LSI Calculator

The saturation index says whether pool water is holding as much calcium carbonate as it can, more than it can, or less. Water that holds less is hungry for calcium, and it takes it from plaster, grout and pebble finishes, which shows up as etching and roughness. Water that holds more drops the excess as scale on tile, in salt cells and inside heaters.

Zero is perfect saturation. The trade treats anything from −0.3 to +0.3 as balanced, the bands out to ±0.5 as leaning one way or the other, and anything beyond ±0.5 as corrosive or scaling. The scale is logarithmic, so the steps are bigger than they look: water at −0.3 holds about half the calcium carbonate it could, and water at −0.6 holds about a quarter.

This calculator uses the revised saturation index equation John Wojtowicz published in 2001, which is the same equation the Pool Runs technician app uses for every pool:

CH is calcium hardness, TA is total alkalinity, TDS is total dissolved solids, and all are in ppm. The temperature term is a continuous line rather than the stepped table older calculators use, so a degree of difference never jumps the index. The last term combines the equation's constant with a correction for how much else is dissolved in the water.

CA is carbonate alkalinity. A total alkalinity test counts part of any cyanuric acid as alkalinity, so the calculator subtracts it using the factors from the PHTA alkalinity fact sheet, interpolating between them for pH in between:

pH is the fastest and cheapest lever, because it enters the equation directly: raising pH by 0.1 raises the index by about 0.1. Alkalinity and calcium enter as logarithms, so they need bigger changes for the same effect. Calcium is also the slowest to fix, because calcium chloride only raises it and the only way down is draining.

Consider a plaster pool filled with soft water, running pH 7.2, 78°F, calcium 150, alkalinity 70 and cyanuric acid 50. The index is −0.78, which is corrosive. pH alone would need to reach 8.0 to balance it, which is outside the usual 7.4 to 7.6 range. Taking pH to 7.5 brings the index to −0.49, raising alkalinity to 90 brings it to −0.36, and raising calcium to 250 brings it to −0.14, which is balanced and keeps every reading in its normal range.

Enter those targets in the Target column above, add your pool volume, and the calculator lists the doses in the order to add them, with the amount for every product that does the job. The pool chemical calculator does the same for chlorine and salt as well, and the muriatic acid calculator covers the opposite case, when the index is too high and pH or alkalinity needs to come down. For a salt pool, enter the salt reading plus about 500 as the dissolved solids, because salt makes up most of what is dissolved in that water.