Depending on how full you are getting the pool (vinyl pools typically go to 80%) you are looking at 3,700 gallons for an 18 foot by 36 inch pool.
48,000 gallons of water
2 gallons
To fill in this size pool you'll need 100.5 cubic yards of material. If you mean how many gallons of water to fill the pool, you'd need 20,300 gallons of water to fully fill it.
If you fill it with 4 feet of water (48 inches), the pool will contain about 5,310 gallons of water.
To fill this pool to the very top you'll need about 1,480 gallons of water - less if you do not fill it all the way up.
This pool can hold up to about 15,148 gallons of water. (this would fill the pool to the very top).
Aproximately 12,000 gallons
The pool holds 7,600 gallons of water
If this is a round pool 18 feet across and you fill it with 4 feet of water, it contains 7,646 gallons of water.
5,287.7 gallons of water.
formula for determining how many gallons in a pool is, length times width times average depth times 7.5 = gallons of water in pool if it round or oval its times 5.9
That will depend on how deep you want the water to be.