In short: the Ryznar Stability Index (RSI) rates whether water will deposit calcium carbonate scale or dissolve it. An RSI below about 6 means scale-forming, roughly 6 to 7 is balanced, and above 7 trends corrosive — the further from that middle band, the stronger the tendency. Enter your own water chemistry above for a figure specific to your supply.
This is a planning estimate from calcium carbonate equilibrium chemistry alone. It doesn't replace corrosion testing, coupon studies, or a treatment engineer's judgment for a specific system.
How the Ryznar Stability Index is calculated
RSI is built on the same saturation pH (pHs) used by the Langelier Saturation Index — the pH at which water is exactly in equilibrium with solid calcium carbonate. Ryznar found that doubling pHs before subtracting the measured pH tracked observed scaling and corrosion behavior more consistently than LSI alone across a wide range of waters:
pHs = (9.3 + A + B) − (C + D), RSI = 2·pHs − pH
Where A = (log₁₀[TDS] − 1) / 10, B = −13.12·log₁₀(°C + 273.15) + 34.55, C = log₁₀[Ca²⁺ as CaCO₃] − 0.4, and D = log₁₀[alkalinity as CaCO₃]. TDS, calcium hardness and alkalinity are all in mg/L. The related Langelier Saturation Index is simply LSI = pH − pHs.
Worked example. For pH 7.8, 25 °C, TDS 300 mg/L, calcium hardness 150 mg/L as CaCO₃ and alkalinity 120 mg/L as CaCO₃: A ≈ 0.148, B ≈ 2.085, C ≈ 1.776, D ≈ 2.079, giving pHs ≈ 7.68. RSI = 2(7.68) − 7.8 ≈ 7.56 (corrosive tendency), and LSI = 7.8 − 7.68 ≈ 0.12 (mildly scale-forming). The two indices don't have to agree in strength — they're calibrated differently from the same underlying chemistry.
What an RSI value means
Ryznar's original ranges, still the standard reference for reading the index:
| RSI range | Tendency | What it means |
|---|---|---|
| Below 5.5 | Heavy scale-forming | Significant calcium carbonate scale likely on pipes and heaters |
| 5.5 – 6.2 | Scale-forming | Water tends to deposit calcium carbonate scale |
| 6.2 – 6.8 | Balanced | Little tendency to scale or corrode; closest to equilibrium |
| 6.8 – 7.5 | Mild corrosion tendency | Slight tendency to dissolve pipe scale and base metal |
| 7.5 – 9.0 | Corrosive | Noticeable tendency to corrode metal piping and fixtures |
| Above 9.0 | Severely corrosive | Aggressive water; corrosion control strongly advised |
Why each input changes the answer
Measured pH feeds directly into both indices. A higher pH pushes LSI up (more scale-forming) and pulls RSI down (also more scale-forming) — the two move in the same direction even though RSI is calibrated the opposite way round.
Temperature lowers pHs as it rises, because warmer water holds less dissolved calcium carbonate before it precipitates. That's why water heaters, boilers and hot-water loops scale faster than the same water run cold — heating a supply doesn't change its RSI classification table, but it does push the calculated value toward scaling.
Calcium hardness and alkalinity both lower pHs as they increase, since more dissolved calcium and carbonate/bicarbonate leaves less "room" before the water is saturated. Hard, high-alkalinity water is more scale-forming; soft, low-alkalinity water tends toward corrosive.
TDS has the smallest effect of the four — it nudges pHs up slightly as total dissolved solids increase, a minor ionic-strength correction rather than a dominant factor.
Three worked scenarios
Same formula, three different water types — showing how far RSI can swing.
Soft, low-alkalinity water
pH 7.2, 15 °C, TDS 150, Ca 40, Alk 80 mg/L
RSI 9.98
Severely corrosive — typical of softened or naturally soft water with little buffering capacity.
Typical municipal tap
pH 7.8, 25 °C, TDS 300, Ca 150, Alk 120 mg/L
RSI 7.56
Corrosive tendency — common for treated surface-water supplies without added alkalinity.
Hard, high-alkalinity well water
pH 8.2, 18 °C, TDS 450, Ca 300, Alk 250 mg/L
RSI 6.22
Near-balanced, borderline scale-forming — typical of hard groundwater in limestone regions.
What else affects real-world scaling and corrosion
RSI and LSI only model calcium carbonate equilibrium. Dissolved oxygen, chlorides and sulfates drive their own corrosion chemistry, especially in copper and galvanized steel piping. Flow velocity, stagnant spots, pipe material and age, and biological activity (like iron- or sulfate-reducing bacteria) all influence outcomes an index alone can't capture. Treat RSI as a first screen — for corrosion-sensitive systems such as boilers, cooling towers, or municipal distribution mains, pair it with the Langelier Saturation Index and site-specific corrosion monitoring.
Frequently asked questions
What is the Ryznar Stability Index?
The Ryznar Stability Index (RSI) is a number derived from a water's pH, temperature, total dissolved solids, calcium hardness and alkalinity that predicts whether the water will tend to form calcium carbonate scale or dissolve it (corrode metal and concrete). It was developed by John Ryznar in 1944 as a refinement of the Langelier Saturation Index.
How is RSI different from the Langelier Saturation Index (LSI)?
Both indices are built from the same saturation pH (pHs). LSI is simply pH minus pHs, so it is positive when water is scale-forming and negative when it is corrosive. RSI is 2 times pHs minus pH, an empirical rescaling that Ryznar found matched observed scaling behavior more closely across a wider range of waters, with lower RSI values meaning more scale-forming.
What RSI value indicates scaling versus corrosion?
As a rule of thumb, RSI below about 6 indicates a tendency to form scale, values in roughly the 6 to 7 range are considered balanced, and values above 7 indicate an increasing tendency toward corrosion. Below 5.5 the scaling tendency is heavy, and above 9 the water is considered aggressively corrosive.
What inputs does the RSI formula need?
You need the measured pH, the water temperature, total dissolved solids (TDS), calcium hardness as CaCO3, and total alkalinity as CaCO3. These feed into the saturation pH (pHs) equation, and RSI is then calculated as 2 times pHs minus the measured pH.
Can I use ppm instead of mg/L for TDS, hardness and alkalinity?
Yes. For dilute water solutions, mg/L and ppm are effectively the same unit, so you can enter values from a water report in either unit without conversion. Just make sure hardness and alkalinity are reported as calcium carbonate (CaCO3) equivalents, which is the standard convention for both the RSI and LSI formulas.
Does RSI predict corrosion or scaling perfectly?
No. RSI and LSI are planning indices based on calcium carbonate equilibrium chemistry alone. They do not account for dissolved oxygen, chlorides, sulfates, flow velocity, pipe material, or biological activity, all of which also affect real-world scaling and corrosion. Treat the result as a starting point, not a guarantee.
Sources & references
- Ryznar, J.W. (1944). A New Index for Determining Amount of Calcium Carbonate Scale Formed by a Water. Journal AWWA, 36(4).
- Langelier, W.F. (1936). The Analytical Control of Anti-Corrosion Water Treatment. Journal AWWA, 28(10).
- American Water Works Association (AWWA) — corrosion control and scale prediction practice for distribution systems.
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)