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LSI Calculator for Pools (Water Balance)

Langelier Saturation Index from pH, temperature, calcium, alkalinity and CYA: corrosive, balanced or scaling.
pH
Water temperature (°F)
Calcium hardness (ppm)
Total alkalinity (ppm)
Stabilizer, CYA (ppm)
Optional. Part of the alkalinity reading is cyanurate; the app takes it off.
LSI
+0.2 · balanced
Between −0.3 and +0.3: kind to plaster, metal and heaters.
Carbonate alkalinity
100 ppm
Factors (temperature, calcium, alkalinity)
0.7 · 2.08 · 2
Estimates. Follow the label on your product; it wins.

How it works

The LSI tells whether your water will etch plaster and metal (negative) or leave scale (positive). Pool water is balanced between −0.3 and +0.3.
LSI = pH + TF + CF + AF − 12.1. TF comes from the temperature (84 °F → 0.7), CF = log10(calcium) − 0.4, AF = log10(carbonate alkalinity). With stabilizer, take about a third of the CYA off the alkalinity first (0.30 × CYA at pH 7.4).
Example: pH 7.5, 84 °F, calcium 300, alkalinity 100 → 7.5 + 0.7 + 2.1 + 2.0 − 12.1 = +0.2, balanced. pH 7.2, 60 °F, calcium 150, alkalinity 75 → −0.8, corrosive.

Frequently asked questions

What is a good LSI for a pool?

Between −0.3 and +0.3. Close to 0 is ideal; plaster pools prefer slightly positive.

How do I raise a low LSI?

Raise calcium (calcium chloride), alkalinity (baking soda) or pH, in that order. Warmer water also raises it.

Why does CYA change the LSI?

A total alkalinity test also counts cyanurate, which does not buffer like carbonate. At pH 7.4 about 30 % of the CYA shows up as alkalinity, so 100 ppm CYA makes a TA of 100 really 70.