Langelier Saturation Index Calculator
Compute the Langelier Saturation Index (LSI) from pH, temperature, TDS, calcium hardness and alkalinity to see if water scales or corrodes.
How it works
LSI = pH − pHs · pHs = (9.3 + A + B) − (C + D) · A=(log₁₀TDS−1)/10 · B=−13.12·log₁₀(°C+273.15)+34.55 · C=log₁₀[Ca]−0.4 · D=log₁₀[alk]
The Langelier Saturation Index measures whether water is in balance with calcium carbonate. It compares the actual pH with the saturation pH (pHs), the pH at which the water would neither dissolve nor deposit calcium carbonate given its temperature, dissolved solids, calcium hardness and alkalinity. A positive LSI means the water is over-saturated and tends to lay down scale; a negative LSI means it is aggressive and tends to corrode metal and etch plaster. Values within about ±0.1 to ±0.3 are considered balanced.
Worked example
At pH 7.5, 25 °C, 500 ppm TDS, 300 ppm calcium and 100 ppm alkalinity: A = 0.170, B = 2.088, C = 2.077, D = 2.000, so pHs = (9.3 + 0.170 + 2.088) − (2.077 + 2.000) = 7.48. LSI = 7.5 − 7.48 = +0.02 — essentially balanced.
Frequently asked questions
What LSI range is safe?
For pools and most water systems, aim for −0.3 to +0.3, with 0 being ideal balance. Below −0.3 the water is corrosive and dissolves plaster and metal; above +0.3 it deposits scale on surfaces and equipment.
Which factor moves the index most?
pH and temperature have the largest swing: warmer water and higher pH both push the LSI toward scaling. Because alkalinity and calcium enter as logarithms, doubling them shifts the index by only about 0.3, whereas a 0.5 pH change shifts it 0.5 directly.
Is this the same as the Ryznar index?
They share the saturation-pH concept but differ in formula and reading. The Ryznar Stability Index uses 2·pHs − pH and is always positive, whereas the Langelier index is a simple pH − pHs difference that is negative when corrosive.
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This calculator is for educational purposes. Double-check important results before acting on them.