What is the formula to calculate how many gallons of water you need for a 48in X 24in X 24in fish tank?

Do you want the formula, or the answer ?

The formula is: Volume = (length) x (width) x (height)

The only remaining step is to handle the units properly.

You need to know that [ 231 cubic inches = 1 US gallon ].

Volume = 48" x 24" x 24" = 27,648 cubic inches

How many 231s is that ?

27,648/231 = 119.688 gallons (rounded)

That's the volume of the tank, and the amount of water needed

to fill it to the rim.