Pool LSI Calculator

Q: What is a good LSI for a swimming pool?

A balanced pool has an LSI between −0.3 and +0.3. Below −0.3 the water is corrosive and can etch plaster and corrode metal; above +0.3 it is scale-forming and can deposit calcium on surfaces and equipment. Aim to keep the LSI as close to 0.0 as practical.

Q: How does cyanuric acid (CYA) affect the LSI?

Cyanuric acid contributes to your total alkalinity reading but not to the carbonate alkalinity that actually buffers the water, so it must be subtracted. This calculator applies the standard correction: adjusted alkalinity = total alkalinity − (CYA ÷ 3). Higher CYA lowers your effective alkalinity and therefore lowers the LSI.

Q: How is the LSI calculated?

The Langelier Saturation Index is LSI = pH + TF + CF + AF − TDS constant, where TF is a temperature factor, CF a calcium-hardness factor, AF an alkalinity factor (using CYA-corrected alkalinity), and the TDS constant is 12.1 for typical pools (12.2 for high-TDS or saltwater pools). This tool uses the same lookup factors as our LSI explainer guide.

Q: What does a negative LSI mean?

A negative LSI (below −0.3) means the water is under-saturated with calcium carbonate — corrosive water. It can dissolve plaster and grout, pit concrete, and corrode metal fittings, ladders, and heaters. Raise the LSI by increasing pH, total alkalinity, or calcium hardness.

Q: What does a high LSI mean?

A high LSI (above +0.3) means the water is over-saturated — scale-forming water. It can deposit calcium scale on tile, plaster, salt cells, and heat exchangers, and make the water cloudy. Lower the LSI by reducing pH, total alkalinity, or calcium hardness.

Q: Is this LSI calculator free?

Yes — it is completely free and runs entirely in your browser. For automatic LSI on every pool, plus chemical dosing recommendations priced from your own inventory, the PoolFlow iOS app calculates this for you in the field, even offline.

Calculate the Langelier Saturation Index in real time with CYA-corrected alkalinity. Slide your readings and watch the balance update instantly.

Balanced water sits between −0.3 and +0.3 LSI.

Your Water Readings

Water Temperature

Calcium Hardness ppm

Total Alkalinity ppm

Cyanuric Acid (CYA) ppm

Total Dissolved Solids ppm

Your LSI

+0.00

Balanced

How it adds up

Temperature factor (TF): +0.00
Calcium factor (CF): +0.00
Alkalinity factor (AF): +0.00
CYA-corrected alkalinity: 0 ppm
TDS constant: −12.1

Get exact dosing in the app

For guidance only. Results use industry-standard lookup factors with linear interpolation and may differ slightly from other calculators. Always confirm with a reliable test kit before adding chemicals.

How the LSI Calculation Works

The Langelier Saturation Index (LSI) predicts whether your pool water will corrode surfaces, stay balanced, or form scale. It is the standard pool-industry measure of calcium carbonate saturation, and it is calculated from six readings:

LSI = pH + TF + CF + AF − TDS constant

pH — entered directly.
TF (temperature factor) — warmer water raises the LSI.
CF (calcium factor) — from calcium hardness.
AF (alkalinity factor) — from CYA-corrected alkalinity.
TDS constant — 12.1 for typical pools, 12.2 for high-TDS or saltwater pools.

The CYA correction

Cyanuric acid inflates your total alkalinity reading without buffering the water, so it has to be removed first:

Adjusted alkalinity = Total alkalinity − (CYA ÷ 3)

For example, 100 ppm total alkalinity with 60 ppm CYA gives an adjusted alkalinity of 100 − 20 = 80 ppm, which is what feeds the alkalinity factor above.

Reading your result

Below −0.3: corrosive — raise pH, alkalinity, or calcium.
−0.3 to +0.3: balanced — maintain.
Above +0.3: scale-forming — lower pH, alkalinity, or calcium.

Want the full breakdown, the factor tables, and worked examples? Read The Langelier Saturation Index, explained, or see our pool water balance guide.

LSI Calculator FAQ

What is a good LSI for a swimming pool?

How does cyanuric acid (CYA) affect the LSI?

How is the LSI calculated?

What does a negative LSI mean?

What does a high LSI mean?

Is this LSI calculator free?

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