Balance your pool water like a pro. Calculate LSI to identify corrosion or scaling risks. Protect your pool surfaces and mechanical equipment from chemical damage.
The Langelier Saturation Index needs five measured values plus water temperature, so gather a full test panel before you start. Measure pH (target 7.4-7.6), Total Alkalinity (TA, 80-120 ppm), and Calcium Hardness (CH, 200-400 ppm) with a reliable drop or photometer test. Read water temperature directly from the pool, not the air, because the same chemistry can read corrosive in 10C (50F) winter water and scaling in 30C (86F) summer water. Enter Cyanuric Acid (CYA) too: stabilizer ties up a fraction of your alkalinity, so the calculator subtracts a CYA correction from raw TA to get the carbonate alkalinity that actually drives the index. Test at a consistent time of day, ideally mid-morning before the sun outgasses carbon dioxide and pushes pH upward. For plaster, vinyl, and fiberglass pools the inputs are identical, but vinyl and fiberglass tolerate a slightly negative index better than bare plaster, which leaches calcium when starved.
LSI = pH + TF + CF + AF - 12.1, where TF is the temperature factor, CF the calcium-hardness factor, and AF the carbonate-alkalinity factor (each a logarithmic lookup the tool computes for you). Worked example: pH 7.5, water 27C/80F (TF approximately 0.7), CH 300 ppm (CF approximately 2.08), and carbonate alkalinity 100 ppm after CYA correction (AF approximately 2.0). LSI = 7.5 + 0.7 + 2.08 + 2.0 - 12.1 = +0.18. That sits inside the balanced window of -0.3 to +0.3, so the water neither dissolves plaster nor deposits scale. Now drop pH to 7.2: LSI falls to roughly -0.12, still safe but trending corrosive. Raising CH to 400 ppm lifts CF to about 2.20, nudging the index up about +0.12. Because each factor is logarithmic, a small pH change moves LSI most, which is why pH is your fastest correction lever.
LSI is the master gauge that protects costly surfaces and equipment, so accuracy compounds: a 0.2 pH test error can flip your result from safe to scaling. Negative water is corrosive, etching plaster, pitting metal heat exchangers, and staining; strongly positive water deposits hard calcium carbonate scale on tile, heaters, and salt cells that is laborious to remove. Common mistakes include entering air temperature, forgetting the CYA correction (which overstates usable alkalinity and hides a corrosive condition), and chasing a perfect 0.00 instead of the practical -0.3 to +0.3 band the index defines. Correct one variable at a time, add chemicals slowly with the pump running, and always retest after a full turnover before stacking another adjustment. In cold or winterized water, deliberately target the upper half of the range, since falling temperature drags LSI down toward corrosion overnight.
An LSI score between -0.3 and +0.3 is considered perfectly balanced. In this range, the water is neither corrosive nor scale-forming.
A negative score indicates corrosive water. You can raise it by increasing your pH, Total Alkalinity, or Calcium Hardness levels.
Yes. Warmer water increases the LSI (more likely to scale), while colder water lowers the LSI (more likely to be corrosive to plaster and metal).
The LSI is a single number that tells you whether your water is balanced, corrosive, or scale-forming. It combines pH, water temperature, calcium hardness, total alkalinity, and total dissolved solids into one figure. A value near zero (about -0.3 to +0.3) is balanced; a negative LSI is corrosive and can etch plaster and metal, while a positive LSI is scale-forming and deposits calcium. Adjust pH and alkalinity first, since they move the index the fastest.