If 20' is the circumference, then the answer is 833.4 gallons.
gallons
A 100-foot round pool filled with 1.5 feet of water would be holding about 88,500 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.
It would fill about 91,000 residential in-ground swimming pools.
7,646 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
Liters or cubic meters are typically used to measure the volume of water in a swimming pool.
you would need to know the height of the pool too.
If you filled it all the way to the top (40") it would hold about 2,830 gallons of water.