Cyanuric acid (CYA) might be the most misunderstood chemical in pool service. Done right, it stretches chlorine dollars and protects pools from UV burn-off. Done wrong — or ignored — it turns into the silent killer that makes a pool look balanced on paper while chlorine does almost nothing in practice. This guide covers everything a working tech needs to know.
Free chlorine (hypochlorous acid, HOCl) is destroyed by UV light rapidly. In direct midday sun, an unstabilized outdoor pool can lose 50–90% of its free chlorine within two hours. Cyanuric acid works as a UV shield by temporarily bonding with hypochlorous acid to form chlorinated isocyanurates — compounds that are protected from photodegradation but can release active chlorine as the pool consumes it.
Think of CYA as a reservoir that buffers chlorine against UV destruction. Without it, every outdoor pool would require constant dosing during the day to maintain any residual. With it, one morning dose of chlorine can last through the afternoon.
| Pool Type | Target CYA Range | Notes |
|---|---|---|
| Outdoor chlorine (tablets or liquid) | 30–50 ppm | Optimal UV protection without over-binding chlorine |
| Salt water generator (SWG) | 60–80 ppm | SWG output is continuous; higher CYA reduces generation demands |
| Indoor pool | 0–20 ppm | No UV exposure; stabilizer adds cost with no benefit |
| Competition / commercial | 0–40 ppm | Regulations often cap at 40 ppm or prohibit stabilizer entirely |
Granular CYA is the most common form. It dissolves slowly and must be handled carefully to avoid clogging returns or settling in one spot.
Liquid stabilizer (usually 50% concentration) acts immediately and won't clog equipment. Pour slowly into skimmer or around the perimeter with the pump running. More expensive per pound of CYA, but preferred for precise small adjustments.
Both trichlor tablets and dichlor granules contain cyanuric acid bound to the chlorine molecule. Every time you add chlorine via these sources, you're also adding CYA. Trichlor is approximately 52–57% CYA by weight. The relationship:
| Chlorine Source | Adds CYA? | Approximate CYA per 10 ppm Cl₂ Added |
|---|---|---|
| Trichlor tablets | Yes | ~6 ppm |
| Dichlor granules | Yes | ~9 ppm |
| Cal-hypo (calcium hypochlorite) | No | 0 ppm |
| Liquid chlorine (sodium hypochlorite) | No | 0 ppm |
| SWG (salt electrolysis) | No | 0 ppm |
A pool maintained exclusively on trichlor tablets can accumulate 30–60 ppm of additional CYA per season just from normal chlorine dosing — even if you never add standalone stabilizer.
| Pool Volume | To raise CYA 10 ppm (granular) | To raise CYA 10 ppm (liquid 50%) |
|---|---|---|
| 10,000 gal | 13 oz (~0.8 lb) | 16 oz |
| 15,000 gal | 19 oz (~1.2 lb) | 24 oz |
| 20,000 gal | 26 oz (~1.6 lb) | 32 oz |
| 30,000 gal | 39 oz (~2.4 lb) | 48 oz |
This is where most pool owners get confused — and where good techs earn their pay. CYA doesn't reduce the reading of free chlorine on a test kit. It reduces its effectiveness. A pool can show 3.0 ppm free chlorine and still be functionally unable to sanitize properly if CYA is at 150 ppm.
The key concept is minimum effective free chlorine (FC) by CYA level. The pool industry uses the "7.5% rule" as a rough guide — for basic sanitation, FC should be at least 7.5% of CYA. For algae prevention, 10–15% is safer.
| CYA Level | Minimum FC (7.5%) | Recommended FC (10%) | SLAM FC Target |
|---|---|---|---|
| 30 ppm | 2.3 ppm | 3.0 ppm | 12 ppm |
| 50 ppm | 3.8 ppm | 5.0 ppm | 20 ppm |
| 80 ppm | 6.0 ppm | 8.0 ppm | 31 ppm |
| 100 ppm | 7.5 ppm | 10 ppm | 39 ppm |
| 150 ppm | 11.3 ppm | 15 ppm | Drain required |
CYA is measured via turbidimetric (cloudiness) comparison. The standard method uses a black dot turbidity tube — you add reagent to the sample and look down the tube until the dot disappears, reading the level at the side scale.
For commercial pools or disputed readings, send a sample to a certified lab. The turbidity method can be confused by high turbidity from algae or other particles — lab analysis uses HPLC for accurate results.
There is no chemical method for reducing CYA that works reliably in a pool environment. Marketing claims aside, the only proven approaches are:
SplashLens logs every chemical test — including CYA history — so you can spot creeping stabilizer levels before they become a drain-and-refill problem. Works offline on the job site.
Open SplashLens Free →For outdoor chlorine pools: 30–50 ppm. Salt chlorine generator pools: 60–80 ppm (higher CYA needed because SWG output varies). Above 90 ppm, chlorine efficacy drops significantly regardless of free chlorine reading.
Granular stabilizer added in a skimmer sock typically dissolves in 24–48 hours with the pump running. Liquid cyanuric acid is immediately available. Do not backwash for at least 48 hours after adding granular CYA.
No practical chemical method exists to reduce CYA in pool water. The only reliable solutions are partial drain-and-refill or reverse osmosis filtration. There is no "CYA reducer" chemical that works reliably at pool scale.
Cyanuric acid is a weak acid. At typical doses, it has minimal impact on pH. However at very high CYA levels (100+ ppm), it can contribute to a slight pH drop and throws off standard alkalinity test kits.
Trichlor tablets are 52–57% cyanuric acid by weight. Every 10 ppm of chlorine added via trichlor also adds approximately 6 ppm of CYA. A pool relying entirely on tablets can gain 30–60 ppm of CYA per season without any additional stabilizer.