I actually found it from when I posted it here a long time ago... I hope it makes sense, plug your numbers in...
The person who developed the formula is Pete Escobal, an engineering graduate from NYU and UCLA:
Flow rate formula for UV sterilizer:
D(new)= D(at 500 gph) x 500/ New flow rate.
50,000 = 6,512(zap dosage for a 25 watt, 3" diameter UV bulb) X 500/S
or the new flow rate would be
S = 6512/50,000 X 500 = 65.12 gph.
For how many hours should the sterilzer run?
T = aG/S Where
T = hours of sterilization required (hrs)
G = gallons in tank (net gallons)
S = Sterilization flow rate (gals/hr)
a = purity coefficient (usually a = 9.2 because we assume that 100 percent of the water will never be sterilized. 9.2 is a good compromise and it implies that in a 100 gallon tank only 1.28 ounces of water will elude sterilization.)
So: T = aG/S
= 9.2 X 90/63 = 13.14 hrs.
That's the time that it will take to cycle the entire tank through the sterilizer. You could put the UV on a timer and cycle it on and off every 13.5 hours. According to the author your Uv light bulb will last longer than six months.