Enter the length and width of the roof and the average annual rainfall into the calculator to determine the total rain harvesting possible.

Rainwater Harvesting Formula

The following formula is used to calculate the potential amount of rain a house can gather from rain. 

RWH = L* W * ARF / 12 * 7.48052
  • Where RWH is the rainfall harvested (gallons)
  • L is the length of the roof (feet)
  • W is the width of the roof (feet)
  • ARF is the annual rainfall in that area (inches)

This formula assumes a 100% catch rate, which is often not likely, so subtracting a few percentages could help provide a better estimate.

This formula also assumes that the house’s roof is a square; if it is an irregular shape, you may need to break down the roof into sections that you can calculate the area of and then add them together.

What is rainwater harvesting?

Rainwater harvesting is the process of “catching” water that falls on a roof’s house and collecting it for further uses such as gardening, bathing, and drinking.

Rainwater harvesting can be an excellent environmentally friendly way to save on the amount of freshwater a person consumes through normal means.

This reduces the load on the water treatment plants and, in turn, reduces the load on the environment to provide those treatment plants with water.

How to calculate rainwater harvest?

The following example problem outlines the steps required in calculating the amount of rainwater a house could catch per year.

First, measure the width of the house. For this example, the width is measured to be 20 feet.

Next, measure the length of the house. The length of the house is found to be 50 feet.

Next, determine the average annual rainfall for the area in inches. This can be easily googled. For this example, the average amount of rain is 46 inches.

Finally, calculate the total amount of rain that can be harvested using the formula above:

RWH = L* W * ARF / 12 * 7.48052

RWH = 50* 20 * 46 / 12 * 7.48052

RWH = 28,675 gallons

This is the maximum amount of water that the house could collect, but as mentioned previously, usually the true amount due to inefficiencies and not being able to catch everything from a heavy storm is closer to 50% of this.