The LSI formula for pools
LSI = pH + TF + CF + AF − TDSF · CF = log₁₀(CH) − 0.4 · AF = log₁₀(TA − CYA ÷ 3) · TDSF = 12.1 below 1,000 ppm TDS, 12.2 from 1,000
The Langelier Saturation Index says whether water is saturated with calcium carbonate. At 0 it neither dissolves nor deposits it. Negative water is aggressive: it pulls calcium out of plaster and grout and corrodes heaters. Positive water lays scale on tile lines, in the heater and on salt cells. pH counts one to one, so 0.1 pH moves the index 0.1; doubling calcium or alkalinity moves it about 0.3. pH 7.5, 84 °F, CH 300, TA 100 and TDS 1,000 give +0.08; pH 7.2, 60 °F, CH 150, TA 80 and TDS 500 give −0.82.
Temperature factor (TF)
Warm water scales sooner. The factor runs from 0.0 at 32 °F to 0.9 at 105 °F, interpolated between the table points: 60 °F 0.4, 66 °F 0.5, 76 °F 0.6, 84 °F 0.7, 94 °F 0.8. A pool that is balanced at 70 °F in spring gains about 0.2 by 90 °F in summer.
| Water | 60 °F | 70 °F | 80 °F | 90 °F |
|---|---|---|---|---|
| pH 7.4, CH 250, TA 80 | −0.50 | −0.36 | −0.25 | −0.14 |
| pH 7.5, CH 300, TA 90 | −0.27 | −0.13 | −0.02 | +0.09 |
| pH 7.6, CH 350, TA 100 | −0.06 | +0.08 | +0.19 | +0.30 |
| pH 7.8, CH 400, TA 120 | +0.28 | +0.42 | +0.53 | +0.64 |
What to change
Too low: raise pH toward 7.6 with soda ash (pool pH calculator), then calcium (calcium hardness calculator) or alkalinity (baking soda calculator). Too high: lower pH and alkalinity with acid (muriatic acid calculator); very hard water only drops by replacing part of it.
Formula and factor tables: the standard pool form of the Langelier index (temperature, calcium and alkalinity factors, TDS constant 12.1/12.2) as used by Swim University and pool trade charts; CYA correction one third of CYA. Read October 23, 2026.
Frequently asked questions
What is a good LSI for a pool?
Between −0.3 and +0.3, and close to 0 is best. Below −0.3 the water is aggressive and slowly etches plaster, grout and metal; above +0.3 it tends to deposit calcium scale on tile, in the heater and on salt cells. Many plaster pool owners aim for 0 to +0.2.
How do I raise the LSI in my pool?
Raise pH first if it is below 7.4, since it moves the index the most: 0.1 pH adds 0.1 LSI. Then raise calcium hardness with calcium chloride or alkalinity with baking soda. Doubling calcium from 150 to 300 ppm adds about 0.3. Warmer water also raises the index.
How do I lower the LSI in my pool?
Lower pH and total alkalinity with muriatic acid; that is the quickest route. If calcium is very high, replace part of the water with softer fill water, since no common chemical removes calcium. Remember that heating the pool raises the index, so a balanced pool in spring can scale in August.
Does stabilizer (CYA) affect the LSI?
Yes. Part of the total alkalinity you measure comes from cyanuric acid, which does not buffer against scale. This calculator subtracts about one third of the CYA from TA before taking the alkalinity factor, so 60 ppm CYA lowers the index by about 0.1 at TA 90.
Does salt affect the LSI?
A little. Salt raises total dissolved solids, and above 1,000 ppm TDS the index drops by about 0.1. A salt pool at 3,200 ppm uses the higher TDS factor here. The bigger effect in salt pools is pH drift upward and the hot salt cell, where scale forms first.