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The Basics of Pool Water Saturation Index

By: Holly Jender on May 29th, 2018

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The Basics of Pool Water Saturation Index

Pool Maintenance

Hey guys, my name is Mike Harper, aka The Pool Care Guy, and in today's post I'm going to be talking about the pool water saturation index. It's a bit of a complex topic that involves lots of numbers, but I'll try to break it down into plain English as best I can. 

The water saturation index, officially known as the LSI (Langelier Saturation Index), is a scientific formula created to calculate the perfect water balance, or essentially whether it is corrosive or scaling. 

Specifically, it's defined as “an approximate indicator of the degree of saturation of calcium carbonate in water.”

The LSI was invented in the 30s by a professor and chemist named W.F. Langelier, who worked at the University of California in Berkeley.

He created it to test water that was kept in large boilers to find out if it would cause etching or scaling of the equipment because calcium scale was a big problem in municipal water storage during that time. But it is particularly helpful in pools today because the same scaling problem can cause permanent damage to some pool structures.

Why is the Water Saturation Index Important?

Like I said, you already know the importance of balanced water by now. Water that is corrosive will corrode your pump and other equipment, and scaling can cause clogging and cloudy water.

And just testing your water with your pool testing kit is usually enough, but what the LSI does is something a little extra.

People often use the saturation index to determine a more perfect chemistry situation when they are having trouble keeping it in balance.

Perfectly balanced water is saturated with the “perfect amount of calcium and dissolved solids, and has a stable pH.” (source).

In other words, the perfect water saturation is 0.0 LSI.

And as you probably already know, balanced water is extremely important for the safety of your swimmers and the maintenance of your pool.

What the saturation index does is give us a little different view of water chemistry. If you know how saturated your water is with mineral and calcium, you can determine how likely your pool is to suffer damage and how far your water chemistry is out of balance.

The LSI Factors

The factors used to calculate LSI are simply the factors you measure in order to balance your pool chemistry, plus the water temperature. Here’s why these factors matter:

pH: this measures the pool's acidity, and even small changes in this level can throw off the balance of every other factor. For this reason, some experts contend that pH is the most important factor to measure in any water chemistry situation.

Temperature: you may possibly keep track of your pool temp for comfort, but you may not realize that it also plays an important part in your pool chemistry. If the water is too hot, it can raise the pH level and speed up chemical reactions. Ideally, the temperature should remain at about 76–104°F.

Total Alkalinity: alkalinity is directly linked to a pool’s pH levels. A higher alkalinity can more easily resist pH shifts.

Calcium Hardness: this just lets you know how much calcium is dissolved in the water and how hard or soft the water is. This alone doesn’t tell you what you need to know about the LSI since it is primarily affected by pH levels.

How to Calculate LSI

The formula to calculate LSI is: (pH) + (Temperature ºF) + (Calcium Hardness) + [(Total Alkalinity) - (CYA correction factor @ current pH)] - (TDS factor) = LSI.

The link provided above has a helpful table to help you find the variable values, but to calculate, let’s use their example:

ph: (7.4)

temperature: 84ºF (0.7)

calcium hardness: 300 (2.1)

alkalinity: 100 (2.0)

cyanuric acid: 100 (pH 7.4 = 0.31)

total dissolved solids: < 1000 (12.1)

So, the calculation would be as follows:

[(7.4) + (0.7) + (2.1) + [(2.0)-(0.31)] - (12.1) = X LSI

[(10.2) + (1.69)] - (12.1) = X LSI

[11.89] - (12.1) = -0.21 LSI

This water would be slightly off balance, leaning a little toward the corrosive side, but still within acceptable range.

But this helps you to see that by increasing some of the factors in the equation to equal 0.21, you could have perfectly balanced water.

Make sense?

But even if you are not a skilled mathematician or scientist, there are still ways to calculate LSI automatically.

Simply plug your numbers into this LSI calculator for an instant reading.


If you stopped by to read this, you probably heard about water saturation somewhere and wanted to know more.

We hope we have answered your questions about it!

The main thing to know is that it is a good way to tweak your pool chemistry in small increments if you are having trouble keeping it balanced.

Other than that, it’s also fun to play around with the formula if you have an app or online calculator to plug your numbers into.

If you feel like we missed something or have some more questions, don’t be afraid to inbox us. We are happy to help!

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