01
The purpose
What does this calculator estimate?
Is your water corrosive, balanced, or scaling? Enter your test results to get the LSI and what to fix.
Water Chemistry · B7
Langelier Saturation Index Calculator
Is your water corrosive, balanced, or scaling? Enter your test results to get the LSI and what to fix.
Measure the inputsAim for an LSI between −0.3 and +0.3; 0.0 to +0.1 is the safest target for plaster.
Read the full resultTemperature matters — the same water can be balanced in spring and scaling in a hot tub.
Check the boundaryIf LSI is negative (corrosive), raise calcium hardness or alkalinity, or nudge pH up.
Useful forWater ChemistryPlanningComparisonVerification
Live calculation
Water balance inputs
✓ Checked May 2026
Enter all five test values for an accurate index.
Recommended dose
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Enter the required values to calculate.
Enter the known values. Let the engine handle the calculation.
✓ Formula checked↻ Live result○ No account needed
Planning estimate only. Confirm product labels, equipment specifications, qualified-professional advice and local requirements before acting.
The result, explained
Use the Langelier Saturation Index Calculator result with context.
Is your water corrosive, balanced, or scaling? Enter your test results to get the LSI and what to fix.
02
The tool
What information does it use?
- pH: enter a measured or verified value
- Water temperature: Warmer water scales more easily.
- Calcium hardness: ppm
- Total alkalinity: ppm
- Total dissolved solids: ppm — affects the correction factor.
- Cyanuric acid: CYA reduces effective alkalinity.
03
The method
How to use this calculator
- pHEnter the value you can verify.
- Water temperatureWarmer water scales more easily.
- Calcium hardnessppm
- Total alkalinityppm
- Total dissolved solidsppm — affects the correction factor.
- Cyanuric acidCYA reduces effective alkalinity.
04
The relationship
The formula in plain English
LSI = pH + Temp Factor + Calcium Factor + Alkalinity Factor − 12.1 (+TDS correction)
CYA lowers effective alkalinity by about a third of its value before the alkalinity factor is applied.05
Worked through
A worked example: pH 7.5, 80°F, CH 250, TA 100, CYA 40
- Effective TA = 100 − (40 ÷ 3) ≈ 87 ppm
- Factors: pH 7.5 + TF 0.66 + CF 1.5 + AF 1.95 − 12.1
- LSI ≈ −0.49 → corrosive; raise CH and TA
| LSI value | Condition | Effect | Action |
|---|---|---|---|
| Below −0.3 | Corrosive | Etches plaster, corrodes metal | Raise CH, TA, or pH |
| −0.3 to 0.0 | Slightly aggressive | Acceptable, watch it | Minor upward nudge |
| 0.0 to +0.3 | Balanced | Ideal — no harm | Maintain |
| +0.3 to +0.5 | Scaling tendency | Cloudiness, early scale | Lower pH |
| Above +0.5 | Scaling | Calcium deposits, cell fouling | Lower pH and CH |
06
Why it matters
How the result supports the next decision
Aim for an LSI between −0.3 and +0.3; 0.0 to +0.1 is the safest target for plaster.
Temperature matters — the same water can be balanced in spring and scaling in a hot tub.
07
Good inputs
Improve the estimate before relying on it
- Aim for an LSI between −0.3 and +0.3; 0.0 to +0.1 is the safest target for plaster.
- Temperature matters — the same water can be balanced in spring and scaling in a hot tub.
- If LSI is negative (corrosive), raise calcium hardness or alkalinity, or nudge pH up.
- If LSI is positive (scaling), lower pH first — it's the fastest, cheapest lever.
- Remember CYA eats alkalinity: high-CYA pools often read more corrosive than the raw TA suggests.
08
The boundary
How to interpret the result
Follow the product label, add chemicals separately and retest before another dose.
The engine reports invalid inputs and safety warnings beside the result. Resolve those messages before using the estimate.
09
Before acting
What should override this estimate?
- Measured site conditions and current test results
- Product labels and manufacturer specifications
- Qualified-professional advice and applicable local codes
10
Common questions
Frequently asked questions
What is the Langelier Saturation Index?
The LSI is a single number that tells you whether pool water is corrosive, balanced, or scaling. It combines pH, temperature, calcium hardness, and alkalinity into one index. A value near zero means the water is in equilibrium with calcium carbonate and won't dissolve or deposit it.
What LSI range is safe for a pool?
Keep LSI between −0.3 and +0.3. Below −0.3 the water is corrosive and etches plaster and corrodes metal. Above +0.3 it scales, leaving calcium deposits on surfaces and inside heaters and salt cells. The center of the range is ideal.
How does temperature affect LSI?
Warmer water has a higher temperature factor, which pushes LSI upward toward scaling. That's why a spa or a heated pool in summer can scale even when the same water was balanced when cool. Always run the LSI at the actual water temperature.
Why does CYA matter for water balance?
Cyanuric acid acts as a weak acid that consumes part of your total alkalinity. The LSI here subtracts about a third of the CYA value from alkalinity before calculating, which is why high-CYA pools often read more corrosive than the test strip alkalinity number alone would suggest.
How do I fix corrosive (negative LSI) water?
Raise the factors that are low. The easiest moves are increasing calcium hardness and total alkalinity, and letting pH rise slightly. Even a small pH increase shifts LSI noticeably, so re-check after each adjustment rather than over-correcting at once.
What is a good LSI for a pool?
A value between −0.3 and +0.3, with 0.0 to +0.1 ideal for protecting plaster and equipment.
Is negative LSI bad?
Below −0.3 it's corrosive — it etches plaster, dissolves grout, and attacks metal. Slightly negative (above −0.3) is acceptable but not ideal.
Does salt water affect LSI?
Salt itself barely changes LSI, but salt pools tend to drift high in pH, which pushes LSI toward scaling. They need regular acid to stay balanced.
Primary sources
Use the result with current guidance.
These official references support safe planning context. Product labels, local requirements and qualified-professional advice take priority over a general calculator estimate.