How CYA Changes Your LSI Calculation
Cyanuric acid (CYA, stabilizer) doesn’t appear in the basic LSI formula. But it changes one of the formula’s inputs — total alkalinity — in a way that matters when CYA is above 50 ppm. Pools with high stabilizer levels have less effective alkalinity than the test kit reads. A pool that appears balanced on a simple LSI calculator may actually be corrosive when calculated correctly.
Most LSI calculators use raw total alkalinity as-is. The full formula requires a correction: some of the alkalinity measured by your test is contributed by cyanurate ions — a byproduct of CYA’s dissociation in water — and cyanurate doesn’t participate in the calcium carbonate equilibrium the same way bicarbonate does. So it needs to be subtracted before the calculation.
What the correction is
The adjustment is simple: effective TA = TA − (CYA ÷ 3)
This CYA-corrected alkalinity is the value that goes into the LSI alkalinity factor — not the raw number from your test. The higher your CYA, the more you subtract, and the lower your LSI will be compared to a calculator that ignores it.
Effective TA = Total Alkalinity − (CYA ÷ 3)
How much it shifts your LSI
The correction grows with CYA level.
| CYA (ppm) | Subtract from TA | Typical LSI shift |
|---|---|---|
| 30 | 10 ppm | Small (−0.05 to −0.08) |
| 60 | 20 ppm | Moderate (−0.10 to −0.15) |
| 90 | 30 ppm | Significant (−0.15 to −0.20) |
| 120 | 40 ppm | Large (−0.20 to −0.25) |
Exact LSI shift depends on the rest of the water chemistry. These ranges assume typical pool conditions.
Concrete example: same pool, two calculators
Pool: pH 7.4, TA 100, CH 250, CYA 80, 82°F
| Calculator | Alkalinity used | Result | Verdict |
|---|---|---|---|
| Basic (no CYA correction) | TA = 100 ppm | LSI ≈ −0.08 | Borderline OK |
| CYA-corrected | Effective TA = 100 − (80 ÷ 3) = 73 ppm | LSI ≈ −0.23 | Corrosive |
These are identical pool readings. The difference is entirely the CYA correction. For a plaster pool, −0.23 is meaningfully corrosive — the water is slowly dissolving the surface. The basic calculator would tell the owner everything is fine. This is one of the four reasons two calculators give different answers for the same pool — see Why Two LSI Calculators Give Different Answers for the full breakdown.
Who it matters most for
Pools that use stabilized chlorine (trichlor tablets)
Trichlor tablets add CYA every time they dissolve. By mid-season, CYA in tablet pools commonly reaches 60–100+ ppm without the owner realizing it. This is exactly the range where the correction meaningfully shifts LSI.
Pools that haven’t been drained or diluted recently
CYA doesn’t degrade naturally — it only leaves through dilution. A pool entering its third season without a significant water exchange can easily have 80–120 ppm CYA, compounding the LSI effect all season.
Indoor pools
Most indoor pools don’t use CYA at all since there’s no UV degradation to protect against. With CYA at 0, the correction term is 0 — the formula uses raw TA. Indoor pools can skip this adjustment entirely.
How to account for it
Use a calculator that includes the correction: The PoolChem Tracker LSI Calculator uses CYA-corrected alkalinity in every calculation. If you’ve been using a simple calculator, your actual LSI may be lower than you thought.
Adjust your TA target upward if CYA is high: To hit an effective TA of 80 ppm with CYA at 90 ppm, you need a raw TA of approximately 110 ppm (80 + 90/3 = 110). The goal isn’t higher total alkalinity — it’s sufficient effective alkalinity after the CYA correction.
Consider a partial drain if CYA is above 80–100 ppm: The only way to lower CYA is dilution. At very high CYA levels, maintaining balanced LSI requires running both TA and CH higher to compensate — which increases scale risk. A partial drain and refill is often the cleaner solution.
LSI calculated correctly — every time
Enter your CYA once. PoolChem Tracker applies the correction to every reading and shows your real LSI — not the one that ignores stabilizer.
Frequently asked questions
Does CYA affect LSI directly or only through alkalinity?
CYA affects LSI only through the alkalinity correction. It lowers the effective alkalinity value that goes into the alkalinity factor of the LSI formula. CYA does not directly change pH, calcium hardness, temperature, or TDS. Its entire LSI impact comes from subtracting CYA/3 from total alkalinity before calculating the alkalinity factor. The higher the CYA, the larger the subtraction, and the lower the resulting LSI.
How much does the CYA correction change my LSI?
At typical CYA levels of 30–60 ppm, the correction shifts LSI by about 0.05–0.15. At higher CYA levels of 80–120 ppm, which are common in outdoor pools late in the season, the shift can be 0.15–0.25. This is large enough to move a pool from the balanced range into corrosive territory. Whether the difference matters for you depends on your CYA level — check your current reading and apply the formula (effective TA = TA minus CYA/3) to see where you actually stand.
Should I raise my alkalinity if I have high CYA?
Yes, if your CYA is high and your effective alkalinity (after the CYA correction) falls below 80 ppm, raising total alkalinity will push LSI back toward balanced. The target is effective TA in the 80–120 ppm range, so your raw TA target rises with CYA. At CYA 90, for example, you’d want raw TA around 110–140 ppm to achieve 80–110 ppm effective TA. That said, at very high CYA levels this becomes harder to manage — a partial drain to reduce CYA is often the better long-term fix.
Do indoor pools need to worry about this?
No. Indoor pools don’t use CYA because there’s no UV light to degrade chlorine. With CYA at zero, the correction term (CYA ÷ 3) is zero — and the formula uses raw total alkalinity as-is. The CYA correction only matters when CYA is present in meaningful amounts (generally above 30 ppm).
Keep reading
- Langelier Saturation Index — Explained — the complete formula and factor definitions
- Why LSI Matters: the real cost of unbalanced water
- Why Two LSI Calculators Give Different Answers — CYA correction is one of the four divergence points
- CSI vs. LSI: Are They the Same Thing? — how formula differences (including CYA correction) define accuracy
- LSI and Saltwater Pools — SWG pools with stabilizer
- LSI by Pool Surface Type — why the correction matters more for plaster pools
- Pool Calcium Hardness — the other LSI input that interacts with CYA management
- LSI Calculator — full formula with CYA correction