Just how much rainwater could I collect?

Calculation using English units

Enter the area of your house's footprint (ft²):
Enter the percentage of the footprint you can collect from:	%
Enter the average annual rainfall for your area (inches):

Don't know your average annual rainfall, click here.
Find use closest weather station and use the row labeled "0.5".
For Texas users, click here.

Calculation using Metric units

Enter the area of your house's footprint (m²):
Enter the percentage of the footprint you can collect from:	%
Enter the average annual rainfall for your area (mm):