A rainwater tank should be at least 600 gallons although more is better.
Common rainwater tank sizes can range from 600 gallons to 50,000 gallons and larger.
The cost of the tank and installation are usually the biggest piece of the rainwater budget.
In most cases, the optimal tank size for a project will be reported as a range of sizes with potential use efficiency.
The amount of rain your roof can collect depends on the size of your roof.
However approximately 550 gallons of rainwater can be collected for every 1000 square feet of collection surface per inch of rain.
To estimate the amount of rainwater collected in one year, take the square footage of your collection surface, divide by 1000, multiply by 550 and then multiply by the average annual rainfall for your area.
For example, on a 2,000 square foot roof, you can collect 2,000 x 0.56 = 1,120 gallons/inch of rain.
If your average rainfall was 25 inches/year, your annual collection potential is 1,120 x 25= 28,000 gallons/year.
Rooftop RWH potential is estimated by multiplying the average annual rainfall, roof area and runoff coefficient.
Runoff coefficient of the concrete roof is considered in the calculation which typically varies from 0.70-0.80.
It's common for about 1/8" inch of rain to fall each hour during a moderate rainstorm.
That means that a 500 square foot roof can fill a properly installed 50-gallon rain barrel in about one hour.
As a general rule of thumb, each square meter of roof space collects around 1 liter of water for every 1 millimeter of rainfall received.
Using the calculation of (x) square meters of roof space available for connection to your tank multiplied by the amount of rainfall will help guide your decision.