If 20' is the circumference, then the answer is 833.4 gallons.
gallons
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.
A 100-foot round pool filled with 1.5 feet of water would be holding about 88,500 gallons.
It would fill about 91,000 residential in-ground swimming pools.
7,646 gallons.
Kiloliters or gallons depending on sizeo f the pool.
gallons
All you have to do to find the volume of a swimming pool is multiply the length x width x average depth x gallons in a cubic foot (7.48). In your case it would be 38x22x5.5x7.48=34,393 gallons.
A 28-foot round pool filled with 4 feet of water would contain about 18,500 gallons.
how many ounces do of roundup do I mix with two gallons of water?
Go to http://www.havuz.org/pool-calculators.htm
you would need to know the height of the pool too.