This is for U.S. Measurements: If your pool is like a box, (rectangular, straight sides, right angles) you can calculate the volume just like you would a box. To find the volume of a box, multiply the length, width, and height (depth is similar to height - but from the top instead of the bottom) convert depth to the same units of measure as the other measurements: 54-inches = 4.5-feet 18-ft X 40-ft X 4.5-ft = 3240-ft3 3240-ft3 x 7.48-gal/ft3=24236.88 Since your pool has rounded corners and the bottom isn't perfectly flat, I would say it is pretty close to 24,000 gallons. If you pay $0.01/gallon (one penny for every gallon), it will cost $240 to fill the pool.