Pool scale, etching & water balance (LSI)
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Crusty scale means your water balance (LSI) is too high; rough, etched plaster means it’s too low. The Langelier Saturation Index combines pH, temperature, calcium, and alkalinity into one number. Keep it near zero.
How the LSI is calculated
The MAHC Annex writes it as LSI = pH + TF + CF + AF − TDSF. TF, CF and AF are factors for temperature, calcium hardness and alkalinity from the table below, and TDSF is 12.1 when dissolved solids are under 1,000 ppm or 12.2 above. Example: pH 7.5 at 84°F (TF 0.7), calcium 300 ppm (CF 2.1), alkalinity 100 ppm (AF 2.0) and TDS under 1,000 gives 7.5 + 0.7 + 2.1 + 2.0 − 12.1 = +0.2, which is balanced.
What each side looks like
The MAHC Annex describes scale as white or lightly colored rough blotches on the walls that also build up on objects, piping and the filter. Low LSI water causes etching, pitting, dissolving and staining of walls, grout and plumbing, and can irritate swimmers.
LSI factors (MAHC Annex Table 5.7.4.6)
Use the factor closest to your reading. Calcium continues to 2,500 ppm (CF 3.0) in the Annex table.
| Temperature | TF | Calcium (ppm) | CF | Alkalinity (ppm) | AF |
|---|---|---|---|---|---|
| 32°F / 0°C | 0.0 | 5 | 0.3 | 5 | 0.7 |
| 37°F / 3°C | 0.1 | 25 | 1.0 | 25 | 1.4 |
| 46°F / 8°C | 0.2 | 50 | 1.3 | 50 | 1.7 |
| 53°F / 12°C | 0.3 | 75 | 1.5 | 75 | 1.9 |
| 60°F / 16°C | 0.4 | 100 | 1.6 | 100 | 2.0 |
| 66°F / 19°C | 0.5 | 150 | 1.8 | 150 | 2.2 |
| 76°F / 24°C | 0.6 | 200 | 1.9 | 200 | 2.3 |
| 84°F / 29°C | 0.7 | 300 | 2.1 | 300 | 2.5 |
| 94°F / 34°C | 0.8 | 400 | 2.2 | 400 | 2.6 |
| 105°F / 41°C | 0.9 | 800 | 2.5 | 800 | 2.9 |
| 128°F / 53°C | 1.0 | 1,000 | 2.6 | 1,000 | 3.0 |
Step by step
- Check your LSITest pH, temperature, calcium hardness, alkalinity and, if you can, TDS, then use the factors above or let the DoseMyPool app compute it. If you use stabilizer outdoors, the MAHC Annex recommends subtracting 30% of your CYA from the alkalinity reading first.
- Lower pH first for scaleAbove +0.3, pH is the fastest lever. The MAHC Annex recommends bringing pH to 7.2–7.6 and alkalinity to 60–180 ppm, and notes that lowering alkalinity usually lowers pH too. Below −0.3, raise pH, alkalinity or calcium instead.
- Fix calcium lastThe MAHC Annex puts calcium at 200–400 ppm for plaster and 80–200 ppm for vinyl or fiberglass. Too high comes down only with a partial drain and refill, and when fill water is hard the Annex suggests working pH and alkalinity instead.
FAQ
What LSI is balanced?
The MAHC Annex treats −0.3 to +0.3 as balanced. Negative is corrosive and positive is scale-forming, and a small positive value is better than a negative one because a thin scale layer does less damage than corrosion.
Does stabilizer affect the LSI?
Yes. The MAHC Annex notes that LSI falls as stabilizer and dissolved solids rise, and recommends subtracting 30% of your CYA from the alkalinity reading before you calculate it in an outdoor stabilized pool.
Sources
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Sections 5.7.3.4 and 5.7.4.6: LSI formula, factor table, range, effects and adjustments
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Section 5.7.4.4.1: alkalinity 60–180 ppm
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