Pool LSI calculator (Langelier Saturation Index)
The Langelier Saturation Index (LSI) combines pH, temperature, calcium hardness, alkalinity and dissolved solids into one number. It tells you whether the water tends to dissolve calcium from plaster and grout, or deposit it as scale.
Reading the result
| LSI | What it means |
|---|---|
| Below −0.3 | Corrosive. The water is undersaturated with calcium and tends to dissolve it: etched or rough plaster, eaten grout, and more corrosion of metal parts. |
| −0.3 to +0.3 | Balanced. The water neither dissolves nor deposits much calcium. |
| Above +0.3 | Scale-forming. Calcium tends to come out of the water as white scale on tile, in heaters and on salt cell plates, and the water may turn cloudy. |
The pH in the "balanced pH" box is the pH at which your water would sit at exactly zero. If your LSI is off, changing pH is usually the quickest fix, since a change of 0.1 in pH moves the LSI by 0.1.
The formula
pHs = (9.3 + A + B) − (C + D)
A = (log₁₀ TDS − 1) ÷ 10
B = −13.12 × log₁₀(water temperature in kelvin) + 34.55
C = log₁₀(calcium hardness) − 0.4
D = log₁₀(carbonate alkalinity)
Carbonate alkalinity ≈ total alkalinity − CYA ÷ 3
This is the standard Langelier equation used in water treatment. Pool tests measure total alkalinity, but part of it comes from cyanurate (the stabilizer), which doesn't take part in calcium carbonate balance. Subtracting about a third of the CYA reading is the usual pool-industry correction at normal pH.
Many pool guides use a simplified version with a lookup table of "factors" and a constant of 12.1. It gives nearly the same answer. This calculator uses the full equation, so you can enter any temperature.
Worked example
pH 7.5, 80 °F water (299.8 K), calcium hardness 300 ppm, total alkalinity 100 ppm, CYA 30 ppm, TDS 1,000 ppm.
- Carbonate alkalinity: 100 − 30 ÷ 3 = 90 ppm.
- A = (3 − 1) ÷ 10 = 0.20. B = −13.12 × 2.477 + 34.55 ≈ 2.05. C = 2.477 − 0.4 ≈ 2.08. D = log₁₀ 90 ≈ 1.95.
- pHs = 9.3 + 0.20 + 2.05 − 2.08 − 1.95 ≈ 7.52. LSI = 7.5 − 7.52 ≈ −0.02: balanced.
Cool the same water to 60 °F and the LSI drops by about 0.2, because cold water holds more calcium carbonate in solution. Heating does the opposite, which is why scale forms first inside heaters.
What to enter for TDS
Total dissolved solids include everything dissolved in the water. For a chlorine pool filled with ordinary tap water, 500–1,500 ppm is typical, and 1,000 ppm is a reasonable guess. For a saltwater pool, add the salt level: a 3,200 ppm salt pool is roughly 4,000 ppm TDS or more. TDS has only a small effect: going from 1,000 to 4,000 ppm lowers the LSI by about 0.06.
Frequently asked questions
What's the CSI, and how is it different?
The Calcite Saturation Index (CSI) is a more detailed version that also accounts for ionic strength and borates, and is used by some pool chemistry apps. For normal pool water the two are usually close, and either is a good guide.
How can I raise a low LSI?
Raise pH toward the top of your range, raise alkalinity, or raise calcium hardness. Calcium is the lasting fix for soft-water plaster pools, since it doesn't drift the way pH does.
How can I lower a high LSI?
Lower pH, and lower alkalinity if it's high. Very hard water may need a partial drain and refill with softer water.