I recommend a tank size of about 1 gallon/square foot of rooftop if you are using rainwater harvesting as a stormwater management tool. This (slightly) exceeds City of Atlanta requirements, and simplifies the math. 1:1:1 is a good ratio, and easy to remember.
1 s.f. of rooftop : 1 gallon of storage : 1 s.f. of turf to irrigate.
Not interested in stormwater management, and just want to collect what you need? I recommend sizing a tank on a 1 month dry spell (happens almost every year), and collecting from up to twice as much rooftop as irrigation area. 3 gallons storage/1 s.f. of turf, collecting from up to 2 s.f/s.f. of turf.
Cooling tower makeup requires 7 gallons/year/square foot of air conditioned office space in Georgia. Also, cooling tower useage alone will use all the rainwater typically collected, for a 7 story building. So if you have a 3-4 story building, you only need to collect from half your rooftop, or you have excess that you can use for irrigation or toilet flushing.
10 gallons/year/square foot for irrigating turf and other thirsty plants
3 gallons/person/day for toilet flushing at an office, or 6 gallons/person/day for continuous occupancy.
50″ rain falls/year in Atlanta, about 25 gallons/year/square foot rooftop is usable
Budget Prices: $2/gallon for an above-ground tank is good budget price. This is for a 10,000 – 100,000 gallon tank. That’s for tanks, pumps, the extra piping, and the filters needed for irrigation or cooling tower makeup.
$3/gallon for a below-ground tank, pumps, extra piping, and filters for irrigation or cooling tower makeup.
