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Calculator · Roof water

How many gallons does a roof shed for a given amount of rain?

Enter a roof area and a rainfall depth to see the volume of water in US gallons, with a diagram alongside the result.

Volume only. It doesn't say how quickly the water arrives or where it goes.

DRAINAGE CALCULATOR

Roof runoff volume

How much water falls on a roof footprint in a given amount of rain?

SEE THE VOLUME

A roof footprint collecting rainfall
A roof footprint collecting rainfall. 1,000 sq ft footprint, 1 inch of rainfall, 100% becomes runoff.RUNOFFSHARE100%
  • 1,000 sq ft footprint
  • 1 inch of rainfall
  • 100% becomes runoff

Illustration not drawn to scale. The blue bar shows the selected share becoming runoff, not a storage tank or flow rate.

Showing the starting values. Change the inputs and calculate to update the picture.

ESTIMATED VOLUME

Enter values to calculate.

Calculated locally in your browser. Your measurements are not sent.

The calculation.

Gallons = horizontal roof area × (rainfall inches ÷ 12) × runoff factor × 7.48051948.

A worked example.

1,000 sq ft × (1 inch ÷ 12) × 1 × 7.48051948 ≈ 623.4 gallons.

What the number means.

Use the horizontal roof footprint, not the sloping roof surface. A factor of 1 assumes all rainfall becomes runoff; it is an upper-volume example. This is a volume, not a flow rate or a local design-storm recommendation.

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