Every time it rains, water pours off your roof, down the downspouts and away into the street.
Once your system is set up, the water costs nothing.
One inch of rain falling on one square foot of roof equals about 0.62 gallons.
The table above shows the key lesson of rainwater harvesting: a roof can collect far more water than most people can store.
Barrels of about 50 to 60 gallons are the easiest place to start.
Choose the downspout that drains the largest roof area and is closest to the garden.
Gravity-fed barrels deliver only low pressure: roughly 0.43 psi for every foot the water sits above the outlet.
Step-by-step advice, charts and free garden calculators at Garden Voila.