you can find the area of the bottom of your pool by multiplying the radius or half the diameter by itself and then multiplying that by pi the formula for area of a circle is as follows:
Area of a circle=pi*r2
since your pool is 15 feet then Area=3.14*12.52 7.52 or about 491 ft2 176.7142.
To answer your next question I would need to know the depth of your pool but the formula for volume of your pool is 491 ft2 176.714 ft2 * depth=volume in ft3. Assuming that your pool is four feet deep this would give a volume of 1962.5 706.85 cubic feet. You can plug in your own depth to get a more accurate answer
Finally to get your answer into gallons use the unit factor 1 cubic foot=7.48 gallons
Thus a four feet deep pool would have a volume of 1962.5 706.85 cubic feet or about 5,287.30 US Gallons.
5,310 gallons.
5,677 gallons
About 4,650 gallons.
6,200 gallons.
5284.62 gallons
Go to http://www.havuz.org/pool-calculators.htm
You need to know the depth of the pool in order to be able to answer this question. Unless it is a round pool and you are referring to 15 as the diameter and 4 as the depth then your pool is about 5,300 gallons
About 3,983 gallons.
3736
5,475 us gal
5300 Gal
3961