Multiply all the numbers to get the volume of the pool in cubic feet. Then divide by 7.5. That assumes the water is filled to a height of four feet. Some above-ground pools have 52" walls, which allow for a water height of 48", which is four feet. The dimensions stated above in the reworded question are for an oval pool, not a round one. I suspect the original question was for a round pool with an 18' diameter and a 4.5' wall. (The slash in 4 1/2 did not show up in the question.) If that is the case, then the answer is 3.14 times 81 times 4.5 divided by 7.5.
How many gallons will fit into a 24 foot diameter x 4 foot high round swimming pool
A round swimming pool 12 feet across by 4 feet deep will hold 452 ft3 which converts to 3,381 gallons
13,346 gallons per vertical foot.
It is 30.6 cubic feet = 190.3 Imperial gallons.
A 12-ft round pool with depth of 2 ft has a capacity of 1,692.2 gallons of water.
There is NO way to calculate this without knowing how deep the pool is filled!
To calculate this the depth would also be needed