10⁻⁵ m/s for a soakaway. A real design requires a site percolation test, not an assumed value. |
| Outflow during the storm | infiltration rate × wetted area × storm duration | BRE Digest 365's storage-sizing principle: the storage volume needed is the runoff volume in, less the volume that infiltrates away through the soakaway's wetted surface during the storm itself. |
This is an early-appraisal planning estimate, not a quotation or a substitute for a surveyed takeoff. Last reviewed 1 Sep 2026.
BRE Digest 365 is the standard UK guidance for soakaway design. This calculator follows its core principle — storage volume needed = storm runoff in, less infiltration out during the storm — as a preliminary estimate, not a full BRE365 design, which requires a site percolation test and checking multiple storm durations for the worst case.
A percolation test on site, carried out to BRE365, gives a real infiltration rate (f). The soil-type options here are representative ranges for early estimating only — always confirm with a real percolation test before finalising a design.
A soakaway disposes of water by infiltrating it into the surrounding ground. A SuDS attenuation tank stores water temporarily and releases it slowly, usually to a sewer or watercourse at a controlled rate — used where the ground can't infiltrate enough water on its own.
Often yes, depending on your local authority's SuDS approval requirements and the scale of the development — check with your local planning authority before construction.
Yes — click or drop an Excel (.xlsx/.xls) or CSV file onto the upload area. The first column should be an item name and the second the measurement for that row; extra columns this calculator uses are read too if present. The file is parsed entirely in your browser — it's never uploaded to our servers. Download the blank template first if you're not sure of the layout.
Yes — use the metric/imperial switch above the calculator. Values you've already entered convert automatically; nothing is lost switching back and forth.
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