How do you calculate the LSI?

Add the pH to the alkalinity factor and the combined calcium-and-temperature factor, then subtract the saturation constant. With pH 7.5, factors of 2.0 and 2.6, and a -12.1 constant, the simplified index is 12.1 in this raw sum before interpretation against the balanced band.

What is a good LSI for a pool?

A balanced pool sits near zero, roughly between -0.3 and +0.3. Above that range the water tends to scale; below it the water becomes corrosive.

What happens if LSI is too high?

Positive LSI means the water is oversaturated and will deposit calcium scale on surfaces, tile lines, and heaters, clouding water and reducing heat transfer.

What happens if LSI is too low?

Negative LSI means the water is undersaturated and aggressive, etching plaster, dissolving grout, and corroding metal components over time.

Why is the saturation baseline entered as a negative number?

The saturation constant is subtracted from the sum of the other factors, so entering it as a negative value, like -12.1 for freshwater, makes the arithmetic add up correctly.

LSI vs total alkalinity, which matters more?

Alkalinity is one input to the LSI. LSI is the overall verdict because it combines pH, alkalinity, calcium, and temperature into a single balance number, so it is the figure to manage.

Pool, Spa & Water Treatment Chemistry calculator

LSI Water Balance Calculator

The Langelier Saturation Index (LSI) is the industry standard for judging whether pool or spa water is balanced, scale-forming, or corrosive. Operators and water-treatment techs use it to protect plaster, tile, grout, and heat exchangers, since water outside the balanced band either deposits scale or dissolves surfaces. This simplified version sums the pH reading with factors for total alkalinity, calcium hardness and temperature, then subtracts a saturation constant. Holding LSI near zero, roughly between -0.3 and +0.3, is the goal for durable, comfortable water.

What this calculator does

Estimate a simplified water-balance index from pH, alkalinity factor, calcium factor, temperature factor, and TDS factor.
Use it to screen scale or corrosion risk before deeper LSI review.
It computes a simplified Langelier Saturation Index by summing pH and the alkalinity, calcium, and temperature factors and subtracting a saturation constant.

Formula used

Simplified LSI = pH + alkalinity factor + calcium and temperature factor + saturation baseline (negative)

Inputs explained

Measured water pH: Use current pH reading.
Total alkalinity factor: Use the factor from your selected LSI method. For 100 ppm total alkalinity, about 1.9 is typical.
Calcium hardness and temperature factor: Combine calcium hardness and temperature correction for the selected method. For 300 ppm Ca at 80 F, about 2.5 is typical.
Saturation constant: Enter as a negative number. Use -12.1 for standard freshwater conditions. This value is subtracted from the sum.

How to use the result

Use it to verify water balance after adjusting pH, alkalinity, calcium hardness, or when water temperature changes seasonally.
It is a simplified index using lookup factors, so it is only as accurate as the factor values you enter and does not replace a full method-specific LSI calculation.

Current U.S. benchmarks

Global copper trades at $13,552 per tonne (IMF via FRED, Jun 2026), up 37.8% in a year, and U.S. industrial electricity averages 8.66 cents per kWh. Both feed electrified-hardware unit economics.

Common questions

How do you calculate the LSI? Add the pH to the alkalinity factor and the combined calcium-and-temperature factor, then subtract the saturation constant. With pH 7.5, factors of 2.0 and 2.6, and a -12.1 constant, the simplified index is 12.1 in this raw sum before interpretation against the balanced band.
What is a good LSI for a pool? A balanced pool sits near zero, roughly between -0.3 and +0.3. Above that range the water tends to scale; below it the water becomes corrosive.
What happens if LSI is too high? Positive LSI means the water is oversaturated and will deposit calcium scale on surfaces, tile lines, and heaters, clouding water and reducing heat transfer.
What happens if LSI is too low? Negative LSI means the water is undersaturated and aggressive, etching plaster, dissolving grout, and corroding metal components over time.
Why is the saturation baseline entered as a negative number? The saturation constant is subtracted from the sum of the other factors, so entering it as a negative value, like -12.1 for freshwater, makes the arithmetic add up correctly.

Last reviewed 2026-05-12.