In short: the Langelier Saturation Index compares your water's actual pH to the pH at which it would be perfectly saturated with calcium carbonate. A value near zero (roughly −0.5 to 0.5) is considered balanced; a clearly positive LSI tends toward scale-forming water, and a clearly negative LSI tends toward corrosive water.
This is a planning-level chemistry estimate for treatment decisions, not a substitute for a corrosion or scaling study on an operating system.
How the Langelier Saturation Index is calculated
LSI is simply the difference between your water's actual pH and its calculated saturation pH, pHs — the pH at which the water is exactly in equilibrium with calcium carbonate:
LSI = pH − pHs
pHs = (9.3 + A + B) − (C + D)
Where A comes from TDS, B from temperature, C from calcium hardness, and D from total alkalinity (all as CaCO₃ where noted):
A = (log₁₀[TDS] − 1) / 10
B = −13.12·log₁₀(°C + 273.15) + 34.55
C = log₁₀[Ca²⁺ as CaCO₃] − 0.4
D = log₁₀[alkalinity as CaCO₃]
Worked example. Typical tap water at pH 7.8, 300 mg/L TDS, 25°C, 120 mg/L calcium hardness and 100 mg/L alkalinity gives A = 0.15, B = 2.09, C = 1.68 and D = 2.00, so pHs = (9.3 + 0.15 + 2.09) − (1.68 + 2.00) = 7.85. LSI = 7.8 − 7.85 = −0.05 — essentially balanced, with a very slight corrosive lean.
Reading the LSI scale
The magnitude of LSI, not just its sign, indicates how strong the scaling or corrosion tendency is. These are the ranges used by the calculator above.
| LSI range | Tendency | What it means |
|---|---|---|
| Below −2.0 | Severe corrosion | Aggressive water; can actively dissolve pipe scale and attack metal |
| −2.0 to −0.5 | Corrosive | Undersaturated; scale tends to dissolve rather than form |
| −0.5 to 0.5 | Balanced | Neither strongly scaling nor corrosive; the usual treatment target |
| 0.5 to 2.0 | Scale-forming | Supersaturated; calcium carbonate tends to deposit as scale |
| 2.0 and above | Heavy scale | Strong scaling tendency; deposits form quickly on hot surfaces |
Worked examples
Typical municipal tap
LSI −0.05
pH 7.8, 300 mg/L TDS, 25°C, 120 mg/L calcium hardness, 100 mg/L alkalinity. Balanced, with a very slight corrosive lean.
Hard, well-buffered water
LSI +0.33
pH 8.0, 500 mg/L TDS, 15°C, 200 mg/L calcium hardness, 150 mg/L alkalinity. Mildly scale-forming, still inside the balanced band.
Soft, low-mineral water
LSI −2.79
pH 6.5, 80 mg/L TDS, 10°C, 30 mg/L calcium hardness, 25 mg/L alkalinity. Severely corrosive; typical of soft, acidic mountain or rain-fed supplies.
What else affects the result
Alkalinity vs. hardness confusion. LSI needs calcium hardness and total alkalinity as two separate CaCO₃ values, not "total hardness." If your report only gives total hardness, use the hardness-to-GPG calculator for units, but you still need a calcium-specific test or lab breakdown for an accurate LSI.
Temperature swings. Because B depends on temperature, the same water can shift from balanced to mildly scale-forming as it heats up — which is exactly why water heaters and boilers scale faster than cold supply lines even from identical source water.
TDS versus conductivity. If your lab reports conductivity instead of TDS, convert it first with the conductivity-to-TDS calculator; LSI is not very sensitive to small TDS errors, since it enters as a divided logarithm.
LSI is an index, not a guarantee. It predicts a chemical tendency toward calcium carbonate scaling or dissolution. Actual scale buildup or corrosion in a real system also depends on flow velocity, dissolved oxygen, chlorine residual, and pipe material, which LSI does not capture.
Frequently asked questions
What is a good Langelier Saturation Index value?
Most treatment guidance targets an LSI between about −0.5 and 0.5. Inside that band the water is considered balanced: it is neither aggressive enough to dissolve pipe scale and metal nor prone to depositing new calcium carbonate scale.
What does a negative LSI mean?
A negative LSI means the water is undersaturated with calcium carbonate, so it tends to dissolve existing scale rather than form new scale. This is described as corrosive, and very negative values (below about −2) point to aggressive water that can attack metal pipes and fittings over time.
What does a positive LSI mean?
A positive LSI means the water is supersaturated with calcium carbonate and tends to deposit it as scale on pipes, heat exchangers and fixtures. Higher positive values mean a stronger scaling tendency, though a thin protective scale layer is sometimes desired in metal piping.
What test results do I need to calculate LSI?
You need pH, total dissolved solids (TDS) in mg/L, water temperature, calcium hardness as CaCO₃ in mg/L, and total alkalinity as CaCO₃ in mg/L. These come from a water test report or a home test kit; a lab report will list calcium hardness and alkalinity separately from total hardness.
Is LSI the same as the Ryznar Stability Index?
No. Both use the same saturation pH, but LSI (pH minus saturation pH) only shows the direction and rough size of the scaling or corrosion tendency, while the Ryznar Stability Index rescales that same information onto a 0–14 practical severity scale. Our Ryznar Stability Index calculator uses the same five inputs.
Can I change my water's LSI?
Yes. Raising pH or alkalinity (for example with soda ash) pushes LSI upward toward scale-forming, while lowering pH or alkalinity, or softening to reduce calcium hardness, pushes it toward corrosive. Treatment plants and pool or boiler operators adjust these levers to keep LSI in a target band rather than changing just one number.
Sources & references
- Langelier, W.F. (1936). The Analytical Control of Anti-Corrosion Water Treatment. Journal AWWA.
- American Water Works Association (AWWA). Water Quality & Treatment reference series.
- APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater.
Sarah Bennett, MS
Water Quality & Sustainability Specialist · Portland, Oregon
Environmental scientist (MS, Water Resources) with 9+ years in residential water quality, treatment, and usage efficiency, and 150+ completed assessments. Sarah writes and reviews the treatment, usage, and cost calculators on watercalculator.us. Read full bio
MS Environmental Science, Water Resources (2018) · BS Environmental Engineering (2016)