Langelier Saturation Index Calculator (Pool and Water Balance)
Whether pool or tap water will dissolve metal and plaster or leave limescale, from pH, temperature, dissolved solids, calcium hardness and alkalinity.
Results
The dashed line marks the value you entered.
What this tool does
The index compares the actual pH with pHs, the pH at which the water would be exactly saturated with calcium carbonate. Below about −0.3 the water is hungry for calcium and attacks plaster, grout and metal fittings; above +0.3 it drops scale on surfaces, heaters and filters; in between it is balanced. Warm water and high calcium or alkalinity push the index up, which is why a hot tub scales faster than a cold pool. It predicts a tendency, not a rate, and the common pool-industry form used here ignores the effect of cyanuric acid on alkalinity.
Formula
LSI = pH − pHs · pHs = 9.3 + A + B − (C + D) · A = (log TDS − 1) ÷ 10 · B = −13.12 log(T + 273) + 34.55 · C = log Ca − 0.4 · D = log alkalinity
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ph | pH | — |
tw | Water temperature | °C |
sd | Total dissolved solids | mg/L |
ch | Calcium hardness (as CaCO₃) | mg/L |
ta | Total alkalinity (as CaCO₃) | mg/L |
L | Langelier Saturation Index | — |
PS | pH at which the water is saturated (pHs) | — |
ST | What it means | — |
Worked example
- pH7.4
- Water temperature28 °C
- Total dissolved solids800 mg/L
- Calcium hardness (as CaCO₃)300 mg/L
- Total alkalinity (as CaCO₃)100 mg/L
- Langelier Saturation Index-0.04
- pH at which the water is saturated (pHs)7.44
- What it meansBalanced
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.