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.
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
One foot of 3 inch pipe will hold approximately 0.625 gallons of water.
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.
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.
You will need a maximum of 11,230 gallons of water.
7,480.52 gallons for every inch of depth