# How to Calculate Roof Runoff

Contents

## How do you calculate a runoff rate?

For a given surface area such as a roof or yard, multiply the area by the inches of rainfall and divide by 231 to obtain the runoff in gallons.

## How do I calculate how much water is coming off my roof?

**How Much Rain Water Runs Off Your Roof?**

- Take the dimensions of the footprint of your roof and convert them to inches. (So, a 50′ x 20′ roof is 600" x 240".)
- Multiply the roof dimensions by the number of inches of rainfall. (In this example, 600" x 240" x 1" = 144,000 cubic inches of water.)

## How much rain water comes off a roof?

Every square foot of roof space collects . 6 gallons of water in a 1 inch rainfall.

## How do I calculate how much rain water I will collect?

To calculate how much rainwater can be harvested, multiply your rainfall (mm) by your roof surface area (m2) being used to catch rainwater. The resulting number represents how many litres of water you can expect to collect.

## How do you calculate stormwater runoff volume?

Therefore, to assure that the first flush is captured and treated, the easiest method to determine the stormwater treatment volume is simply to multiply the project size or contributing drainage area times the treatment volume.

## What is total runoff?

The total runoff is equal to the total precipitation less the losses caused by evapotranspiration (loss to the atmosphere from soil surfaces and plant leaves), storage (as in temporary ponds), and other such abstractions.

## How many gallons are in an inch of rain per square foot?

An inch of rainfall on a square foot of surface area yields . 623 gallons.

## How much is 1 inch of rain in gallons?

**6 gallons**

Per the USGS Rainfall Calculator, one inch of rainfall equals 6 gallons of water per square yard or 27,154 gallons of water per acre!

## How do you calculate catchment area?

Catchment areas can be calculated by simple buffer zones, walking or driving time to the location, and even mobility pattern data, painting a vivid picture of where your customers visit your business from.

## How do you calculate rain per square foot?

**Amount of Rain Falling on Your Property**

- 1 square foot = 144 square inches (12 x 12)
- 1 acre = 43,560 square feet or 6,272,640 square inches (43,560 x 144)
- 1 inch of rain falling on 1 square foot = 144 cubic inches (12 x 12 x 1)

## How is rainwater downpipe calculated?

To calculate the minimum number of downpipes, divide the roof catchment area by the allowable maximum catchment per downpipe. To calculate the average catchment per downpipe, divide the roof catchment area by the number of downpipes.

## How much water is in an inch of rain?

Volume and weight One inch of rain falling on 1 acre of ground is equal to about 27,154 gallons and weighs about 113 tons. An inch of snow falling evenly on 1 acre of ground is equivalent to about 2,715 gallons of water.

## What is roof catchment area?

The roof of a building is referred to as the catchment area. When you are designing a roof drainage system for a building you will need to determine the size of the catchment area. The catchment area is the total area of the building that can capture and direct rainfall to the roof drainage system.

## How do you calculate storm drainage?

To calculate the volume of water that needs to be stored, multiply the amount of runoff from each drainage zone by 15. The runoff for each zone was in gallons per minute. Multiplying by 15 minutes leaves you with the amount of gallons to be stored.

## What are examples of run off?

One example of runoff is during a torrential rainstorm. The rain falls so fast, the soil does not have enough time to absorb all of it. Thus, water starts flowing down ditches and street curbs to reach either a storm drain, creek, river or lake.

## What are examples of runoff?

Runoff from nonpoint sources includes lawn fertilizer, car exhaust, and even spilled gasoline from a car. Farms are a huge nonpoint source of runoff, as rainwater and irrigation drain fertilizers and pesticides into bodies of water. Impervious surfaces, or surfaces that can’t absorb water, increase runoff.

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