Rain Barrel Calculator

A rain barrel captures the same water any harvesting system does, so the tool starts from the roof: footprint area times rainfall depth times a collection efficiency gives the volume that reaches the downspout. Because that volume is what actually flows toward the barrel, comparing it to the barrel's size tells you how many barrels it would take to hold a whole storm — and how quickly a single barrel overflows.

The capture math leans on the same constant every rainwater tool does: about `0.623` gallons per square foot of roof per inch of rain, trimmed by an efficiency that accounts for splash-out and the first flush that rinses the roof. Divide that captured volume by your barrel's capacity and round up, because you cannot buy a fraction of a barrel.

The headline most people want is sobering: even a modest roof in a light rain overwhelms one barrel almost immediately. That is not a flaw in the plan — it is why overflow routing matters. Size the barrels for the water you can actually use between storms, and send the rest to a safe overflow, not against the foundation.