Estimate how much rainwater a rain barrel can collect from your roof. Enter rainfall depth, roof area, barrel size, number of barrels, and collection efficiency to calculate collected water, storage capacity, overflow, and how many barrels may be needed.
Rainfall depth:
Enter how much rain fell or is forecast, such as 1 inch or 25 millimeters.
Roof collection area:
Enter the roof area that drains into the barrel. For a simple estimate, use the roof footprint area.
Barrel size and count:
Enter the size of each barrel and how many barrels are connected.
Collection efficiency:
Use this to estimate losses from gutters, screens, first-flush diverters, leaks, splash, and overflow.
A rain barrel calculator is useful for garden watering, rainwater harvesting, barrel sizing, downspout planning, stormwater reduction, lawn care, and estimating how much free water a roof can provide.
Actual results can vary based on roof material, gutter layout, rainfall intensity, wind, debris screens, overflow plumbing, and how much water is already in the barrel.
Your result shows how much water could be collected from the roof area after applying collection efficiency. It also compares that amount with your total rain barrel capacity to estimate tank fill, overflow, unused capacity, and the number of barrels needed.
A rain barrel can fill quickly. One inch of rain on 100 square feet produces about 62 gallons before losses, enough to fill many standard barrels.
Multiply rainfall depth by roof collection area, then multiply by collection efficiency to estimate collectable rainwater.
Divide the estimated collected gallons by the size of one barrel. Round up to estimate how many barrels would hold the water without overflow.
Many rough estimates use 75% to 95%, depending on the roof, gutters, screens, first-flush loss, leaks, and system setup.