How many gallons of water are in a 18 round by 48 inch swimming pool?

Answer 1

Here's a formula for round pools:

distance across*distance across*depth*5.9

for rectangles:

length*width*depth*7.5

or for a deep end:

length*width*((shallow end depth + deep end depth)/2)*7.5

so in your case, 18*18*4*5.9=7500 gallons