# Surface water interception calculator

> Convert measured areas and rainfall depths into rainfall volumes, then compare allocated interception provision with a chosen event basis.

- Canonical HTML: https://rospower.co.uk/resources/surface-water-interception-calculator/
- Documentation index: https://rospower.co.uk/llms.txt
- Updated: 2026-11-04

> This Markdown version contains the published explanatory content. Use the canonical HTML page for calculators, builders and other interactive controls.

## Page content

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

**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.

## 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](https://rospower.co.uk/resources/suds-compliance/)
- [Percolation test and soakaway sizer](https://rospower.co.uk/resources/percolation-soakaway-sizer/)
- [Warehouse SuDS and drainage](https://rospower.co.uk/blog/warehouse-suds-drainage/)