Roof Rainwater Runoff Calculator
Last reviewed How we calculate
A roof collects the rain that falls on it and sends it to a handful of downspouts. This works out how much water that is, how much of it you could realistically collect in a tank or barrel, and the peak flow your gutters have to deal with in a heavy storm.
Rainwater runoff from your roof
How much rain a roof sheds, how much of it you could collect, and the peak flow your gutters have to carry.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Water you could collect
- Total that falls on the roof
- Peak flow off the roof What the gutters and downspouts have to carry in the heaviest rain.
One inch of rain on one square foot of roof is about 0.62 US gallons, so even a modest roof sheds a lot of water. Whether you may store it, and what you may use it for, depends on local rules, and water collected from a roof is not safe to drink without treatment.
How the calculation works
Multiply the area the roof covers by the depth of rain and you have a volume. One inch of rain on one square foot is 144 cubic inches, which is about 0.62 US gallons. The collection efficiency takes off what splashes, evaporates or is diverted to clear the first dirty water. The peak flow uses the same sum at your heaviest rainfall rate, per minute, with nothing taken off.
A worked example
A roof covering 1,500 sq ft receives about 935 gallons from one inch of rain. At 80 per cent efficiency about 748 gallons could be collected, which would fill thirteen 55-gallon barrels. In a storm dropping 2 inches an hour, water comes off that roof at about 31 gallons a minute.
Why pitch does not matter
Rain falls more or less vertically, so a roof catches the rain that would otherwise have fallen on the ground beneath it. That is why this calculator asks for the plan area, the footprint the roof covers, and not the larger sloped area used for ordering materials.
Before you store it
Rules on collecting rainwater vary from place to place, so check what applies where you live. Roof runoff picks up dirt, bird droppings and whatever the roofing material sheds, and it is not safe to drink without proper treatment. For watering a garden it is ideal.
Frequently asked questions
How much water comes off a roof in one inch of rain?
About 0.62 US gallons for every square foot of roof plan area. A roof covering 1,000 square feet sheds about 623 gallons in an inch of rain.
Should I use the sloped roof area or the footprint?
The footprint. Rain falls more or less vertically, so a roof catches what would have fallen on the ground beneath it, however steep it is.
Is rainwater from a roof safe to drink?
Not without proper treatment. Roof runoff picks up dirt, bird droppings and whatever the roofing material sheds. For watering a garden it is ideal.