What’s your Langelier Saturation Index?

Water that is not balanced — that is too acidic or too basic — can cause bather discomfort, cloudy water and damage to the pool and spa surfaces and equipment.

Stable water quality is based on proper use of the saturation index and formula. Bringing the pH, total alkalinity, total hardness, temperature, and total dissolved solids together using the index gives you the complete answer to pool water quality and balance. Various factors are used to calculate saturation levels. The results provide you the information needed to balance the pool and correct its corrosive or scale forming condition.


pH: measured from test kit

TF: temperature factor, measured at pool

CF: calcium hardness, measured from test kit

AF: alkalinity factor, measured at pool

BALANCE:  Water with a calculated index between: -0.5 and 0.5 is balanced.

OVER:   0.5 is scale-forming (over-saturated).

BELOW:  -0.5 is corrosive (under-saturated).


A swimming pool has a F.A.C. of 2.0 ppm, total available chlorine (T.A.C.) residual of 2.5 ppm, with a pH of 7.0. The total alkalinity is 50 ppm and calcium hardness is 200 ppm; water temperature is 84 F. Is the pool water scale-forming?

SATURATION INDEX = (pH) 7.0 + (TF) 0.7 + (CF) 1.9 + (AF) 1.7 + 12.1 = -0.8

In this equation we are given the pH of 7.0.

 Table 1 shows that when:

■ Water temperature is 84 F, the temperature factor (TF) is 0.7.

■ Calcium hardness is 200 ppm the CF is 1.9.

■ Alkalinity is 50 ppm, the AF is 1.7.

The equation solution equals -0.8. From the saturation index information above, this indicates a corrosive water condition.

Use our handy online LSI Calculator.

Our LSI Calculator lets you punch in the numbers from your pool water test kit, then receive an instant readout of your water’s LSI value. If the result is in the sweet spot between –0.3 and +0.3, your water is being as kind as possible to your equipment. Fortunately, if it’s not, there are simple ways to fix it.

Easy ways to recover your balance.

•Raise your LSI by adding sodium bicarbonate or baking soda.

•Lower your LSI by adding muriatic acid.

Red Square Pools serving S. Nevada, Las Vegas, Henderson & Summerlin.  Services include weekly cleaning, repair, equipment sales, acid wash and tile cleaning.


About Red Square Pools

Thank you for checking out our blog...we appreciate you taking the time to read our posts. To be quite honest, our posts come from your questions. Red Square Pool's wants to clear up the confusion and frustration of maintaining your pool and spa. We simply want to make Pool Maintenance and Repair SIMPLIFIED!! Let us know what your thinking by dropping us an email at info@redsquarepools.com. Have a safe and happy swimming experience!

7 responses »

  1. Do really assume this is true?

    • Dear Hotshot Bald Cop,

      Thank you for your comment! The Langelier Saturation index (LSI) method for testing water is a great tool when test strips and drops are not giving you successful results. The “catch” is the formula does require accurate data variables to determine proper results. The Taylor DPD Test Kit will help you determine most of the variables to plug into the formula. For the day to day maintenance, using the Taylor DPD Test Kit by itself should suffice and give reliable and accurate information.

      Red Square Pools

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