Rainwater Barrel Calculator

Summary

Enter your roof area, rainfall amount, and barrel size to see how much water you can collect and how quickly your barrel fills.

How it works

1. Enter the catchment area of your roof in square feet or square meters.
2. Enter the rainfall amount in inches or millimeters for the event or time period.
3. Enter your barrel capacity in gallons or liters.
4. Optionally enter the number of barrels you have connected.
5. The calculator applies a runoff coefficient (default 0.85 for typical asphalt shingles) and computes the collectable volume.
6. Results show total runoff, usable collection, fills per event, and overflow volume.

Use cases

• Size a rain barrel setup before purchasing.
• Estimate how much water is available for garden irrigation.
• Determine if one barrel is enough or if you need a second tank.
• Calculate overflow risk for heavy rainfall events.
• Plan storage capacity to cover dry spells between rains.
• Evaluate water savings and environmental benefit of rainwater harvesting.

Frequently Asked Questions