That’s it! One method for calculating gallons will work in every pool, no matter what the shape or size. Rectangle, oval, or amoeba-shaped, you’ll have greater accuracy if you calculate pool gallons chemically.

NOTE: The formula as written below will work best on pool sizes of <30K gallons, but can easily be re-tooled for larger bodies of water.

Take a sample of pool water in a clean container and put it to the side. This is our ‘Before’ sample. Next, broadcast 4 lbs of baking soda evenly across the pool’s surface with the pump running. Allow the pump to circulate the water for an hour to an hour and a half. Take a new sample of water – our ‘After’ sample. We will want precisely five times the amount of water that we usually use in our Total Alkalinity drop test (this is a three-bottle test). For example, if we typically use 25 ml of water, we will now use 125 ml. This means we will need an additional clean vial of large enough size to hold our 125 ml sample.


Because we are using five times the amount of water for our test, we will also need to use five times the amount of drops that the instructions dictate. But, we only need this excess amount for the first two of the three test solutions (i.e., We usually add two drops of the first test solution, we now add ten drops; we typically add five drops of the second test solution, we now add 25 drops).

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The third bottle will still be one drop at a time, swirling the sample between each drop and counting the drops necessary to make the expected color change. However, instead of each drop being worth ten ppm, as usual, they will now only have a value of 2 ppm each.

You will want to test the Total Alkalinity reading for both your ‘Before’ and ‘After’ sample using this method and then calculate the difference between the two. Now, this is where the magic happens – Complete the following equation.

285,600 ÷ ppm Change = Gallons

    TA Factor                      (Sample B ppm – Sample A ppm)         Your Pool Gallons

Gallons of H2O in pool  _______________________________________________

Take our FREE online tutorial on water testing techniques and test kit interferences HERE


Rudy Stankowitz is a 30-year veteran of the swimming pool industry and President/CEO of Aquatic Facility Training & Consultants

This Post Has One Comment

  1. Great “rule of thumb” for making this calculation. Very few clients remember the gallon volume of their swimming pool.

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