This question cannot be answered without more information. Does 21 foot refer to the length of a side of a square or rectangular pool, the radius or circumference of a circular pool? Is the pool of some other shape? What is its depth?
This pool holds a maximum of about 10,360 gallons of water.
The pool holds 7,600 gallons of water
I had a pool liner replaced in a pool of that size; however, my pool also has an 8' diving well. The contractor said that I used 12,000 gallons of water.
13,594 US gallons of water.
About 18,402 gallons.
About 24 gallons.
This pool, when filled with 4.5 feet of water, will contain about 15,300 gallons of water.
About 14,351 US gallons of water.
A 2-foot round pool with 12 inches of water is holding about 23.6 gallons of water.
1 cubic foot is 7.48 US gallons
If you fill it with 4 feet of water (48 inches), the pool will contain about 5,310 gallons of water.
Go to http://www.havuz.org/pool-calculators.htm