The LSI formula
LSI = pH + TF + CF + AF - 12.1
TF, CF and AF are the temperature, calcium hardness and alkalinity factors from the tables below. AF uses carbonate alkalinity: TA minus about a third of your CYA. The 12.1 constant assumes about 1,000 ppm TDS and rises to about 12.2 at 3,000 ppm.
Worked example: pH 7.6, 82°F, calcium 300 ppm, TA 90 ppm, CYA 50 ppm. Carbonate alkalinity is 90 - 50 x 0.33 = 73.5 ppm. TF is 0.68, CF is 2.1 and AF is 1.88, so LSI = 7.6 + 0.68 + 2.1 + 1.88 - 12.1 = 0.16. Balanced. Without the CYA correction the same water reads 0.24, closer to scaling than it really is.
LSI factor tables
| °F | Factor |
|---|---|
| 32 | 0.0 |
| 37 | 0.1 |
| 46 | 0.2 |
| 53 | 0.3 |
| 60 | 0.4 |
| 66 | 0.5 |
| 76 | 0.6 |
| 84 | 0.7 |
| 94 | 0.8 |
| 105 | 0.9 |
| ppm | Factor |
|---|---|
| 25 | 1.0 |
| 50 | 1.3 |
| 75 | 1.5 |
| 100 | 1.6 |
| 150 | 1.8 |
| 200 | 1.9 |
| 300 | 2.1 |
| 400 | 2.2 |
| 800 | 2.5 |
| ppm | Factor |
|---|---|
| 25 | 1.4 |
| 50 | 1.7 |
| 75 | 1.9 |
| 100 | 2.0 |
| 150 | 2.2 |
| 200 | 2.3 |
| 300 | 2.5 |
| 400 | 2.6 |
| 800 | 2.9 |
The calculator interpolates between rows instead of rounding to the nearest one.
What your LSI means
| LSI | Water | What you see |
|---|---|---|
| Below -0.5 | Very corrosive | Etched or rough plaster, pitting, grout loss, metal staining from corroded heaters and rails. |
| -0.5 to -0.3 | Mildly corrosive | Slow etching, dull plaster, early staining. |
| -0.3 to +0.3 | Balanced | No etching or scale. Aim for 0.0 to +0.2 on plaster. |
| +0.3 to +0.5 | Mildly scaling | Cloudy water, white crust at the waterline, a scaled salt cell. |
| Above +0.5 | Heavy scaling | Rough surfaces, scale in heaters and pipes, a salt cell that stops making chlorine. |
More on the symptoms: pool staining, white flakes in pool water and calcium hardness too high.
LSI for saltwater pools
Inside a salt cell the water is warmer and the pH is higher than in the pool, so a pool that reads +0.2 can be scaling inside the cell. Keep salt pools at -0.2 to 0.0, enter your salt reading as TDS, and keep CYA at 60 to 80 ppm (check it with the stabilizer calculator and salt with the salt calculator).
Plaster vs vinyl vs fiberglass
Plaster, pebble and gunite are made of calcium carbonate, so corrosive water dissolves them: keep them at 0.0 to +0.3. Vinyl and fiberglass have nothing to etch, but corrosive water still attacks heaters, ladders and pump seals, and scaling water still stains fiberglass; -0.3 to 0.0 is fine. Read more in plaster vs pebble.
LSI vs RSI vs CSI
LSI tells you which way water leans (negative corrodes, positive scales). The Ryznar Stability Index (RSI = 2 x pHs - pH) reads the other way: 6 to 7 is balanced, below 6 scales, above 7 corrodes. The Calcium Saturation Index (CSI) is a refined LSI used by some pool professionals; it reads like LSI. The calculator shows LSI and RSI side by side. For the background, read the LSI explained.