Cyanuric Acid (CYA / Stabilizer) Explained
Quick Answer
Cyanuric acid (CYA) is a pool stabilizer that shields free chlorine from UV light degradation. Without CYA, outdoor pools lose nearly all their chlorine within a few hours of sunlight. Keep CYA at 30–50 ppm. Above 100 ppm causes chlorine lock — the only fix is dilution.
- CYA protects outdoor pool chlorine from UV light; without it, FC is destroyed in hours
- Ideal range: 30–50 ppm for traditional pools; 60–80 ppm for salt water pools
- Above 100 ppm causes "chlorine lock" — FC is present but largely ineffective
- CYA does not break down naturally; the only way to lower it is partial drain and refill
What Is Cyanuric Acid
Cyanuric acid (CYA), also known as pool stabilizer or pool conditioner, is a heterocyclic organic compound that forms a weak, reversible bond with hypochlorous acid in pool water. This bond protects chlorine molecules from being immediately destroyed by UV radiation from sunlight, extending their effective life in outdoor pools dramatically. When chlorine encounters a pathogen or organic compound, the CYA bond breaks and releases the chlorine to do its sanitizing work, then CYA reforms the bond with remaining or newly-added chlorine.
Why It Matters
Without CYA, an outdoor pool can lose 75–90% of its free chlorine within 2–4 hours of direct sunlight, making maintenance both expensive and ineffective. CYA allows chlorine to last throughout the day rather than requiring constant re-dosing. However, excessive CYA creates the opposite problem — it binds chlorine so tightly that FC loses effectiveness even at high concentrations, a condition known as chlorine lock. Maintaining CYA in the 30–50 ppm range balances UV protection with chlorine availability.
Ideal Range
| Parameter | Value | Notes |
|---|---|---|
| CYA (outdoor pools) | 30–50 ppm | Below 20 ppm: significant UV loss; above 80 ppm: reduced effectiveness |
| CYA (salt water pools) | 60–80 ppm | Slightly higher to compensate for continuous SWG output |
| CYA (indoor pools) | 0 ppm | No UV exposure; CYA unnecessary and may impair sanitization |
Symptoms When Too Low
| Symptom | What It Means | Fix |
|---|---|---|
| Rapid FC loss during the day | UV destroys unprotected chlorine within hours of sunlight | Add stabilizer/conditioner; raise CYA to 30–50 ppm |
| FC at zero by midday | Pool unprotected during peak swim hours | Add CYA immediately; also check for algae demand |
| Frequent expensive chlorine dosing | Burning through chlorine due to UV degradation | Raise CYA once; chlorine consumption will drop significantly |
Symptoms When Too High
| Symptom | What It Means | Fix |
|---|---|---|
| Chlorine lock (FC ineffective despite normal readings) | CYA over-binds FC; effective sanitizer drops dramatically | Drain 30–50% of pool water; refill; rebalance all parameters |
| Persistent algae despite normal FC | Chlorine present but too stabilized to kill algae | Partial drain to lower CYA; rebalance before shocking |
| Test kit reads FC present but pool is green | CYA masks true FC availability from colorimetric tests | Drain and refill to lower CYA below 80 ppm |
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Frequently Asked Questions
What is cyanuric acid and why do I need it?
Cyanuric acid (CYA) is a pool stabilizer that shields free chlorine from UV light degradation. Without it, outdoor pools lose up to 90% of FC in a few hours of direct sunlight. CYA forms a reversible bond with chlorine that protects it from UV while still allowing it to react with pathogens. Target 30–50 ppm for outdoor pools using traditional chlorine.
How do I add cyanuric acid to my pool?
Add granular CYA directly to the skimmer with the pump running, or dissolve in a bucket of warm water first and pour slowly around the pool perimeter. CYA dissolves slowly — allow 24–48 hours for it to fully incorporate before testing. CYA levels rise slowly so avoid multiple additions close together. Never mix with other chemicals.
What happens if CYA is too high?
Above 80–100 ppm, CYA begins to over-stabilize chlorine in a condition known as chlorine lock. FC tests as present but is largely unavailable to kill pathogens, algae can grow despite apparent normal FC levels, and test results become unreliable. The only solution is to drain 30–50% of pool water and refill — CYA does not degrade naturally.
How much CYA should a salt water pool have?
Salt water pools typically benefit from slightly higher CYA — 60–80 ppm — because the continuous chlorine generation from the SWG can compensate for some efficiency loss from higher stabilizer levels. This range provides excellent UV protection. Above 80 ppm in a salt water pool still carries chlorine lock risk.
Does CYA affect pool chemistry tests?
High CYA can cause interference with OTO-based test kits (yellow/orange colorimetric chlorine tests), producing inconsistent or low FC readings. Use DPD-based test kits (pink color scale) for accurate FC measurement in CYA-stabilized pools. Digital photometers provide the most reliable results when CYA is above 30 ppm.
Can CYA build up in hot tubs?
Yes. Dichlor (sodium dichloro-s-triazinetrione), the standard hot tub sanitizer, contains about 54% CYA by weight. Every dose adds a small amount of stabilizer to the water. After months of dichlor use, CYA can accumulate to 100+ ppm — one reason hot tub water should be completely drained and replaced every 3–4 months.
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- Typical range: 1–3 ppm chlorine
- Recommended pH: 7.2–7.6
- Test water regularly
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Last updated: April 2026