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Rational Method Stormwater Runoff Calculator

Peak stormwater runoff Q = C·i·A for a site from area, runoff coefficient by surface type and design rainfall intensity — composite C for mixed surfaces, flow in cfs and L/s, the pipe or culvert size to carry it and the volume of a design storm.

Roofs and paving (C ≈ 0.90)

Gravel / compacted surfaces (C ≈ 0.60)

Lawns and landscaping (C ≈ 0.25)

Woods / undeveloped (C ≈ 0.15)

Design rainfall intensity (from the local IDF curve for the time of concentration)

Time of concentration / storm duration

Outlet pipe slope

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How it works

Q (cfs) = C × i (in/h) × A (acres); composite C = Σ(Cᵢ Aᵢ) ÷ ΣA; storm volume = Q × duration; full-pipe capacity (Manning): Q = 0.463/n × D^(8/3) × S^(1/2)

The rational method is the workhorse for small drainage areas (under about 200 acres): peak flow equals a runoff coefficient — the share of rain that runs off rather than soaking in — times the rainfall intensity times the area, and with US units of inches per hour and acres the answer comes out almost exactly in cubic feet per second. The coefficient is weighted by surface: pavement and roofs shed nearly everything, lawns a quarter or less. Intensity comes from the local intensity–duration–frequency curve at a duration equal to the site’s time of concentration — the time for water from the far corner to reach the outlet — because a shorter, more intense burst does not have time to arrive all at once. Sizing an outlet pipe with Manning’s equation and comparing post- with pre-development flow shows how much detention a permit is likely to demand.

Worked example

A 3-acre site with 1.2 acres of roof and paving, 0.3 of gravel and 1.5 of lawn has a composite C of 0.55. In a 15-minute, 4.5 in/h design storm the peak is 7.4 cfs (208 L/s) and about 6,600 ft³ runs off; an 18″ concrete pipe at 1% carries it (a 15″ falls just short). Pre-development the same storm produced 2.7 cfs, so roughly 4.7 cfs must be detained.

Frequently asked questions

Where do I get the intensity?

From NOAA Atlas 14 (US) or the equivalent national IDF data, for the return period the code specifies (often 10-year for pipes, 100-year for overflow) at the time of concentration.

Why not use it for large watersheds?

It assumes uniform rainfall over the whole area for the full time of concentration and ignores storage in channels; for anything beyond ~100–200 acres use NRCS TR-55 or a hydrograph model.

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