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Why PoolHealth calculates the LSI differently
The Langelier Saturation Index (LSI) is one of the most used numbers in pool chemistry, and one of the most easily misread. Here is what it is, where the usual way of calculating it falls short, and what we do instead.
What the LSI is
The Langelier Saturation Index (LSI) estimates whether water will dissolve calcium carbonate or deposit it. Negative water tends to etch plaster and corrode metal. Positive water tends to leave scale on surfaces, heaters and filters. A value near zero, commonly between −0.3 and +0.3, is treated as balanced.
W. F. Langelier published it in 1936 for drinking-water systems, not for pools. It has been used for pools ever since because it needs only a few measurements.
The standard way to calculate it
The LSI is the difference between the actual pH and the pH at which the water would be exactly saturated with calcium carbonate:
LSI = pH − pHs
The saturation pH (pHs) depends on water temperature, calcium hardness (CH), alkalinity (TA) and total dissolved solids (TDS). In practice most calculators do not solve this chemistry. They read each factor from a table, a few rows per variable, and add the numbers up. Between the rows they interpolate.
Where tables fall short
A table is a convenient summary of the chemistry, not the chemistry itself. It has three weak points:
- Steps. Each factor changes in steps, so the result can jump when a reading crosses the edge of a range.
- Range. Tables are built for typical water. Warm hot tubs and high dissolved solids (TDS) fall at the edge of them or outside them.
- Alkalinity (TA). A pool test measures total alkalinity (TA). In a pool stabilised with cyanuric acid (CYA), part of that total is cyanurate, not carbonate. Only carbonate alkalinity takes part in scaling and etching. The usual shortcut is to take off a fixed share of the CYA, but that share is not fixed. It changes with pH.
What PoolHealth does differently
PoolHealth does not read the factors from tables. It calculates the index from the same equilibrium chemistry the tables were built to approximate:
- Carbonate alkalinity. We take your CYA into account with a correction that follows pH continuously, instead of one fixed rule for every pool. If you do not use CYA, nothing is subtracted.
- Temperature. The solubility of calcium carbonate and the carbonic acid equilibrium are calculated directly from the water temperature you enter. There is no temperature table to step through.
- Dissolved solids (TDS). The TDS you enter sets the ionic strength of the water, and from it we calculate how strongly calcium and bicarbonate are held back in solution.
- Ion pairs. Some calcium and bicarbonate in the water exists as ion pairs and is not available to form scale. We apply a small correction for this. It is based on published work, but we fitted it empirically, because the measurements it would ideally use (such as magnesium and sulphate) are not part of a normal pool test.
The result is a smooth number, with no jumps at table edges. It stays reasonable in warm water and at higher TDS, where tables begin to struggle.
We do not publish the coefficients. They come from our own work with real pools and hot tubs, and we keep improving them.
LSI at every visit
PoolHealth calculates the LSI at every visit, as part of day-to-day maintenance, not only when you run a full test.
Some readings change from visit to visit: pH and water temperature. Others should stay stable for weeks: alkalinity (TA),1 calcium hardness (CH), CYA and total dissolved solids (TDS). At a regular visit you only enter what you actually measured. For the stable values, PoolHealth uses the last known reading, so the index is still calculated and you are not asked to repeat tests that would give you the same number.
Once a month we ask you to measure all the values again. It does two things. It keeps the last known values close to the real water. And it lets you see the trend over a longer period: a calcium hardness (CH) that creeps up, or CYA that builds as stabilised chlorine is added, shows in the history long before it becomes a problem in the water.
1 We recommend measuring total alkalinity (TA) at every visit. It is one of the main inputs of the index, and unlike the other stable values it moves with what you do to the pool and with the water itself. Acid additions lower it, and the source water (mains, well or desalinated) sets where it starts and what it becomes after every top-up.
Where accuracy is lower
No LSI calculation is equally accurate everywhere. Our method is most reliable across the range of readings found in ordinary pools and hot tubs. At the extremes, with very low or very high alkalinity (TA), calcium hardness (CH), dissolved solids (TDS) or temperature, the error is higher. There are two reasons. The empirical correction for ion pairs was fitted on typical readings, and the approximations used for dissolved salts become less exact as the water becomes more concentrated.
Such readings are rare in practice. If one of yours falls far outside the usual range, retest first: an extreme value is often a measurement error. Treat the index as a rough guide in that case.
Read it in context
A balanced LSI does not guarantee balanced water. Depending on the type of chemistry you use, the season and the condition of the pool system, the water can still be aggressive or scale-forming. A pool that has just been drained and refilled, one with fresh plaster or an old heater, and one that is heated all summer or left cold all winter can each behave differently at the same index. We recommend reading the LSI in that context, not as a verdict on its own.
The LSI is also only as good as the readings behind it, so test carefully and use a photometer and a TDS meter where you can. On its own it tells you nothing about disinfection, equipment or the chemicals you use. That is why PoolHealth shows it next to the rest of the picture and turns it into a step for the day.
Sources
- W. F. Langelier (1936). The analytical control of anti-corrosion water treatment. Journal of the American Water Works Association, 28(10), 1500-1521
- J. A. Wojtowicz (1997). Swimming Pool Water Balance, Part 7: A revised and updated saturation index equation. Journal of the Swimming Pool and Spa Industry, vol. 3, no. 1, pp. 28-34
- J. A. Wojtowicz (1997). Swimming Pool Water Balance, Part 9: Corrections, potential errors, and significance of the saturation index. Journal of the Swimming Pool and Spa Industry, vol. 3, no. 1, pp. 37-40
- L. N. Plummer and E. Busenberg (1982). The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90 C. Geochimica et Cosmochimica Acta 46(6), 1011-1040
This article is general information, not professional advice. Always follow product labels and the rules that apply where you live.
Spotted something outdated or inaccurate? Tell us at info@pool-health.com and we will check and correct it. Last reviewed: November 2026.