Surface Water Runoff Calculator

Runoff risk and SuDS-aware drainage recommendations.

All calculators

Inputs

Surface

Project location

Length(m)

Width(m)

4 advanced inputs hidden. Switch to Advanced to fine-tune.

Confidencehigh· 86Complexitylow· 16

Catchment area: 15m²
Perimeter m: 16m
Peak runoff (l/hr): 712
Peak runoff (m³/hr): 0.71
Runoff coefficient (C): 0.95

Heads-up: Where the water goes matters more than how much. Discharging onto a neighbour, the pavement, or the highway is an offence — design the runoff to a legal point on your own land. Why it matters: Part H of the Building Regs covers surface water; councils can require you to dig it up if you skip it.

This app provides general UK guidance and material estimates only. It is not legal, planning, engineering or building-control advice. Always confirm requirements with your local planning authority, building control, utility providers, manufacturers or qualified professionals.