How many gallons does 1 inch of tank have in 275 gallon tank?

To figure out that amount, what you need is the Length, Width, and Height of the 275 gallon. Then the equation if you were calculating an aquarium volume is LxWXH divided by 231 (this takes cubic inches to gallons)
So, if you want to figure out how many gallons is in 1 inch of water in that 275 gallon tank, you simply apply the calculation of LxWx1 (inch) divided by 231