Rainwater Tank Annual Yield
Calculator

It multiplies your roof's catchment area by your region's total annual rainfall and a runoff efficiency factor (typically 80-90%), giving an estimated total gallons or liters collectable over a full year if all of it were captured and none overflowed.

Tank capacity limits how much can be stored at once, so during heavy rain events the tank fills and overflows before all the potential water is captured; unless your tank is large relative to your roof area and rainfall pattern, some of the theoretical annual yield is always lost to overflow.

Yes significantly — two locations with the same annual rainfall total can have very different harvestable yields if one gets it in frequent moderate events (easier to capture without overflow) and the other in a few intense storms (more overflow loss), so annual totals alone can be misleading.

As a rough guideline, sizing a tank to hold roughly 1-2 weeks of your average water usage, or enough to bridge typical dry spells between rain events in your climate, captures most of the practical benefit without paying for storage capacity that rarely gets used.

Yes, if the same tank supplies both household and garden use, calculate your annual harvest first, then subtract non-irrigation uses to see what's actually left for the garden, since assuming the full harvested volume is available for plants overstates how much irrigation water you can rely on.