Cyanuric acid: the stabiliser that keeps your chlorine alive, until it doesn’t
What CYA does, why it matters, what the right level is, how to test, and what to do when it’s too high. The complete pool-stabiliser guide.
Last updated: 2 June 2026
Cyanuric acid (CYA) is the most misunderstood chemical in residential pool care. It does an important job, stabilising chlorine against UV degradation, but build it up too far and it becomes the cause of every “I can’t keep chlorine in my pool” problem. Most pool owners don’t test for it. Many test kits don’t include it. Most chlorine products contain it and silently raise it month by month.
This guide covers what cyanuric acid actually does, why it matters, how to test for it correctly, what the right level is, and, most importantly, what to do when it’s too high. The full chemistry of what happens when CYA goes wrong is in our chlorine lock guide. This page covers managing CYA before it gets to that point.
What cyanuric acid is and what it does
Cyanuric acid is a stabiliser. Without it, UV light from the sun breaks down free chlorine in pool water within about 2 hours of midday sun exposure. With it, the same chlorine lasts 8–12 hours. CYA binds to chlorine in a reversible way, “parking” it in a form that UV can’t easily destroy. The chlorine releases back into active form on demand, when there’s something for it to oxidise.
That mechanism is what makes outdoor chlorinated pools possible. A pool without CYA at 30+ ppm in the Australian summer would need to be dosed three times a day to maintain sanitary chlorine levels. With CYA in range, once-daily dosing (or a saltwater chlorinator running on a normal schedule) is enough.
Why CYA also causes problems
The reserve relationship between CYA and chlorine is non-linear. As CYA climbs, the percentage of your free chlorine that’s actually active as hypochlorous acid (the killer form) drops sharply:
| CYA level (ppm) | Active chlorine as % of measured FC | Minimum FC to maintain sanitation |
|---|---|---|
| 0 | ~30% | 0.5 ppm |
| 30 | ~4–6% | 2 ppm |
| 50 | ~3% | 4 ppm |
| 80 | ~1.5% | 6 ppm |
| 100 | ~1% | 8 ppm |
| 150 | <1% | 12 ppm |
The minimum FC is what’s needed just to prevent algae. The full target FC is roughly 1.5× the minimum (11.5% of CYA, vs the 7.5% minimum). Most consumer test kits and most pool product labels don’t communicate this. They print “1–3 ppm” on the bottle regardless of CYA. That’s the cause of the most common residential pool problem: the owner sees “chlorine 2 ppm, fine!” while the pool is actually below the algae-prevention threshold.
The ideal CYA level for different pool types
| Pool type | CYA range | Why |
|---|---|---|
| Indoor pool | 0 ppm | No UV to stabilise against. CYA serves no purpose. |
| Outdoor chlorine pool | 30–50 ppm | Enough to slow UV degradation, low enough to keep FC targets reasonable. |
| Outdoor saltwater pool | 60–80 ppm | SWG cells run continuously; higher CYA prevents UV from outpacing the cell. |
| Outdoor spa | 20–30 ppm (or 0) | Spas use much less chlorine per litre; low CYA is fine, zero is OK for covered spas. |
| Hot Australian summer pool | 50–70 ppm | Higher UV intensity in QLD/NT/WA pools justifies slightly higher CYA. |
How CYA builds up over time
Cyanuric acid doesn’t degrade. Unlike chlorine, which oxidises away, or pH which drifts back to neutral, CYA stays in your water indefinitely until you physically remove it (by draining and refilling).
It builds up because most popular chlorine products contain CYA:
- Trichlor tablets, every 1 kg of trichlor adds about 600 g of CYA to the pool.
- Dichlor granular shock, every 1 kg adds about 570 g of CYA.
- Cyanuric acid stabiliser, 100% CYA, added intentionally.
The math is sobering. A typical 50,000 L Australian pool that’s chlorinated with trichlor tablets through summer will gain roughly 30 ppm of CYA per month of trichlor use. From a starting CYA of 30 ppm in October, by February you’re at 150 ppm and the pool is in chlorine-lock territory.
The single best CYA-management tip: Use trichlor tablets only when you need to ramp CYA up from low. Once CYA is in the 30–50 ppm range, switch to unstabilised chlorine sources, liquid chlorine (sodium hypochlorite) or cal-hypo, for ongoing chlorination. This keeps CYA stable instead of climbing.
How to test for CYA correctly
CYA is tested via a turbidity reaction, you add a reagent that causes CYA to cloud the water, then read how cloudy it gets against a graduated cylinder.
Reagent/drop test (Taylor K-2006C is the standard)
Accurate to about ±10 ppm. The K-2006C uses a small comparator tube, you fill it with the test solution and look down through the top until you can no longer see the dot at the bottom. The level at which the dot disappears gives you the CYA reading.
