Skip to content
Septic & DrainageCalculators

Drainage

Rainfall Runoff Calculator

Work out how much water a surface sheds in a given rainfall. Enter the catchment area, the depth of rain and a runoff coefficient for the surface, and the result comes back in gallons, cubic feet and cubic meters.

Your measurements

Results update as you type. Every unit dropdown converts for you.

Use the flat footprint of the surface, not the sloped surface area.

Presets fill the runoff coefficient below. You can overwrite it.

The share of rainfall that runs off rather than soaking in or evaporating. 1.0 is total runoff.

Runoff volume

887.8gallons

Volume in cubic feet
118.7 cu ft
Volume in cubic meters
3.36 m³
Volume in liters
3,361 L
Rainfall used
1.00 in
Coefficient used
0.95
Show the calculationOpen
  1. Runoff volume1,500 sq ft × 1.00 in × 0.623 × 0.95=887.8 gal
  2. Convert to cubic feet887.8 gal ÷ 7.48052=118.7 cu ft
  3. Convert to cubic meters118.7 cu ft × 0.0283168=3.36 m³

Reading your result

The gallon figure is the total water shed by that surface for the whole rainfall depth you entered — an event total, not a rate.

Compare it against whatever you are sizing. A rain barrel measured in tens of gallons will overflow in almost any real storm; a dry well is sized in cubic feet, which is the row below.

If you are looking at several surfaces draining to one point, run each and add them rather than estimating a combined coefficient.

How this calculator works

One inch of rain on one square foot is about 0.623 gallons. Multiply that by your area and your rainfall depth and you have the water that lands on the surface.

The runoff coefficient is the share of that water which actually runs off rather than soaking in, evaporating or clinging to the surface. A metal roof sheds nearly everything, so its coefficient is close to 1. A lawn on sandy soil absorbs most of what falls on it.

Use the flat footprint of the surface, not its sloped area. A pitched roof catches rain according to its plan area, because rain falls vertically.

The result is total volume for the whole rainfall event, not a flow rate. Sizing a pipe or a channel needs an intensity figure — inches per hour — which is a different calculation.

The formula

Runoff in gallons
area (sq ft) × rainfall (in) × 0.623 × coefficient
Cubic feet
gallons ÷ 7.48052
Cubic meters
cubic feet × 0.0283168

Worked example

A 1,500 sq ft roof footprint in one inch of rain, at a coefficient of 0.95.

  1. 01Runoff1,500 × 1 × 0.623 × 0.95=888 gal
  2. 02Cubic feet888 ÷ 7.48052=118.7 cu ft
  3. 03Cubic meters118.7 × 0.0283168=3.36 m³

One inch of rain sheds about 888 gallons off that roof — which is why downspout placement matters so much near a foundation.

How to take the measurements

  • Measure the roof footprint at ground level, or take the building's outside dimensions and add the eave overhang.
  • For a roof drained by several downspouts, divide the footprint between them to size each discharge point.
  • Use rainfall depth for the storm you care about. Local weather records give both typical and extreme figures.
  • Pick a coefficient that matches the surface, and lean higher when soil is already saturated — wet ground behaves much more like pavement.
  • Add up all surfaces draining to the same point rather than calculating them separately.

Practical considerations

  • Runoff volume is what decides whether storage overflows. A rain barrel holds a small fraction of what a roof sheds in a single storm.
  • Where the water goes matters as much as how much of it there is. Discharging onto adjoining property can create a dispute and may not be permitted.
  • Downspout extensions are the cheapest fix for water against a foundation, and are often enough on their own before anything gets excavated.
  • Consider frequent storms as well as extreme ones. A system sized only for a rare event can still leave you wet most weeks.

Assumptions and limitations

  • This gives volume for a rainfall depth, not a flow rate. Sizing a pipe or channel needs intensity in inches per hour, which is a different calculation.
  • The runoff coefficient is a simplification of a complex process. It varies with slope, soil, antecedent moisture and storm intensity.
  • Where the water goes afterwards — infiltration capacity, discharge points, and whether directing runoff onto adjoining property is permitted — is outside the calculation.

Frequently asked questions

How many gallons does an inch of rain produce?

About 0.623 gallons per square foot. A 1,000 sq ft surface sheds roughly 623 gallons per inch of rain before any runoff coefficient is applied.

Do I use roof surface area or footprint?

Footprint. Rain falls vertically, so a steep roof catches the same volume as a flat roof with the same plan area.

What coefficient should I use for a lawn?

It varies with soil and slope, and rises sharply once the ground is saturated. The presets give a starting range; adjust based on how your yard actually behaves in a storm.

Can I use this to size a drain pipe?

Not directly. Pipe sizing works from flow rate — volume per unit time — while this gives total volume for a rainfall depth. It is useful for sizing storage, such as a rain barrel or dry well.

Do I use roof surface area or footprint?

Footprint. Rain falls vertically, so pitch does not increase the volume a roof catches.

Can I use this to size a drain pipe?

Not directly. Pipe sizing works from flow rate, while this gives total volume for a rainfall depth. It is well suited to sizing storage such as a rain barrel or dry well.

What coefficient should I use for a lawn?

It varies with soil and slope and climbs when the ground is already wet. The presets give a starting range; adjust based on how your yard actually behaves in a storm.

Related calculators