Surface water interception calculator

Turn measured catchment areas and rainfall depths into volumes, then make a transparent provision comparison.

Updated 4 November 2026

Scope: this calculator works out rainfall volume from values you enter and compares allocated provision with one volume basis you choose. It does not size a drainage system or decide whether a design meets a standard.

Sustainable drainage systems (SuDS) are drainage features that manage runoff close to where rain falls. The calculator does not model infiltration, evaporation, reuse demand, antecedent wetness, drainage routing or how often an interception feature performs.

1. Enter catchments

Give each contributing area one row. Use the net area draining to the feature, without counting the same surface twice.

Catchment name	Area (m²)	Rainfall depth (mm)	Provision allocated (m³)	Remove row
Catchment name	Catchment area in square metres	Rainfall depth in millimetres	Interception provision allocated to this catchment in cubic metres

Area and rainfall depth are required for every row. Enter a provision allocation for each row, including 0 where none is allocated. Add each shared provision volume once only, then split it between catchments using your recorded project basis. You can enter up to 20 catchments.

2. Choose the comparison volume basis

Choose the rainfall depth used to calculate the volume that your proposed provision is compared with. This is a single event volume comparison.

Use each catchment's entered rainfall depthThe comparison basis uses the rainfall depth entered in each catchment row. Use one common comparison depthEnter the same depth for every catchment below. If you are illustrating the first 5 mm in England's Standard 2, enter 5 mm yourself.

Common comparison depth (mm)

For England, the 2025 non-statutory national standards describe the SuDS approach for planning decisions. Clause 2.1 says the approach should prevent at least the first 5 mm from running off the site for the majority of rainfall events. Clause 2.5 sets 80% summer and 50% winter interception performance. Clause 2.6 requires evidence for each positively drained surface that at least 5 mm is collected for use, infiltrated, or captured, conveyed and stored in SuDS features where runoff is naturally absorbed or retained and lost to soils or the atmosphere without discharging off site. Clause 2.7 lists methods deemed compliant for zero runoff from the first 5 mm for those shares of events. These are event proportions, not volume multipliers. A single event volume balance cannot demonstrate the seasonal outcomes.

Read the detailed England SuDS standards, including Standard 2. The standards apply in England. Check the applicable requirements and approving body for your project and jurisdiction.

Volume comparison

Catchment	Area (m²)	Entered rainfall (mm)	Rainfall volume (L)	Rainfall volume (m³)	Comparison depth (mm)	Comparison basis (m³)	Provision allocated (m³)	Difference (m³)

A positive difference means the entered allocation exceeds the chosen basis volume arithmetically. The total nets the rows against one another, so a surplus in one catchment can offset a shortfall in another in that total. A negative row still has its own shortfall and is not cancelled as a design assessment by a surplus elsewhere. Row differences are rounded individually; the total difference uses unrounded totals and is rounded once, so it can differ from the sum of displayed row differences by up to 10 mL across 20 catchments. This is not a pass, design approval or evidence of interception performance. Volume retained temporarily before later discharge is not, by itself, proof of Standard 2 interception.

Displayed and exported volume values are rounded to the nearest millilitre (0.001 L or 0.000001 m³).

Worked example: two catchments at a 5 mm depth

For illustration only, an 800 m² roof at 5 mm gives 4,000 litres (4 m³). A 1,200 m² paved area at the same depth gives 6,000 litres (6 m³). Together, the event volume is 10,000 litres (10 m³). If a project records 4.5 m³ for the roof and 5.5 m³ for the paved area, the roof row is +0.5 m³, the paved-area row is −0.5 m³, and the arithmetic total is zero. The total nets the rows; it does not remove either surface's individual comparison.

The example is not a project default, design recommendation or compliance finding. The calculation treats each catchment area and allocated provision once. It does not test the seasonal event proportions in England's Standard 2.

How the calculation works

Rainfall volume in litres = catchment area in m² × rainfall depth in mm.
Rainfall volume in m³ = area × depth ÷ 1,000.
Provision allocation is added once for each catchment. The site total is the sum of the row allocations.
Difference = provision allocated − selected comparison volume. A negative value is a shortfall against that arithmetic basis; it is not a design verdict.

No data is saved by this page. Export a CSV if you want to keep the current values. Keep the source measurements, drainage routing and allocation basis with the project record.

Related guidance and tools

SuDS compliance guide Percolation test and soakaway sizer Warehouse SuDS and drainage

Surface water interception calculator

1. Enter catchments

2. Choose the comparison volume basis

Volume comparison

Worked example: two catchments at a 5 mm depth

How the calculation works

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