Langelier Saturation Index (LSI) calculator
Use 1,000 if unknown; add salt for salt pools.
Quick answer
The Langelier Saturation Index shows whether pool water will dissolve or deposit calcium carbonate. LSI = pH − pHs, where pHs is calculated from temperature, calcium hardness, carbonate alkalinity and TDS. Between −0.3 and +0.3 is balanced; below −0.3 the water is corrosive and etches plaster; above +0.3 it forms scale.
- Balanced: −0.3 to +0.3. Corrosive: below −0.3. Scaling: above +0.3.
- pH has the biggest effect — 0.1 pH ≈ 0.1 LSI.
- Use carbonate alkalinity: total alkalinity minus about CYA ÷ 3.
- Hot spas scale more easily than pools at the same chemistry.
The LSI formula
LSI = pH − pHs, where pHs = (9.3 + A + B) − (C + D):
- A = (log₁₀ TDS − 1) ÷ 10
- B = −13.12 × log₁₀(°C + 273) + 34.55
- C = log₁₀(calcium hardness as CaCO₃) − 0.4
- D = log₁₀(carbonate alkalinity as CaCO₃)
| Example water | LSI | Result |
|---|---|---|
| pH 7.5, 82°F, CH 250, TA 90, CYA 30, TDS 1,000 | −0.13 | Balanced |
| pH 7.2, 78°F, CH 200, TA 80, no CYA | −0.57 | Corrosive |
| pH 7.8, 84°F, CH 400, TA 120, no CYA | +0.57 | Scale-forming |
| Spa: pH 7.5, 104°F, CH 200, TA 80, TDS 1,500 | −0.02 | Balanced |
How to correct an out-of-balance LSI
- Corrosive (negative): raise pH toward 7.6, raise calcium hardness with calcium chloride, or raise alkalinity with sodium bicarbonate — use the chemical calculator.
- Scaling (positive): lower pH or alkalinity with acid (pH calculator), or dilute very hard water.
- Cal-hypo pools gain calcium with every dose; check LSI monthly.
- Log it. Record LSI with your weekly balance tests so trends are visible.
How to calculate the Langelier Saturation Index
- Test the water. Measure pH, temperature, calcium hardness, total alkalinity, CYA and TDS.
- Correct alkalinity. Carbonate alkalinity = total alkalinity − about CYA ÷ 3.
- Calculate pHs. pHs = (9.3 + A + B) − (C + D) using the formulas above.
- Find LSI. LSI = measured pH − pHs. Adjust if it falls outside −0.3 to +0.3.
Frequently asked questions
What is a good LSI for a pool?
Between −0.3 and +0.3. Many commercial operators aim for 0 to +0.3 to protect plaster and grout, while avoiding scale on heaters.
Why subtract CYA from alkalinity for LSI?
Test kits count cyanurate as alkalinity, but it does not take part in calcium carbonate balance. At normal pool pH about one-third of the CYA reading shows up as alkalinity, so carbonate alkalinity ≈ total alkalinity − CYA ÷ 3.
Does temperature affect LSI?
Yes. Warmer water makes calcium carbonate less soluble, so the same chemistry has a higher LSI in a 104°F spa than in an 80°F pool.
What is the difference between LSI and CSI?
The Calcite Saturation Index (CSI) is a more detailed version that accounts for ionic strength and ion pairing. For normal pool chemistry the two give similar answers; LSI is the one taught in most CPO courses.
This page is an educational summary, not legal advice. Pool and spa codes differ by state and county and change over time — always confirm requirements with the health department that inspects your facility.