Salt pools: use ~salt level + 1,000
LSI
-0.04
Balanced
Water is within the balanced range (−0.3 to +0.3). Keep doing what you're doing.
How LSI is calculated
The Langelier Saturation Index measures how saturated water is with calcium carbonate:
LSI = pH + Temperature factor + Calcium factor + Alkalinity factor − TDS constant
- Temperature factor — from the standard table (0.0 at 32°F up to 0.9 at 105°F); warmer water scales more easily.
- Calcium factor — log₁₀(calcium hardness) − 0.4.
- Alkalinity factor — log₁₀ of carbonate alkalinity. This calculator subtracts one-third of your CYA from total alkalinity first, because cyanurate alkalinity doesn't participate in scale formation — skipping this correction overstates LSI on stabilized pools.
- TDS constant — 12.1 up to 1,000 ppm TDS, 12.2 above (which is why salt pools read slightly more corrosive at the same chemistry).
Winter matters most. The temperature factor is why pools that sit balanced all summer quietly turn corrosive in January — a pool at 7.5 pH that's fine at 84°F can dip below −0.3 at 50°F with nothing else changing. Pros who manage LSI seasonally run calcium and alkalinity higher going into winter.
Reading the result
| LSI | Meaning | What it costs |
|---|---|---|
| Below −0.3 | Corrosive / aggressive | Etched plaster, dissolved grout, heater damage |
| −0.3 to +0.3 | Balanced | — |
| Above +0.3 | Scale-forming | Scaled tile, clogged salt cells, cloudy water |
LSI on a weekly route
You don't need to compute LSI at every stop — but you do need the readings that feed it, logged every visit, so trends are visible before they cost a heater. Tadpole's tech app records chemistry at every stop and keeps it on the pool's history. See how logging works → And for everyday dosing, the chlorine dosing calculator handles the most common adjustment of all.
