Once you fully understand the testing procedures and balance ranges, next is to know the amount of water in pools, spas, and hot tubs. Most sanitizers and other chemicals you will use are formulated for a certain number of gallons of water. If you do not know how much water, then you do not know how much sanitizer or whatever to add to do the job properly. Pool owners/store customers are bad news when it comes to trying to find out how much water is in their pool. They may have a 16,000 gallon pool, but they tell you it holds up from 1,500 to 30,000 gallons. We do not have to be exact on the number of gallons, but we must have a reasonably close estimate. Below are basic formulas to use in determining at least approximate volumes of water

Pools
Rectangular: Multiply the length, times the width, times the average depth, times 7.5.
Example: 15’ X 30’ pool, approximately 5’ average depth

 15 X 30 X 5 X 7.5= 16,875 gallons

Round: Multiply the diameter, times the diameter, times the average depth, times 5.9.

Oval: Multiply the maximum length, times the maximum width, times the average depth times 5.9.

Spas
Follow the same procedures as above, but allow for steps and benches.

Hot Tubs
4’ round X 3’ deep=250 gallons
4’ round X 4’ deep=350 gallons
5’ round X 4’ deep=500 gallons
6’ round X 4’ deep=750 gallons