Water storage: Rainfall/tank size equation

In one book or another, I recall reading an equation that inputs the annual
rainfall and roof size in square feet (apologies to metric-based folks) and
outputs the # of gallons of water that can be reclaimed from said roof in a
year.  I'm looking to determine the best size for a cistern for this
rainwater for our wet season/dry season climate in northern California.

If anyone has this equation a little closer to hand than I seem to, I'd
appreciate hearing it from you.

Thanks in advance,

Loren Davidson
The First Amendment went too far.  It should have said, "Congress shall 
make no law".