Enter your current readings to get your Langelier Saturation Index — with the full factor breakdown and CYA-corrected alkalinity.
Your LSI is in the target range. Water is neither corroding surfaces nor depositing scale. Keep testing regularly — temperature swings and rain can shift LSI without any chemical changes.
| LSI Value | Status | What it means for your pool |
|---|---|---|
| < −0.5 | Highly corrosive | Aggressively dissolves plaster, etches grout, attacks metal fittings and heater coils |
| −0.5 to −0.3 | Corrosive | Slow but measurable surface damage over weeks and months |
| −0.3 to +0.3 | Balanced ✓ | Target range — water is satisfied, neither dissolving surfaces nor depositing scale |
| +0.3 to +0.5 | Scale-forming | Calcium starts precipitating on surfaces and inside equipment |
| > +0.5 | Heavily scale-forming | Visible scale on tile and grout, salt cells clogging, heater coil damage, cloudy water |
Most LSI calculators use a simplified version of the Langelier formula. This one uses the exact formula from the PoolChem Tracker app, with two details that other tools commonly skip:
Industry standard tools often use raw total alkalinity and a step-table temperature factor. The CYA correction matters most for pools with stabilizer — at CYA 80 ppm, skipping it overstates the alkalinity factor by about 0.13 LSI units. See the full LSI guide for a worked example showing the difference.
The target range is −0.3 to +0.3. Zero is perfect balance. Below −0.3, water is corrosive; above +0.3, it is scale-forming. Many pool professionals target −0.1 to +0.1 as a tighter goal.
Two reasons. First, most calculators use raw total alkalinity instead of CYA-corrected alkalinity. At CYA 60 ppm, this error alone overstates LSI by about 0.09 units. Second, the temperature factor varies by implementation — stepped lookup tables produce small jumps at integer temperatures while a polynomial is continuous. These differences are why pool owners sometimes see conflicting readings from different tools.
Your water is corrosive — undersaturated with calcium. It will pull minerals from whatever surfaces it contacts, etching plaster, dissolving grout, and corroding heat exchanger coils over time. Raise calcium hardness, pH, or alkalinity to bring LSI up. Low calcium hardness is the most common cause of persistently negative LSI.
Your water is scale-forming — it holds more dissolved calcium than it can keep in solution. Calcium precipitates out as white deposits on tile, inside pipes, on heater coils, and clogging salt cell plates. Lower pH with muriatic acid first (fastest lever), then alkalinity if elevated. If calcium is above 500 ppm, a partial drain and refill is the only solution.
Yes. CYA adds cyanurate alkalinity that shows up on your TA test but does not buffer pH the same way carbonate alkalinity does. The correct formula subtracts CYA ÷ 3 from TA before plugging in. Pools running CYA 80 ppm with TA 100 ppm have an effective alkalinity of about 73 ppm — not 100. Skipping this makes water appear more balanced than it is. See the full LSI guide for a worked step-by-step example.
Every time you test your water. If you are already measuring pH, alkalinity, calcium hardness, and temperature, LSI calculation takes no extra effort — it is just math on numbers you already have. Pay special attention after heavy rain, seasonal temperature swings, adding calcium or acid, and after any partial drain and refill.
PoolChem Tracker runs this same formula automatically every time you log a reading — no copy-paste, no manual entry. It also tracks your history and shows how LSI has shifted over time.
