How do I calculate surface water runoff?

Surface water runoff is calculated using the Rational Method: Q = C × I × A, where Q is runoff rate (L/s), C is the runoff coefficient (1.0 for roofs, 0.9 for concrete), I is rainfall intensity (mm/hr) and A is catchment area (m²). The standard UK design rainfall is 75mm/hr for a 1-in-2-year storm.

How many gullies do I need?

As a rule of thumb, one road gully serves approximately 50m² of impermeable area. For linear drainage, gullies should be spaced at 20-30m intervals on roads and 10-15m on car parks. Each gully connects to the underground pipe via a trapped outlet to prevent odour.

What is Manning's equation for pipe flow?

Manning's equation calculates pipe flow velocity: V = (1/n) × R^(2/3) × S^(1/2), where n is the roughness coefficient (0.011 for uPVC), R is the hydraulic radius (pipe diameter/4 for full flow), and S is the pipe gradient. Flow rate Q = V × pipe cross-sectional area.

Can I connect surface water to the foul sewer?

In areas with combined sewers, yes. In areas with separate sewer systems, surface water must NOT be connected to the foul sewer — it is illegal under the Water Industry Act. Check with your water company which system serves your area. SuDS (soakaways, permeable paving) are preferred.

What pipe gradient do I need for surface water?

The minimum gradient for surface water drainage is 1:100 for 100mm pipe and 1:150 for 150mm pipe. Self-cleansing velocity of 0.75 m/s should be achieved. Steeper gradients increase capacity — at 1:40, a 110mm pipe carries roughly twice as much water as at 1:100.

TradeCalculator

Surface Water Drainage Calculator — Runoff, Pipe Capacity & Gullies

Calculate surface water runoff from impermeable areas, pipe capacity using Manning's equation, and gully count for domestic and commercial drainage.

Impermeable Area

Total catchment area to be drained

Surface Type

Pipe Size

Gully Spacing (optional)

Leave 0 for auto-calculation

Price Per Metre (optional)

Installed cost per metre of drainage

How We Calculate This

This calculator determines surface water drainage requirements using the Rational Method and Manning's equation for pipe sizing.

The formula

Runoff Q (L/s) = C × I × A / (3600 × 1000)

Pipe velocity V = (1/n) × (D/4)^(2/3) × S^(1/2)

Runoff coefficients

Roof: 1.0
Concrete/asphalt: 0.9
Block paving: 0.7
Gravel: 0.4

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.