Volume is going to be based on water height, so you want to use the planned depth of the water, not the height of the walls, if you're plugging into an online calculator.
If you have 4 feet of water in the pool I get 17130 gallons:
radius of pool is diameter/2 or 13.5 feet, pi * radius ^^2 * 4 feet deep = 2290 cubic feet, 2290 x 7.48 (# US gallons per cubic foot) = 17130
Measure the actual water depth in inches and adjust accordingly, eg if 50 inches deep then volume is 17130 * 50 / 48.
This assumes the inside of the pool is exactly 27 feet... these days companies seem to claim whatever they can get away with so it's probably 25-1/4 feet or something
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