You should know how much water is in your pool for calculating chemicals you need to add for maintenance. The formlulas are pretty standard – and Pentair has this handy Pool Volume Calculator as well.
Rectangle Pool: Volume in Gallons = Length x Width x Average Depth* x 7.5
Oval/Round/Freeform** Pool: Volume in Gallons = Length x Width x Average Depth x 5.9
*average depth is just that, if your shallow end is 3′ 5″ deep and your deepest part of your pool is 6′ Deep, add them together 3.5+6 = 9.5 /2 = 4.75 for our average depth figure.
**We could get all technical on the volume differences in ovals/round/freeform in all their shapes, however, for this exercise we want to keep it simple you can get a quick estimate of your water for your chemical calculations. We’ll have you round the number as well, so a close estimate is going to get you where you need to go.
If your pool has a sun shelf, you can figure the formula out for just the length/width/depth of the sunshelf based on its shape, and then the rest of the pool.
When you have your number, it’s going to be useful to either round up or down to a number easily used in chemical calculations, as most product instructions are for 10k gallons/15k gallons/20k gallons etc.
Happy Calculating!