If 20' is the circumference, then the answer is 833.4 gallons.
About 7300 gallons
Approximately 4,400 gallons.
A round swimming pool 12 feet across by 4 feet deep will hold 452 ft3 which converts to 3,381 gallons
To calculate this the depth would also be needed
I would say probably around 50-200 gallons.
A 100-foot round pool filled with 1.5 feet of water would be holding about 88,500 gallons.
To fill your 18-foot round pool to a depth of 48" would require about 7,646 gallons of water. If you filled it to 36", the total would decrease to about 5,735 gallons.
It would fill about 91,000 residential in-ground swimming pools.
There are 13,536 gallons in a 24 foot diameter pool that has 4 feet of water. Not true for a pool that is 4 feet in height. This would hold less than 4 feet of water.