Cyanuric acid (pool stabilizer) explained
Cyanuric acid (CYA), sold as pool stabilizer or conditioner, shields chlorine from sunlight so it lasts through the day.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (8 sections)
Without it, chlorine breaks down in sunlight. The trade-off is slower-acting chlorine: the more CYA you have, the more free chlorine (FC) you need, so most chlorine pools do best at 30–50 ppm and salt water pools at 60–80 ppm.
What CYA does (sunlight shield)
Free chlorine in an outdoor pool is broken down by the ultraviolet part of sunlight. Without stabilizer, a sunny afternoon can destroy most of it, and you would have to add chlorine several times a day to keep any in the water. Cyanuric acid binds loosely to chlorine. Bound chlorine is protected from UV and acts as a reserve: as the small free, active part is used up, the bound part releases more.
That is why nearly every outdoor chlorine pool uses some stabilizer, and why it's added automatically with trichlor tablets and dichlor granules. Indoor pools, which get little UV, and hot tubs don't need it. The CDC advises against CYA in hot tubs.
The trade-off: slower chlorine (MAHC data)
The same binding that protects chlorine also keeps most of it from working. What kills germs and algae is hypochlorous acid (HOCl), and the CDC's Model Aquatic Health Code (MAHC) 2024 Annex shows that its concentration depends mostly on the ratio of free chlorine to CYA, not on free chlorine alone. Its table (pH 7.5, 77 °F) gives about 0.020 ppm of HOCl at a CYA:FC ratio of 20:1, about 0.0097 ppm at 40:1, and about 0.0038 ppm at 100:1.
Our engine estimate for a pool holding 3 ppm of free chlorine shows how large the effect is:
| CYA | 0 | 20 | 30 | 40 | 60 | 80 | 100 |
|---|---|---|---|---|---|---|---|
| HOCl, ppm | 1.46 | 0.059 | 0.039 | 0.029 | 0.020 | 0.015 | 0.012 |
Approximation fitted to the MAHC Annex table: HOCl ≈ 0.39 × FC ÷ CYA when CYA is much higher than FC.
Going from 30 to 60 ppm CYA halves the active chlorine at the same test reading. That's the reason a pool can "read 3 ppm" and still turn green. The fix isn't more stabilizer; it's a free chlorine level scaled to your CYA.
How much you need
- Chlorine pools (liquid chlorine, bleach, tablets): 30–50 ppm. Taylor lists 30–50 as ideal; ANSI/APSP-11 (as cited by PHTA) sets 100 ppm as the maximum.
- Salt water pools: 60–80 ppm is common because the cell makes chlorine slowly all day. Cell makers differ (Hayward allows up to 80 ppm; Pentair's IntelliChlor guide recommends 30–50 ppm), so check your manual.
- Hot tubs and indoor pools: none.
Enter your CYA to see the chlorine levels that go with it. The table below applies the same rule to every level from 0 to 100 ppm.
| CYA | Minimum FC | Target FC | Salt pool minimum | Salt pool target |
|---|---|---|---|---|
| 0 ppm | 1 | 2–4 | 1 | 2–4 |
| 10 ppm | 2 | 2–4 | 2 | 2–4 |
| 20 ppm | 2 | 2–4 | 2 | 2–4 |
| 30 ppm | 2.5 | 2.5–4.5 | 2 | 2–4 |
| 40 ppm | 3 | 3–5 | 2 | 2–4 |
| 50 ppm | 4 | 4–6 | 2.5 | 2.5–4.5 |
| 60 ppm | 4.5 | 4.5–6.5 | 3 | 3–5 |
| 70 ppm | 5.5 | 5.5–7.5 | 3.5 | 3.5–5.5 |
| 80 ppm | 6 | 6–8 | 4 | 4–6 |
| 90 ppm | 7 | 7–9 | 4.5 | 4.5–6.5 |
| 100 ppm | 7.5 | 7.5–9.5 | 5 | 5–7 |
How we apply the CDC/MAHC principle that minimum chlorine rises with CYA: minimum ≈ 7.5% of CYA (5% for salt water generators), at least 2 ppm with CYA. To shock, follow the directions on your product’s label. See methodology.
Notice the conflict at the top: many EPA-registered pool chlorine labels say not to swim above 4 ppm, while the minimum at CYA 60 is already 4.5 ppm. Keeping CYA at 30–50 is the simplest way to satisfy both. The reasoning is on pool chlorine levels.
