If your pool has regular shapes such as rectangles or circles, it's relatively easy to come up the pool size. For example, for a rectangular pool, its size can be estimated by:
Average depth * width * lengthIf you measure your three dimension in feet, you will come up total cubic feet of your pool size.
Converting Cubic Feet to Gallons
To convert cubic feet to gallons, you can use the following formula:
1 ft3 * 7.48 gallon/ft3 = 7.48 gallons
To simplify, you may use 7.5 instead of 7.48 in you calculation. If you have an irregular shape, say, kidney shape as shown below, it takes a bit more effort.
Kidney or Irregular Shapes
There are different methods used to calculate the capacity of irregular shapes. First, you can imagine the pool as a combination of smaller, regular shapes. Measure these various areas and use the calculations described in  for each square or rectangular area and for each circular area. Add these volumes together to determine the total capacity.
Here we will introduce you another method.
Volume in gallons = (Estimated surface area * Average Depth) * 7.5where
Average surface area = (A+B) * C * 0.45
The total of measurement A plus measurement B multiplied by 0.45 multiplied by the length (i.e., C) gives you the surface area of the kidney shape.