Cyanuric Acid (CYA)
Cyanuric acid (CYA) is a chemical added to pool water that protects chlorine from being destroyed by sunlight — acting as a sunscreen for chlorine. It's the most important and most misunderstood number in pool chemistry.
Target ranges
| Pool type | Target CYA | Notes |
|---|---|---|
| Traditional chlorine pool (outdoor) | 30–50 ppm | The standard target |
| Salt water pool (SWG) | 60–80 ppm | SWG cells run efficiently at higher CYA |
| Indoor pool | 0–20 ppm | No UV exposure, so little stabilizer needed |
| Above 80 ppm | Caution | Chlorine becomes increasingly ineffective |
| Above 100 ppm | Problem zone | Algae blooms common despite "normal" FC readings |
What CYA does
Without CYA, UV from sunlight destroys free chlorine within hours. A pool with no stabilizer can lose 90% of its FC in a single sunny day. CYA binds to chlorine molecules, shields them from UV, and slowly releases them back as active chlorine — effectively a slow-release sunscreen for your sanitizer.
But CYA is a double-edged tool. The same binding that protects chlorine also reduces how strongly chlorine sanitizes. The more CYA you have, the more total free chlorine you need to maintain the same actual killing power. This is the FC/CYA relationship — the single most important concept in modern pool chemistry.
The 7.5% rule of thumb
Free chlorine should be at least 7.5% of your CYA level. So at CYA 50, aim for FC of about 4 ppm. At CYA 80, you need 6 ppm. A "normal" FC reading can be dangerously low if CYA is high. See the FC/CYA chart for every level.
How CYA enters your water
CYA gets added to pools in two ways:
- Stabilized chlorine products — trichlor tablets and dichlor granules contain CYA built in. Every pound you add raises CYA permanently.
- Direct addition — CYA granules (sold as "stabilizer" or "conditioner") added straight to the pool to reach an initial target.
Liquid chlorine (sodium hypochlorite), cal-hypo, and salt water generators do not add CYA. This is why switching from tablets to liquid chlorine is the standard fix once your CYA reaches its target.
Why CYA causes most pool problems
CYA doesn't break down naturally. It doesn't evaporate. It doesn't react out of solution. Once CYA is in your pool, it stays there until you remove water. Pool owners using stabilized chlorine all season often end up with CYA above 100 ppm by mid-summer without realizing it.
When CYA climbs too high:
- Effective chlorine drops below what's needed to kill algae
- FC readings look "normal" but the water still goes green
- Shocking becomes ineffective — the chlorine is locked up
- Even adding huge amounts of chlorine doesn't fix it
How to lower CYA
There is no chemical that removes CYA from pool water. The only practical methods are:
- Partial drain and refill — the standard fix. Drain 25–50% of the pool, refill with fresh water, retest CYA. This is the only reliable home option.
- Reverse osmosis service — a specialist truck can filter CYA out without draining. Expensive, available in limited regions.
How to raise CYA
Add CYA granules (sold as "stabilizer" or "conditioner") directly to the pool. Dissolve slowly in the skimmer over a few days. As a rough guide, 1 lb per 10,000 gallons raises CYA by about 12 ppm.
Want to...
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