What the saturation index actually tells you
Water wants a certain amount of dissolved calcium carbonate, set by its pH, alkalinity, calcium, temperature and TDS together. Undersaturated water takes calcium from wherever it can — plaster, grout, the metal in your heater. Oversaturated water gives it back — as scale on the waterline, cloudiness, and crust inside your salt cell and heat exchanger. The index measures the distance from equilibrium:
| CSI | Meaning | What you'd eventually see |
|---|---|---|
| below −0.6 | Aggressive | Etched, rough plaster; pitted grout; metal corrosion; vinyl unaffected |
| −0.6 to −0.3 | Slightly corrosive | Slow plaster wear — fine for vinyl and fiberglass |
| −0.3 to +0.3 | Balanced | Nothing — this is the target band |
| +0.3 to +0.6 | Slightly scaling | Waterline crust first, salt-cell scale in warm weather |
| above +0.6 | Scaling | Visible scale, cloudy water that filters can't fix, choked heater |
The reason to check the index instead of policing each reading separately: the factors trade off. CH 500 sounds alarming, but with pH 7.4 and TA 60 the water can still be perfectly balanced — while "in-range" readings can combine into scaling water on a hot August afternoon. The index is the verdict; the individual readings are just the evidence.
The classic Langelier index (LSI) uses your total alkalinity as measured. But in a stabilized pool, part of that TA is cyanurate — alkalinity that belongs to your CYA and doesn't participate in calcium chemistry. CSI subtracts it (at typical pH, roughly a third of your CYA), which is why a high-CYA pool is more corrosive than its raw LSI suggests. This calculator always applies the correction when you enter CYA — same approach as Taylor's watergram and PoolMath.
Which lever to pull
The calculator's "what moves it" panel shows exactly how far each adjustment shifts your water, but the hierarchy is consistent: pH is the big lever — the index tracks it one-for-one, and it's the cheapest thing to change. Alkalinity is the medium lever and drifts pH anyway, so fixing TA often fixes the index twice. Calcium is the slow lever — easy to raise, drain-to-lower — so aim it once and manage around it. Temperature you don't control, which is exactly why it deserves attention:
Cold water is aggressive water: the same chemistry that's balanced at 84 °F in July can sit near −0.5 at 45 °F — which is why plaster pools should close for winter with CSI slightly positive, and why spring often reveals etching nobody can explain. The reverse happens inside your heater: exchanger surfaces run far hotter than the pool, so scale forms there long before you see it at the waterline. If you run a heater hard, keep the index in the lower half of the balanced band.
Who should watch it, and how closely
Plaster, pebble and tile pools live and die by the index — check it whenever you test, especially at season edges. Salt-water pools should watch the scaling side: the cell's plates are the hottest, highest-pH spot in the system and scale there first (most cell "failures" are scale). Vinyl and fiberglass owners get more slack — nothing in the surface dissolves — but heaters don't care what your walls are made of. If your fill water is hard, the index is the tool that tells you whether a high CH is actually a problem this season or just a number.