The question the indices answer
Will this water deposit calcium carbonate scale, or will it dissolve what is already there (and attack cement and metal along the way)? The answer depends on five things at once — pH, calcium hardness, alkalinity, temperature and TDS — which is exactly why a single-number index is useful: it does the balancing for you.
Langelier Saturation Index (LSI)
The LSI compares the water's actual pH with the pH at which it would be exactly saturated with calcium carbonate (pHs):
LSI = pH − pHs
- LSI positive → supersaturated → scale-forming. The further above zero, the faster scale lays down.
- LSI negative → undersaturated → the water dissolves calcium carbonate rather than depositing it, so it strips cementitious linings and leaves metal without a protective carbonate film.
- LSI ≈ 0 (±0.3) → roughly balanced.
Because pHs falls as temperature rises, the same water becomes more scale-forming when heated — which is why scale appears in the geyser first. Always calculate LSI at the working temperature, not the tap temperature.
Ryznar Stability Index (RSI)
The Ryznar index uses the same inputs, weighted from field data on real pipework:
RSI = 2·pHs − pH
It reads in the opposite direction and has no zero crossing:
| RSI | Tendency |
|---|---|
| < 6 | Scale-forming |
| 6 – 7 | Approximately stable |
| > 7 | Increasingly corrosive |
| > 8.5 | Aggressively corrosive |
Old hands quote RSI because it tracked field experience well; most modern datasheets quote LSI. Quoting both costs nothing — our calculator returns both from one set of inputs.
What to do with the answer
- Scale-forming water ahead of heaters, boilers or heat exchangers: soften, dose antiscalant, or accept a descaling schedule.
- Scale-forming water ahead of RO: the index matters in the concentrate, not the feed — at 75% recovery the concentrate is four times more concentrated, so a feed that looks balanced can still gypsum-and-carbonate the tail elements. Antiscalant selection starts here.
- Negative LSI: the water will dissolve cementitious linings, and bare metal loses the carbonate film that would otherwise shield it — one of the conditions that lets corrosion proceed, though not a measurement of it. pH correction via calcite media or dosing moves the index positive; so does blending.
- Municipal supply is usually delivered slightly positive on purpose, to lay a thin protective carbonate film in the mains.
The limits
LSI and RSI only speak for calcium carbonate. They say nothing about sulphate scales (gypsum), silica, iron fouling or chloride corrosion of stainless steel — all of which need their own checks on difficult waters. The indices are also equilibrium predictions: they say which way the water wants to go, not how fast it will get there.
The bigger limit is the one the name invites you to forget. These are saturation indices, not corrosion indices. A negative LSI tells you the water will not lay down a protective carbonate film; it does not tell you a pipe is corroding, or how fast. Corrosion rate depends on dissolved oxygen, chloride and sulphate, temperature, flow velocity, the metal itself and what is already on its surface — none of which appear in the calculation. Chloride-rich water can eat stainless steel at a perfectly balanced LSI, and a positive LSI is no guarantee against corrosion. Where the question is genuinely metal loss rather than scale, use an index built for it — the Larson–Skold ratio for chloride and sulphate against alkalinity on steel — and, on anything that matters, a coupon test.
Rule of thumb: calculate the index at the highest temperature and highest concentration the water will actually see in the system. Feed-water LSI at 20 °C flatters almost every design.