The NPC LSI Calc is the newest, most advanced and most accurate calculator for determining the Langelier Saturation Index (LSI) for swimming pool and spa water. It was developed by noted pool and spa water chemistry expert and author, Robert W. Lowry. It is the only LSI calculator that currently has exact adjustments for cyanuric acid and borate.

The LSI is used to predict if pool or spa water is corrosive, balanced or scaling. The NPC LSI Calc uses the original mathematical equations which use logarithms, square roots, and interpolated constants for calculations. We do not use inaccurate lookup tables and factors. The actual water test results are used for calculations.

The NPC LSI Calc is easy to use, you input the customer or pool name and then the water test results for pH, Total Alkalinity (TA), Cyanuric Acid (CYA) Borate (B) Calcium Hardness (CH), Total Dissolved Solids (TDS), and the water Temperature (T) in degrees Fahrenheit. The LSI with adjustments is automatically calculated and displayed. The background color of the LSI value changes from green to amber to red depending on whether the water is Corrosive, Acceptable, Ideal, or Scaling. This gives a visual or at a glance notice of the water’s condition.

The original LSI was never intended for predicting corrosive, balanced or scaling water conditions for pools or spas. In fact, it was developed in 1936 which was many years before the development of Cyanuric Acid (CYA) which became popular around 1960 and remains so. The very popular trichlor chlorine tablets actually contain 56 % CYA and so as chlorine is dispensed, the CYA builds up in the water. CYA has a direct influence on Total Alkalinity (TA) so an adjustment had to be made to the original LSI calculation.

Another family of chemicals that have become popular recently as a pH buffer and algaestat in swimming pool and spa water are borates. Borates also have an influence of TA. Using borates in pool or spa water keeps the pH from rising but also requires an adjustment to TA and therefore has an effect on the LSI calculation too.

We have determined the exact affect borate and CYA will have on LSI calculations and built in the mathematical adjustments to the NPC LSI Calc App.

The goal mathematically is to have an LSI of near 0.00 but any value from –0.30 to + 0.50 is either Acceptable or Ideal. Positive values show increasing tendencies for forming scale and values more than +0.50 indicate scaling water conditions that require action. Negative values show increasing tendencies for being corrosive or aggressive and values greater than –0.30 indicate corrosive water that requires action.

We do not recommend which water conditions to change or chemical amounts to adjust the water balance or LSI. Which condition to adjust is best accomplished by the service tech, pool owner or pool store. There may be reasons that one condition or another cannot be changed or perhaps a condition is easier to change than another. But you can input new amounts for any condition to see what affect it will have on LSI.

Results can be saved and reviewed. The number of records saved is only limited by your phone’s memory. It is easy to delete of records.

Protecting the pool or spa surface, plumbing and equipment and having balanced water for your customers, your family or guests are easy with the NPC LSI Calc.

What’s New

Version 1.1

Bug Fixes & Improvements

Ratings and Reviews

3.4 out of 5
5 Ratings

5 Ratings

Trav2001 ,

Great App

I own a pool service company in AZ and I use this app all the time to get my LSI readings at each pool. Even though it doesn’t give you suggested amounts of chems to use to adjust the numbers that’s ok for now, although I’d like to see that calculator on here someday. It’s dead in accurate every time.

Reme18MZ ,

Now good to go on iOS 12

With the latest update the app is working as advertised. Back in business, thanks! I will add that the arrow graphic at the top is pretty much unreadable with way too small fonts. But the rest of the app is good.

Developer Response ,

We apologize, there was an issue with the latest version of iOS, but it has been corrected. Please update the app and let us know if there are still any issues. thank you.

Merlin4900 ,

My bad.

App great

App Privacy

The developer, National Plasters Council, Inc, has not provided details about its privacy practices and handling of data to Apple. For more information, see the developer’s privacy policy.

No Details Provided

The developer will be required to provide privacy details when they submit their next app update.


  • Family Sharing

    With Family Sharing set up, up to six family members can use this app.

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