Pro tip: this test is sensitive to temperature. Cold water gives a falsely high reading. Bring your sample to room temperature (around 20 °C) before testing for accurate results, especially important in Australian winter pools.
Test strips with CYA
Some 7-way and 9-way test strips include CYA. Less accurate than a drop test (±20–30 ppm typical) but fine for a “high vs low” go/no-go check.
Digital photometers
Some digital photometers include CYA testing, others don’t. Verify before buying. The Lamotte ColorQ Pro 7 includes it; the cheaper ColorQ Pro 5 doesn’t.
What to do when CYA is too high
Three options, in order of cost-effectiveness:
1. Partial drain and refill (the standard fix)
Replacement percentage scales linearly with CYA. For a typical pool going from 100 ppm CYA back to 40 ppm:
| Starting CYA | Target CYA | % of water to replace |
|---|---|---|
| 80 ppm | 40 ppm | 50% |
| 100 ppm | 40 ppm | 60% |
| 150 ppm | 40 ppm | 73% |
| 200 ppm | 40 ppm | 80% |
Drain via your filter’s waste port (if equipped) or by sump-pump. Refill with mains water. Total cost: water bill (~$5 per kilolitre in most Australian cities, so ~$150–250 for a major drain on a 50,000 L pool) plus the time to do it.
2. Bio-Active CYA reducer (slow alternative)
A bacterial culture that biologically breaks down CYA over 7–10 days. Effective at reducing 30–50% of your starting CYA, not more. Best used when you’re at 80–120 ppm and want to avoid draining. Doesn’t work well at >150 ppm.
3. Reverse osmosis filtration (premium option)
Mobile RO services run pool water through a membrane on-site, stripping CYA, calcium, salt, and dissolved solids. Common in WA and parts of QLD where water restrictions make draining problematic. Cost: roughly $600–$1,200 for a typical Australian pool.
What doesn’t work: “CYA reducer” liquid additives that aren’t Bio-Active or similar bacterial cultures. Most “miracle CYA reducer” products on Australian shelves are at best a mild chelator with no measurable effect on CYA. Stick to the three options above.
Sources & references
- Trouble Free Pool, ABCs of Pool Water Chemistry
- Richard Falk, The Chlorine/CYA Relationship
- ANSI/APSP-11, Water Quality Standards in Public Pools
- AS 3633-1989, Private swimming pools (Australian Standard)
Frequently asked questions
What is cyanuric acid in a pool?
Cyanuric acid (CYA) is a chemical added to chlorinated pool water to stabilise the chlorine against UV degradation from sunlight. Without CYA, midday sun would destroy your free chlorine in about 2 hours. With CYA at 30+ ppm, the same chlorine lasts 8–12 hours.
What is the ideal CYA level?
For outdoor chlorine pools, 30–50 ppm. For outdoor saltwater pools, 60–80 ppm because the salt cell runs continuously. For indoor pools and most spas, 0 ppm, there’s no UV to protect against. Hot-climate outdoor pools (QLD, NT, WA) tolerate the higher end of these ranges.
How do I lower cyanuric acid in my pool?
Partial drain and refill is the standard fix. Replace 50–80% of your water (depending on current CYA level) with fresh water. Alternative: Bio-Active CYA reducer (bacterial product) for moderately high CYA. Or mobile reverse osmosis filtration for premium pools or water-restricted regions. Generic ‘CYA reducer’ liquids on shelves typically don’t work.
Does cyanuric acid expire or degrade?
No. CYA doesn’t degrade in pool water. The only way to lower it is to physically remove water (drain + refill), use a bacterial breakdown product, or run reverse osmosis filtration. This is why it builds up over a season if you’re using stabilised chlorine products.
What chlorine products don’t contain CYA?
Liquid chlorine (sodium hypochlorite, 12.5%), cal-hypo (calcium hypochlorite), saltwater chlorinator-generated chlorine, and pure sodium hypochlorite shocks. Anything labelled ‘stabilised chlorine’, trichlor tablets, or dichlor granular shock contains CYA.
How much CYA does each kg of trichlor add?
Trichlor tablets are about 60% available chlorine and 54% CYA by weight. So 1 kg of trichlor delivers 600 g of chlorine and adds 540 g of CYA to your pool. In a 50,000 L pool, that’s about 11 ppm of CYA per 1 kg of trichlor consumed.
Can my pool have CYA without me adding it?
Yes, if you’ve ever used trichlor tablets, dichlor shock, or ‘stabilised’ chlorine of any kind, CYA was added. Most pool owners who say ‘I don’t use stabiliser’ are surprised to test and find their CYA at 80+ ppm, it came from their chlorine source.
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