Why two LSI calculators can give you different answers
The core problem
You ran the same numbers through two LSI calculators and got different results. Both claim to use the Langelier Saturation Index. The problem: not all calculators use the same formula. Four specific differences can shift your result by 0.1 to 0.3 points — enough to flip a ‘balanced’ reading to ‘corrosive.’
The four ways calculators diverge
1. CYA alkalinity correction — the biggest source of divergence
Most calculators use raw total alkalinity as their input. The correct formula for pools with stabilizer uses CYA-corrected alkalinity: total alkalinity minus CYA divided by 3.
| Input | Simplified (no CYA correction) | Full formula (CYA-corrected) |
|---|---|---|
| Total Alkalinity | 90 ppm | 90 ppm |
| CYA | not used | 60 ppm |
| Alkalinity input | 90 ppm | 90 − (60 ÷ 3) = 70 ppm |
| Alkalinity factor (AF) | log₁₀(90) = 1.954 | log₁₀(70) = 1.845 |
| Difference | 0.109 points flows directly into LSI | |
That 0.109 difference in the alkalinity factor flows directly into the LSI result with no scaling or rounding. At CYA 80 ppm — typical for a saltwater pool — the correction is even larger: 26.7 ppm subtracted from alkalinity. This is why calculators without a CYA field are unreliable for any stabilized pool.
2. Temperature factor method
Two approaches exist for the temperature factor. The first uses a coarse lookup table with linear interpolation between 5°F or 10°F increments. The second uses a polynomial curve fitted to industry reference data that computes continuously at any temperature.
The table method introduces rounding errors, especially at temperatures above 88°F or below 55°F, where the temperature effect on LSI changes more steeply. At typical pool temperatures of 70–85°F, the difference is usually 0.01–0.03 points. At extremes — a very cold spring opening or a desert pool in August — the gap between methods grows to 0.03–0.05 points.
3. TDS — included or ignored
The full LSI formula adds a Total Dissolved Solids factor: +0.012 × log₁₀(TDS ppm). Many calculators omit it entirely.
For freshwater pools, TDS is negligible — typically around 0.02 points. For a saltwater pool at 3,200 ppm, TDS adds roughly +0.042 to LSI. That is a small but real nudge toward scale-forming conditions that tools without a salt field systematically miss. See LSI and saltwater pools for more on how this interacts with pH drift in SWG pools.
4. Factor table rounding vs. direct calculation
Older tools and printed resources use pre-computed lookup tables for the calcium hardness and alkalinity factors, rounded to one decimal place. Direct log₁₀ calculation is more precise. At values between table entries — for example, TA 73 ppm or calcium hardness 275 ppm — table-based interpolation introduces small errors that add up across the full formula.
Concrete comparison: same pool, two approaches
Pool: pH 7.4, TA 90 ppm, CH 250 ppm, CYA 60 ppm, temp 82°F, no salt.
| Approach | Alkalinity input | Alkalinity factor (AF) | LSI result | Verdict |
|---|---|---|---|---|
| Simplified (no CYA correction) | 90 ppm | 1.954 | ≈ −0.13 | Essentially balanced |
| Full formula (CYA-corrected) | 70 ppm | 1.845 | ≈ −0.24 | Corrosive |
With CYA 80 ppm, the corrected alkalinity drops to 63 ppm and the gap between approaches widens to about 0.15 points. What the simplified calculator calls balanced, the full formula identifies as corrosive — a difference that matters for plaster and equipment protection.
How to tell which calculator is accurate
Three questions to ask before trusting an LSI result:
- Does it ask for CYA? If not, it uses raw total alkalinity — wrong for any stabilized pool. The error grows with CYA level.
- Does it ask for salt or TDS? Relevant for saltwater pools. Without a salt or TDS field, the TDS factor is either assumed zero or guessed.
- Does it show the factor breakdown? A calculator that displays the pH factor, temperature factor, calcium factor, alkalinity factor, and TDS factor separately lets you verify the math against known values and catch entry errors.
LSI calculated with the full formula
PoolChem Tracker uses CYA-corrected alkalinity, a polynomial temperature curve, and salt-derived TDS — and shows you the complete factor breakdown so you can verify the math on every reading.
Frequently asked questions
Why does my pool store printout show different LSI than my app?
Pool store software typically uses raw total alkalinity without the CYA correction, and often relies on pre-computed lookup tables rather than direct logarithmic calculation. If your pool has any CYA — which most outdoor pools do — the raw-TA method overstates the alkalinity factor and reports a higher (less corrosive) LSI than the full formula produces. The gap is typically 0.1 to 0.2 points at common CYA levels.
Does the CYA correction really matter that much?
Yes. At CYA 60 ppm, the correction subtracts 20 ppm from your alkalinity input. At CYA 80 ppm it subtracts about 27 ppm. This shifts the alkalinity factor by roughly 0.1 to 0.15 in the LSI formula — enough to move a reading from ‘essentially balanced’ to ‘corrosive.’ Any pool with stabilizer in the water is affected; the higher the CYA, the larger the correction and the larger the error if a calculator skips it.
Which LSI formula is the official standard?
The Langelier Saturation Index was defined by Wilfred Langelier and has been refined by water treatment references including AWWA publications. The original formula was developed for municipal drinking water. The CYA alkalinity correction is specific to pool chemistry — it accounts for the fact that cyanuric acid ties up a portion of total alkalinity that does not participate in the carbonate equilibrium the LSI formula measures. The correction is standard in accurate pool chemistry tools but is absent from calculators built for industrial or drinking-water applications.
My calculator doesn't have a CYA field — can I still use it?
Only if your CYA is near zero. At CYA 0, the correction is zero and there is no error. In practice, most outdoor pools have CYA 30–80 ppm from stabilized chlorine products, even if no CYA was added directly. At CYA 30, the error is modest (about 10 ppm subtracted from TA). At CYA 60–80 — typical for most pools — the error is significant enough to flip a ‘balanced’ reading to ‘corrosive’ in the full formula. If your calculator has no CYA field, treat its output as an approximation only.
Does water temperature affect which calculator is more accurate?
Yes. The temperature factor difference between a lookup table and a polynomial curve is small at moderate temperatures (around 0.01–0.02 points from 65°F to 85°F) but grows meaningfully at extremes. Above 90°F or below 55°F, the table-based method introduces rounding errors that can reach 0.03–0.05 points — because table entries are spaced at 5°F or 10°F increments and temperatures between those values are interpolated. In very hot climates or during cold-weather operation, the polynomial method is materially more accurate.
Keep reading
- Langelier Saturation Index — Explained — the complete formula and factor definitions
- Why LSI Matters: the real cost of unbalanced water — what corrosive and scale-forming water actually does to pool surfaces and equipment
- LSI and Saltwater Pools — TDS and the SWG pH drift problem
- LSI Calculator — uses the full formula with factor breakdown
- Pool Calcium Hardness — the CH factor in LSI
- How to Balance Pool Water in 4 Steps — the correct order for adjusting chemicals
- Pool Chemistry for Beginners — the five numbers every pool owner should know
- CSI vs. LSI: Are They the Same Thing? — what each term means and why the name matters less than the formula