How to calculate storage capacity for rainwater tanks?

Calculating Roof Catchment Area

To determine the roof catchment area, measure the length and width of the roof in feet. Multiply the length and width to get the total square footage. For example, a 20x20 foot roof has a catchment area of 400 square feet. However, if your roof is irregularly shaped, you may need to break it down into smaller sections for more accurate calculations.

Determining Rainfall Intensity

Rainfall intensity varies depending on your location. In general, a rainfall intensity of 0.3 inches per hour is a good estimate for most areas. For more accurate calculations, refer to local rainfall data or consult with a professional. For our example, let’s assume an intensity of 0.3 inches per hour.

Calculating Storage Capacity

To calculate storage capacity, divide the roof catchment area by 4,500 square feet. This gives you the rainfall collection potential in gallons per hour. For our 400 square foot roof, this would be: 400 ÷ 4,500 = approximately 0.089 gallons per hour. To determine the total storage capacity, multiply this value by the desired storage period in hours. For example, if you want to store rainwater for 48 hours, you would multiply 0.089 gallons per hour by 48 hours, resulting in approximately 4.3 gallons of storage capacity. However, this may not be enough for your needs, so you may need to increase the tank size or adjust your storage period.

Tips and Considerations

When choosing a rainwater tank, consider the material, size, and shape. A larger tank may be necessary to accommodate your storage needs, but it may also increase costs and space requirements. Additionally, consider the type of roof you have and the amount of rainfall it can collect. Gutter systems can also impact rainwater collection, so ensure they are properly sized and installed to direct water into the tank.