Pool LSI Calculator (Langelier Saturation Index / Water Balance)

Between −0.3 and +0.3, with 0 best. Above +0.3 the water tends to deposit scale and turn cloudy; below −0.3 it tends to etch plaster and corrode metal. Maryland and New York public-pool rules accept −0.5 to +0.5.

Add pH, the temperature factor, the calcium factor and the alkalinity factor, then subtract 12.1. Example from a county health handout: pH 7.6, 86 °F (0.8), hardness 380 ppm (2.2), alkalinity 70 ppm (1.9): 7.6 + 0.8 + 2.2 + 1.9 − 12.1 = +0.4, slightly scale-forming.

The health-department method rounds each reading up to the next table row, which can add about 0.1 per factor. Interpolating between rows gives +0.26 for the same example. Use the table method when you need to match an inspector's worksheet.

Lower pH or alkalinity first; they are the easiest to move. The calculator shows the pH that would bring the index to 0 with your other readings unchanged (7.2 in the example). Use the pool alkalinity calculator for acid doses.

The state formula uses total alkalinity as tested and has no stabilizer term. Some pool industry calculators adjust alkalinity for cyanuric acid, so their result can be a little lower.