How to Calculate Rainfall for Harvesting

Harvesting rainwater is an easy way for gardeners to supplement, or even replace, water requirements for their gardens. It’s free, it’s clean, and it helps conserve a resource. And every time it rains, there are thousands of gallons being wasted. It rolls off roofs and other structures, onto the ground, into ditches and storm drains. Instead of wasting it, people are collecting it. But in order to have the right-sized containers or catchment systems, it’s important to know how much water is coming off the roof. Follow these simple directions to arrive at the correct volume of water.

Things You'll Need

• Rain gauge
• Paper
• Pencil
• Calculator
• Measuring tape.

Instructions

How to Calculate Rainfall for Harvesting

1. The first thing to do is to figure the area of the structure. Begin by using the measuring tape, and measure your roof from one eave to the next. You can do this on the ground because it doesn’t matter whether your roof is flat or sloped. So let’s say the measurement is 30 feet X 50 feet. Write this down.

2. You now have the measurements necessary to find the square footage of your roof.Multiply 30 feet times 50 feet. That’s 1,500 square feet. And that is your catchment area. Write that down. Area = length (feet) X width (feet).

3. Now, check the rain gauge. For simplicity, let’s say that it rained one inch. Write down the number of inches.

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

5. Now, multiply .623 gallons by the number of surface square feet. In this case it would be 30 feet X 50 feet= 1,500 square feet X 1 inch X .623 gallons per square foot per inch of rainfall. That comes to 9.345 gallons per square feet.

6. Let’s say instead it rained 2.5 inches. Then the calculation would read 30 feet X 50 feet=1,500 square feet X 2.5 inches X .623 gallons per square foot per inch of rainfall.The total number of gallons here is 2,336.25 inches of rainfall.

Tips & Warnings

• There’s no reason to climb up on the roof to do these measurements.