Cyanuric acid (CYA) is a chlorine stabilizer that shields free chlorine from being destroyed by the sun’s UV rays. Without it, direct sunlight can wipe out most of the chlorine in an outdoor pool within a few hours. The target range for most outdoor pools is 30 to 50 ppm. Too little and your chlorine disappears faster than you can add it. Too much and your chlorine becomes so sluggish it can barely sanitize at all. It’s one of those chemicals where the dose really is the medicine.
Why Does Sunlight Destroy Chlorine?
Free chlorine – the active sanitizing form – is chemically unstable when hit with UV radiation. UV energy breaks the chlorine molecule apart, turning it into chloride ions that provide zero sanitation. On a bright summer day, an unprotected outdoor pool can lose 75 to 90 percent of its free chlorine in about two hours. That’s not a slow leak; that’s almost everything, gone before lunch. Cyanuric acid works by temporarily bonding with chlorine molecules and holding them in a protected state. UV rays can’t break down the CYA-chlorine bond the same way they attack free chlorine, so much more of your sanitizer survives the day.
How Does CYA Actually Work?
Cyanuric acid forms a loose chemical bond with hypochlorous acid (the active killing form of chlorine) and acts as a kind of reservoir. When the pool water calls for active chlorine – say, a swimmer introduces contaminants, or bacteria enter the water – the bond breaks and releases free chlorine to do its job. Once the CYA releases that chlorine and it does its work, the CYA molecule is still there, ready to stabilize the next dose you add. This is why CYA is sometimes called a “slow feeder” for chlorine: it doesn’t eliminate the chlorine, it just slows the release and protects it from UV burnoff at the same time.
The catch is that CYA slows chlorine’s reaction speed even when you want it to work fast. At 30 ppm CYA, chlorine is still quite effective. At 100 ppm CYA, the available active chlorine is so suppressed that you’d need to run a free chlorine level of 10 ppm or higher to get the same kill time as 2 ppm of unstabilized chlorine. That’s why high CYA is sometimes called “chlorine lock,” though that’s a bit of an oversimplification.
What CYA Level Should You Keep in Your Pool?
For a standard outdoor chlorine pool, target 30 to 50 ppm. This range gives you solid UV protection without significantly throttling chlorine’s effectiveness. If you run a salt chlorine generator, most manufacturers recommend 60 to 80 ppm because SWGs produce chlorine continuously, and they benefit from extra protection to reduce how hard they have to work. Above 80 ppm you’re in diminishing-returns territory. Above 100 ppm, most pool professionals consider the water compromised enough that a partial drain is the only real fix.
Some pool owners discover their CYA has crept to 150, 200, even higher over years of using stabilized chlorine tabs (trichlor) without periodic dilution. Every trichlor tablet or granule you add contains roughly 50 percent CYA by weight, so the CYA climbs all season long and doesn’t go anywhere when the season ends. If you’ve been running tabs for years without tracking CYA, get it tested.
How to Add Cyanuric Acid to Your Pool
CYA comes as a dry granular product or a liquid stabilizer. The granular form is more common and tends to be cheaper. Here’s how to add it properly:
- Test your current CYA level first. Don’t dose blind.
- Calculate how much you need. To raise CYA by 10 ppm in a 10,000-gallon pool, you need roughly 1.3 oz (about 37 grams) of granular CYA.
- Pre-dissolve granular CYA in a bucket of warm water before adding it. CYA dissolves slowly and can sit on the pool floor and bleach the surface if you add it dry.
- Pour the dissolved CYA into the pool near a return jet with the pump running.
- Run the pump for at least 24 hours before retesting. CYA takes time to fully incorporate and your first retest after an hour or two will read low.
AquaDoc makes a granular stabilizer a lot of pool owners use for this exact purpose – it dissolves more evenly than some bulk-store versions, which helps avoid the “white patch on the floor” problem people run into.
Do Indoor Pools or Hot Tubs Need Cyanuric Acid?
No. If there’s no sunlight hitting the water, CYA provides zero benefit. For indoor pools, it only adds unnecessary chemical load and reduces chlorine effectiveness. Hot tubs are the same story – the CYA would just make your sanitizer work harder for no return. Stick to unstabilized chlorine (like sodium hypochlorite or lithium hypochlorite) for any application that isn’t an outdoor pool exposed to direct sun. This is one of the most common mistakes new pool owners make when they treat all chlorine products as interchangeable.
What Happens If Your CYA Gets Too High?
Once CYA is dissolved in pool water, it doesn’t break down. It doesn’t evaporate. It doesn’t get filtered out. It just accumulates. The only way to reduce it is to remove water. A partial drain of 25 to 50 percent of the pool volume, followed by refilling with fresh water, is the standard fix. If your CYA is at 200 ppm, you may need two rounds of this to get back to range. Some products claim to break down CYA enzymatically, but independent testing has not shown reliable results from these. Dilution is the solution. For guidance on managing overall water chemistry season to season, the team at River Pools and Spas has published solid practical resources worth bookmarking.
Frequently Asked Questions
What level of cyanuric acid should I keep in my pool?
For a standard chlorine pool, keep CYA between 30 and 50 ppm. Salt water pools can run 60 to 80 ppm. Above 100 ppm, chlorine loses so much effectiveness that you essentially need to dilute the pool to fix it.
Does cyanuric acid raise or lower pH?
Cyanuric acid is a weak acid, so adding it can slightly lower pH. The effect is usually minor, but test your pH after adding CYA and adjust if needed.
How do you lower cyanuric acid in a pool?
The only reliable way to lower CYA is to drain part of the pool and refill with fresh water. There are no chemicals that reliably remove CYA once it’s in solution.
Do indoor pools need cyanuric acid?
No. Indoor pools have no UV exposure, so CYA provides no benefit. Using it indoors can actually make your chlorine less effective for no gain.
Can high CYA make a pool look cloudy?
High CYA alone does not typically cause cloudiness directly, but it weakens chlorine enough that algae and bacteria can take hold – and that does cause cloudy water. The CYA is the hidden cause, not a direct one.
CYA is one of those pool chemicals that does exactly one job and does it well – until you have too much of it, at which point it quietly works against you. Test it a few times a season, keep it in the 30 to 50 ppm range, and switch to unstabilized chlorine if it starts climbing. That’s really all there is to it.