Using the LSI Calculator

Before You Calculate

Use pH, water temperature, calcium hardness, total alkalinity, cyanuric acid (CYA), and total dissolved solids measured from the same water at about the same time. Enter calcium hardness and total alkalinity as ppm as CaCO3 and TDS as ppm. If the alkalinity value has already been corrected for CYA, enter CYA as 0 so it is not subtracted twice.

LSI is a calcium carbonate saturation index. It does not measure sanitizer effectiveness, water clarity, metal corrosion, or the amount of scale that will form. EPA guidance cautions that LSI indicates the direction of calcium carbonate tendency but does not predict the actual precipitation amount or establish a complete corrosion assessment.[1]

Open LSI Calculator

The displayed Balance target pH is the same theoretical saturation pH. It is not automatically the correct operating target. Sanitizer requirements, swimmer comfort, surface type, local code, and the rest of the water balance still apply. LSI and saturation pH display to two decimal places; the status is assigned from the unrounded result.

Interpret The Sign And Status

Negative LSI means the water is undersaturated with calcium carbonate; positive LSI means it is supersaturated and tends toward calcium carbonate deposition. Zero is theoretical equilibrium. The MAHC Annex identifies −0.3 through +0.3 as the water-balance range for public aquatic venues.[3]

When PoolKit Checks The Readings

PoolKit does not report LSI when CYA-adjusted carbonate alkalinity is below 1 ppm and warns below 10 ppm, where small test errors can materially change the result. These are PoolKit calculation checks, not water-quality standards.

It also asks for review when pH is outside 6.8–8.6, temperature outside 50–110°F, calcium hardness outside 25–1,500 ppm, total alkalinity above 300 ppm, CYA above 200 ppm, or TDS outside 100–10,000 ppm. Treat these messages as prompts to confirm units and retest, not as recommended ranges or proof that a measurement is wrong.

The calculator can accept values outside routine operating guidance. For example, CDC recommends pool pH 7.0–7.8, which is narrower than PoolKit’s 6.8–8.6 review band.[4] Use the site plan and applicable local requirements for operating targets.

Use LSI As One Part Of The Review

Confirm any unexpected result with fresh tests before changing water chemistry. Review pH, calcium hardness, alkalinity, temperature, TDS, surface material, and recent treatment history together. At very low carbonate alkalinity or unusual TDS, temperature, CYA, or salt levels, the simplified equation becomes more sensitive to measurement and model assumptions.

Do not change pH solely to make the displayed Balance target pH match. Plan corrections around sanitizer performance, adopted requirements, product directions, and all affected readings.