How LSI is calculated
LSI = pH + TF + CF + AF − 12.1
- TF: temperature factor, from 0.0 at 32 °F up to 1.0 at 128 °F.
- CF: calcium factor = log₁₀(calcium hardness) − 0.4.
- AF: alkalinity factor = log₁₀(carbonate alkalinity).
- 12.1: constant for TDS up to 1,000 ppm. Use 12.2 above that, which covers most salt pools.
Why CYA matters
A total-alkalinity test also counts cyanurate from stabilizer, but cyanurate doesn't hold calcium in the water the way carbonate does. The calculator subtracts the cyanurate part, about one-third of the CYA reading at pH 7.5, before taking the log. Skipping this step makes stabilized pools look more scale-forming than they are.
Reading the result
| LSI | Water is… |
|---|---|
| Below −0.3 | Corrosive: etches plaster, corrodes metal |
| −0.3 to +0.3 | Balanced |
| Above +0.3 | Scale-forming: deposits calcium |
What to adjust
pH moves LSI fastest. Each 0.1 of pH is 0.1 of LSI, so it's usually the first thing to change. Calcium is the slow fix: raising it protects plaster in soft-water areas, and the only way to lower it is to drain. Temperature matters too. A pool that's balanced at 70 °F in spring can scale at 90 °F in August, especially in heaters and on salt cells, which run hotter than the pool. Use the chemical calculator for the dose that gets you there.