What "balanced water" means
Water dissolves things until it is saturated. If pool water is under-saturated with calcium carbonate, it takes calcium from wherever it can: plaster, grout, concrete and tile bedding, leaving etched, rough surfaces, and it is more corrosive to heaters, ladders and pump seals. If it is over-saturated, calcium carbonate comes out of the water as scale on tiles, in salt cells and inside heater tubes, and the water can turn cloudy. Balanced water sits close to the saturation point, where it neither dissolves nor deposits.
The Langelier Saturation Index (LSI), developed by Wilfred Langelier in 1936 for drinking-water systems, is the standard way to put a number on this. The pool industry uses a simplified version with lookup factors for temperature, calcium and alkalinity:
LSI = pH + TF + CF + AF − C
TF = temperature factor · CF = log₁₀(calcium hardness) − 0.4
AF = log₁₀(carbonate alkalinity) · carbonate alkalinity = total alkalinity − cyanuric acid × correction
C = 12.1 (TDS below 1,000 ppm) or 12.2 (TDS 1,000 ppm and above)
Reading the result
| LSI | Tendency | Typical signs |
|---|---|---|
| Below −0.3 | Corrosive / aggressive | Etched plaster, pitted grout, metal staining, wrinkling vinyl |
| −0.3 to +0.3 | Balanced | Clear water, surfaces stable |
| Above +0.3 | Scale forming | White crust on tiles, cloudy water, scaled salt cell or heater |
The −0.3 to +0.3 band is the range most commonly used in the pool trade. Some professionals aim for slightly positive values in plaster pools to protect the surface.
Worked examples
- Typical outdoor pool: pH 7.5, 82 °F, calcium hardness 250, alkalinity 100, stabilizer 40, TDS 500. TF 0.68, CF 2.00, carbonate alkalinity 87.2 (AF 1.94). LSI = 7.5 + 0.68 + 2.00 + 1.94 − 12.1 = +0.01, balanced.
- Soft water, low pH: pH 7.2, 78 °F, hardness 100, alkalinity 60, no stabilizer. LSI = −0.90, aggressive: raise hardness and alkalinity, then pH.
- Hot salt pool in summer: pH 7.9, 86 °F, hardness 450, alkalinity 150, stabilizer 30, TDS 3,500. LSI = +0.82, scale forming: the salt cell will scale quickly. Lowering pH and alkalinity is the quickest fix.
- Hot tub at 102 °F: pH 7.6, hardness 150, alkalinity 90, TDS 800. LSI = +0.10, balanced. Hot water raises the LSI, which is why spas scale more easily than pools with the same chemistry.
How to move the LSI
- pH changes the index one-for-one and is the quickest lever: lowering pH by 0.2 lowers LSI by 0.2.
- Alkalinity and calcium hardness change it on a log scale: doubling either adds about 0.3. Use the alkalinity and calcium hardness calculators for the doses.
- Temperature raises it: heating the pool from 78 °F to 88 °F adds roughly 0.1. Check balance again after you start heating.
- Stabilizer lowers the carbonate part of the alkalinity reading, so high CYA makes the water slightly more aggressive than the alkalinity test suggests.
Common mistakes
- Ignoring temperature. Water that is balanced in spring can scale in a heated summer pool.
- Forgetting TDS in salt pools. Salt pools are above 1,000 ppm TDS, which changes the constant from 12.1 to 12.2.
- Balancing only pH. Very soft water with "perfect" pH can still etch plaster.
The LSI is an indicator of tendency, not a guarantee. Follow your pool surface and equipment manufacturer's water-chemistry requirements.
Temperature factors used
| Water temperature | Factor |
|---|---|
| 32 °F (0 °C) | 0.0 |
| 46 °F (8 °C) | 0.2 |
| 60 °F (16 °C) | 0.4 |
| 66 °F (19 °C) | 0.5 |
| 76 °F (24 °C) | 0.6 |
| 84 °F (29 °C) | 0.7 |
| 94 °F (34 °C) | 0.8 |
| 105 °F (41 °C) | 0.9 |
The calculator interpolates between these values. The cyanuric acid correction subtracts about 0.22 × CYA at pH 7.0 rising to about 0.36 × CYA at pH 8.0 (about one-third at pH 7.5) from total alkalinity before taking the logarithm.
A balancing plan that works
- Set total alkalinity to its target.
- Set calcium hardness for your surface.
- Adjust pH last and fine-tune the LSI with it.
- Recheck after heating starts or after big rain.
Frequently asked questions
What is a good LSI for a pool?
Between −0.3 and +0.3 is commonly treated as balanced. Below −0.3 the water tends to etch and corrode; above +0.3 it tends to form scale.
How do I raise the LSI?
Raise pH, total alkalinity or calcium hardness. pH changes the index fastest; doubling alkalinity or hardness adds about 0.3.
Does temperature affect pool water balance?
Yes. Warmer water has a higher LSI. A hot tub at 102 °F scales more easily than a pool with the same chemistry at 80 °F.
Why does cyanuric acid matter for LSI?
Cyanuric acid shows up in the total alkalinity test. Only the carbonate part counts for the LSI, so about one-third of the CYA reading is subtracted from alkalinity.
Is a negative LSI bad?
Slightly negative (to about −0.3) is usually fine. Lower values mean the water tends to dissolve plaster and grout and corrode metal.
Sources
- Langelier, W. F. (1936). The analytical control of anti-corrosion water treatment. Journal of the American Water Works Association, 28(10).
- SPASA Australia: Water balance.
- Temperature, calcium and alkalinity factors as published in common pool water-chemistry references (e.g. Taylor Technologies' pool and spa water chemistry guide).
Last reviewed 28 October 2026. Results are estimates; see our disclaimer.