I do not remember the formula

I bought an air tank without the compressor or motor. I do not know how many gallons it holds and for the life of me, I do not remember the formula
I tried googling it but seems everything I google anything it keeps transferring from one web search page to another.
I can not find the formula. Can someone help????
The tank is 24 inches in diameter and 48 inches high. I am guessing it is about 60 gallons. It needs refurbishing but for $45 how can you go wrong.