How to Calculate LSI Saturation Index for Swimming Pools and Hot Tubs
The Langelier Saturation Index (LSI) is the most useful single number in pool water chemistry. It tells you whether water will deposit scale or dissolve calcium from surfaces — two problems that cost your clients real money in equipment damage and resurfacing.
Most pool engineers test pH, chlorine, and temperature at every visit. Fewer calculate LSI regularly, because the formula involves log calculations that aren't practical to do by hand at the poolside. This guide breaks down the calculation, provides the factor tables you need, and shows you how to interpret and correct the results.
The LSI Formula
LSI = pH − pHs
Where pHs (the saturation pH) is the theoretical pH at which water is perfectly balanced with respect to calcium carbonate:
pHs = (9.3 + A + B) − (C + D)
| Factor | Formula | What it represents |
|---|---|---|
| A | (log₁₀[TDS] − 1) / 10 | Total dissolved solids factor |
| B | −13.12 × log₁₀(°C + 273) + 34.55 | Temperature factor |
| C | log₁₀[Calcium hardness] − 0.4 | Calcium factor |
| D | log₁₀[Total alkalinity] | Alkalinity factor |
All concentrations are in mg/L as CaCO₃.
Interpreting the result
| LSI | Status | What's happening | Risk |
|---|---|---|---|
| Below −0.3 | Corrosive | Water is undersaturated — dissolves calcium | Etched plaster, corroded metals, pitted grout |
| −0.3 to +0.3 | Balanced | Water is at or near saturation equilibrium | Ideal — surfaces protected, no scaling |
| Above +0.3 | Scale-forming | Water is oversaturated — calcium precipitates | White deposits on heat exchangers, tiles, pipework |
Worked Example: Residential Pool
A residential pool on your Tuesday round reads:
- pH: 7.4
- Temperature: 28°C
- Calcium hardness: 250 mg/L
- Total alkalinity: 120 mg/L
- TDS: 1,000 mg/L (use this default if you don't test TDS)
Step 1: Calculate each factor
- A = (log₁₀(1000) − 1) / 10 = (3 − 1) / 10 = 0.20
- B = −13.12 × log₁₀(301) + 34.55 = −13.12 × 2.479 + 34.55 = 2.03
- C = log₁₀(250) − 0.4 = 2.398 − 0.4 = 2.00
- D = log₁₀(120) = 2.08
Step 2: Calculate pHs
pHs = (9.3 + 0.20 + 2.03) − (2.00 + 2.08) = 11.53 − 4.08 = 7.45
Step 3: Calculate LSI
LSI = 7.4 − 7.45 = −0.05
Result: Balanced (−0.05 falls within the ±0.3 range). This water is very slightly corrosive but well within acceptable limits. No correction needed.
Worked Example: Hot Tub
A hot tub on the same round reads:
-
pH: 7.6
-
Temperature: 38°C
-
Calcium hardness: 180 mg/L
-
Total alkalinity: 100 mg/L
-
TDS: 1,500 mg/L (hot tubs accumulate TDS faster)
-
A = (log₁₀(1500) − 1) / 10 = (3.176 − 1) / 10 = 0.22
-
B = −13.12 × log₁₀(311) + 34.55 = −13.12 × 2.493 + 34.55 = 1.85
-
C = log₁₀(180) − 0.4 = 2.255 − 0.4 = 1.86
-
D = log₁₀(100) = 2.00
pHs = (9.3 + 0.22 + 1.85) − (1.86 + 2.00) = 11.37 − 3.86 = 7.51
LSI = 7.6 − 7.51 = +0.09
Result: Balanced (+0.09). The higher temperature and pH push toward scaling, but it's within range. If pH drifts to 7.8, LSI jumps to +0.29 — right at the boundary. Hot tubs need tighter pH control.
How to Correct an Out-of-Range LSI
LSI too low (corrosive)
Options, in order of effectiveness:
- Raise pH — the most immediate lever. Increasing pH from 7.0 to 7.2 shifts LSI by roughly +0.2.
- Raise calcium hardness — add calcium chloride. This is the permanent fix for soft-water areas where mains water has low calcium.
- Raise alkalinity — add sodium bicarbonate. This also stabilises pH.
LSI too high (scale-forming)
Options:
- Lower pH — add sodium bisulphate (dry acid). Most effective immediate correction.
- Reduce alkalinity — lower alkalinity with acid, then re-adjust pH separately if needed.
- Partial drain and refill — if calcium is very high (hard water areas), dilution is the only practical fix.
What you can't change
Temperature is set by the client's preference (or pool type). TDS is determined by water source and chemical accumulation. Focus on pH, alkalinity, and calcium as your controllable variables.
Skip the Manual Calculation
The formula is accurate but tedious at the poolside. Our free chemistry calculator runs the full Langelier calculation from your readings — enter pH, temperature, calcium, alkalinity, and TDS, and get the LSI result with a visual indicator and correction recommendations.
PoolRound will calculate LSI automatically from every set of readings you log on-site. No manual formulas, no lookup tables — just accurate water balance data for every pool on your round. Join the waitlist.
Sources
- PWTAG Standards — Water quality standards for pools and spas, including water balance guidance