Mastering Pool Water Balance with LSI
Why the Langelier Saturation Index is the ultimate metric for pool health.
Water is a hungry solvent. If it does not carry enough dissolved calcium and carbonate, it will pull them out of your pool's plaster, grout, and heat-exchanger metal. If it carries too much, it dumps the excess as cloudy scale on tile and equipment. The Langelier Saturation Index (LSI) is the single number that tells you which way your water is leaning, and it is the metric our LSI calculator solves so you can stop guessing and start balancing on purpose.
The Six Factors of LSI
LSI is built from six water measurements: pH, total alkalinity, calcium hardness, cyanuric acid (CYA), water temperature, and total dissolved solids (TDS). Each one nudges the saturation point of calcium carbonate up or down. The calculator implements the standard formula LSI = pH − pHs, where the saturation pH (pHs) = 9.3 + A + B − C − D. Here A is a TDS term, (log10(TDS) − 1) / 10; B is a temperature term, −13.12 × log10(tempC + 273) + 34.55; C is a calcium term, log10(calcium hardness) − 0.4; and D is a corrected-alkalinity term, log10(TA − CYA/3). That CYA/3 correction matters in stabilized pools: high cyanuric acid borrows from your measured alkalinity, so a pool reading 100 ppm TA with 60 ppm CYA behaves like one with only 80 ppm of carbonate buffering.
Reading the Index
An LSI of 0.0 is perfect balance. The calculator flags water below −0.3 as corrosive (aggressive) and water above +0.3 as scaling, treating the −0.3 to +0.3 band as balanced. Corrosive water etches plaster, dissolves grout, and pits metal; scaling water clouds the pool and crusts heater elements. Staying inside the band is the cheapest equipment insurance you can buy, and it keeps the water comfortable for swimmers rather than aggressive on skin and eyes.
A Worked Example
Take a typical 20,000-gallon plaster pool at pH 7.5, 26 °C (about 79 °F), 250 ppm calcium hardness, 100 ppm total alkalinity, 1,200 ppm TDS, and 40 ppm CYA. The corrected alkalinity becomes 100 − 40/3 ≈ 86.7 ppm. Plugging those into the formula yields a pHs near 7.64, so the LSI lands at about −0.14 — comfortably inside the balanced band. Now drop the calcium hardness to 120 ppm, the kind of reading a fresh fill or a softener-fed pool can show, and the C term shrinks, pushing pHs up to about 7.96 and the index down to roughly −0.46, below the −0.3 corrosive threshold. The water is now corrosive even though pH and alkalinity never changed, which is exactly why calcium hardness deserves a seat at the table.
Which Knob to Turn First
Because all six inputs interact, fix them in the order that is easiest to control. Bring total alkalinity into the 80–120 ppm range first, since it stabilizes pH and feeds the D term. Next set pH to 7.4–7.6, the swimmer comfort zone and a strong LSI lever. Then raise calcium hardness if it is low — calcium chloride is the standard additive and our calcium hardness calculator sizes the dose. Temperature you mostly accept rather than control, but knowing that warmer water scales more explains why your heater fouls before the pool does. Re-run the LSI calculator after each change so you can see the index walk toward zero instead of overcorrecting blind.
FAQ
Does temperature affect LSI?
Yes, significantly. The temperature term (B) rises with heat, so warmer water becomes more scaling at the same pH and alkalinity. This is why scale appears in heaters and on solar panels first — the water is hottest there — even when the bulk pool reads balanced.
Why does CYA lower my effective alkalinity in the LSI math?
Cyanuric acid contributes to a total-alkalinity titration but does not buffer calcium carbonate saturation the way bicarbonate does. The formula subtracts CYA/3 to estimate the true carbonate alkalinity, so a stabilized pool needs a slightly higher measured TA to reach the same LSI as an unstabilized one.
What is the fastest way to rescue corrosive water?
If the index is below −0.3, the quickest safe move is usually raising calcium hardness and total alkalinity, since both push pHs down and lift the index without touching pH. Add calcium chloride and sodium bicarbonate in measured doses, circulate, retest, and recompute the LSI before adding more.