Here's a formula for round pools:
distance across*distance across*depth*5.9
for rectangles:
length*width*depth*7.5
or for a deep end:
length*width*((shallow end depth + deep end depth)/2)*7.5
so in your case, 15*15*4*5.9=5000 gallons
Twelve feet of water in a 10-foot round pool is approximately 7,080 gallons.
Exactly 5264.1669 gallons
Three feet of water in a 12-foot round pool is about 2,550 gallons.
3961
An 18-foot round pool with 3 feet of water contains 5,734.8 gallons of water. If you fill it to 4 feet deep it increases to 7,646 gallons of water.
That all depends on how deep the water has to be, which the question neglects to specify.
141.06 gallons
(3.5 feet) * PI * ((9 feet)^2) = 6 662.46122 US gallons
13,594 US gallons of water.
About 18,402 gallons.
About 24 gallons.
367.2 (rounded)