I assume you mean a circular pool. So start by calculating the volume 7 x 7 x 3.1416 x 3.5. That will give you the volume in cubic feet, the it is either 6.24 imperial gallons per foot or more fore US gallons, which are smaller.
Round Pool: Depth x Diameter-squared x 5.9 = Volume in Gallons
Go to http://www.havuz.org/pool-calculators.htm
4,047.4 US gallons of water.
Depends how deep
3300
16 and 7/8
If the bottom is flat and level, then it takes 239.38 gallons for each inch of the water's depth.
Exactly 5264.1669 gallons
7,646 gallons.
3961
that is about 105 cubic feet, and if you fill with water,which has 7.48 gallons per cu ft, that is 7.48 x 105 = 782 gallons
615,142 gallons of water are needed to fill this cylinder.
31.987 gallons per foot.
It would require 2,937.6 US gallons of water to fill the pipe.
The volume of water in a 4-inch diameter, 1-foot section of pipe is 0.6528 gallons of water.
A three-inch diameter pipe one foot long holds 0.367 gallon.
You will need a maximum of 11,230 gallons of water.