Check whether pool water is corrosive, balanced, or scale-forming. Enter your test results and the Langelier Saturation Index updates as you type.
Typical range 7.2 – 7.6
Measure the water, not the air
Typical range 200 – 400
Typical range 80 – 120
Enter 0 if the pool has no stabilizer
Small effect; salt pools read higher
Langelier Saturation Index
Between -0.3 and +0.3. The water is close to saturated with calcium carbonate, so it should neither etch surfaces nor leave scale.
LSI = pH + TF + CF + AF − TDSF
The Langelier Saturation Index (LSI) predicts whether water will dissolve calcium carbonate or deposit it. Water always pulls toward balance: if it holds less calcium carbonate than it could, it takes some from whatever it touches, like plaster, grout, and tile. If it holds more, the extra comes out as scale.
An LSI of 0 means the water is exactly saturated. Negative numbers mean it is hungry for calcium (corrosive); positive numbers mean it has more than it can hold (scale-forming). Most pool operators aim for -0.3 to +0.3.
pH has the most direct effect: LSI moves one-for-one with it, so raising pH by 0.2 raises LSI by 0.2. It is also the reading that drifts most between visits, which is why a pool can swing from balanced to scale-forming in a week.
Warm water holds less calcium carbonate, so LSI rises as the water heats up, roughly 0.1 for every 10°F. A pool that is balanced in winter can be scale-forming in summer or on a heated spa, and cold water leans corrosive.
More calcium pushes the water toward scale. The effect is logarithmic: doubling calcium hardness (say from 200 to 400 ppm) adds about 0.3 to LSI. Low-calcium water leans corrosive and pulls calcium out of plaster.
Alkalinity works like calcium: doubling it adds about 0.3. But a total alkalinity test also counts cyanurate from stabilizer, which does not form scale. The pool-industry correction subtracts one third of the CYA reading to get carbonate alkalinity, and that is the number this calculator uses.
Dissolved solids slightly lower LSI. Going from 1,000 to 4,000 ppm lowers it by only about 0.06, so TDS rarely changes the reading, but salt pools, which carry several thousand ppm, should use their real number.
It uses the Carrier form of the Langelier equation, written the way pool references show it: LSI = pH + TF + CF + AF − TDSF. The temperature factor is 13.12 × log(water temperature in kelvin ÷ 273.15), the calcium factor is log(calcium hardness) − 0.4, the alkalinity factor is log(total alkalinity − CYA ÷ 3), and the TDS factor is about 12.1 for typical pool water, rising slightly with TDS.
Test kits, lookup charts, and apps round these factors differently, so use LSI as a guide and retest before making big adjustments. Want to track readings over time? Print the free weekly pool chemical log sheet.
LSI, the Langelier Saturation Index, is one number that tells you whether pool water tends to dissolve calcium carbonate (corrosive) or deposit it as scale. It combines pH, water temperature, calcium hardness, carbonate alkalinity, and total dissolved solids.
Between -0.3 and +0.3 is generally treated as balanced. Below -0.3 the water tends to be corrosive, and above +0.3 it tends to form scale.
A total alkalinity test also picks up cyanurate from stabilizer. The pool-industry correction subtracts one third of the CYA reading to estimate carbonate alkalinity, which is the alkalinity the LSI formula needs.
Calculators use different versions of the formula. Some use rounded lookup tables for temperature and TDS; this one uses the continuous Carrier equation. Results can differ by a tenth or more, so treat readings near -0.3 or +0.3 as borderline.
Skimlog has an LSI calculator and dosing suggestions built in, and saves each pool's readings with every service. Try it free for 14 days.
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