To raise CYA, use granular or liquid stabilizer; the stabilizer calculator gives the dose. As a reference point, 13.4 oz of granular stabilizer raises 10,000 gallons by 10 ppm, and 0 to 40 ppm in 15,000 gallons takes about 5.0 lb. It dissolves slowly: put it in a sock in the skimmer basket or pre-dissolve it in a bucket, and retest after about five days before adding more.
How it builds up
Every stabilized chlorine product adds CYA, and it never gets used up the way chlorine does. PHTA puts it at 0.6 ppm CYA per 1 ppm of chlorine from trichlor and 0.9 ppm from dichlor. In practice:
- A pool that uses 2 ppm of chlorine a day on tablets alone gains about 8.4 ppm CYA a week, or roughly 36 ppm a month.
- Starting a season at 30 ppm, that pool passes 50 ppm in the third week and 80 ppm in the sixth.
- Evaporation doesn't remove CYA. Top-up water replaces the water, but the stabilizer stays.
The chlorine tablets calculator projects this week by week for your pool and chlorine use.
How long it lasts
CYA leaves mainly with water: splash-out, backwashing, overflow after heavy rain, and draining. A covered pool in a dry climate that never backwashes can hold its stabilizer for years. Pools that lose a lot of water through backwashing and rain lose CYA in proportion: replace 10% of the water and you lose about 10% of the CYA.
Some owners see CYA fall without draining, especially over a long closed season or after a period with no chlorine. Bacteria can break cyanuric acid down (a 2022 study in the Journal of Industrial Microbiology and Biotechnology used a bacterial enzyme for exactly that), and chlorine may oxidize it slowly. We found no reliable rate for home pools, so treat unexplained drops as something to confirm with a careful retest. Turbidity-based CYA tests read low in warm samples (PHTA), which can look like a loss.
Too low / too high
Too low (under about 20–30 ppm outdoors): chlorine disappears quickly in sunlight, often dropping to zero by late afternoon. You add chlorine every day and still find it gone. Raise CYA to your target.
Too high (over about 60 ppm for chlorine pools, 90 for salt pools): the chlorine you need climbs above what labels allow for swimming, algae can start growing at "normal" readings, and every daily chlorine dose has to be larger to do the same work. The MAHC 2024 Annex treats a CYA:FC ratio above 45:1 as a threshold for closing public pools. There is no chemical that reliably removes CYA; the fix is a partial drain. See how to lower CYA, and the full list of targets on the pool chemistry chart.
Never add more stabilizer than your target calls for: only draining brings it down. Store stabilizer dry and away from chlorine products. Always follow the product label.
FAQ
What happens if cyanuric acid is too low?
Sunlight destroys unprotected chlorine quickly, so free chlorine drops to near zero during the day even if you added plenty the night before. That leaves the pool unprotected and lets algae grow. Test CYA; if it's under 30 ppm in an outdoor chlorine pool, raise it with stabilizer.
How long does cyanuric acid last in a pool?
Until the water it is in leaves the pool. CYA doesn't evaporate and isn't consumed like chlorine, so it stays through the season and often across seasons. Splash-out, backwashing, rain overflow, and draining lower it in proportion to the water removed.
What depletes cyanuric acid in the pool?
Mostly water loss: backwashing, splash-out, overflow, and partial drains. Bacterial breakdown and slow oxidation by chlorine may lower it too, especially in closed or neglected pools, but the rate isn't well documented for home pools. Rule out test error first: retest with a diluted sample at room temperature.
Sources
- CDC, Model Aquatic Health Code 2024, Annex (HOCl vs CYA:FC table, closure ratio): cdc.gov (PDF)
- CDC, Home pool and hot tub water treatment and testing: cdc.gov
- PHTA fact sheet, Dichlor (CYA added per ppm of chlorine): phta.org
- Taylor Technologies, K-1005 instruction manual: PDF
- Hayward, AquaRite manual: PDF
- Pentair, IntelliChlor Quick Start Guide and Tips: PDF
- Removal of cyanuric acid in swimming pools using a cyanuric acid hydrolase, J Ind Microbiol Biotechnol 49(2), 2022: pmc.ncbi.nlm.nih.gov