PooloraCyanuric acid (stabilizer): how much your pool needs
Stabilizer keeps the sun from burning off your chlorine, but too much of it quietly makes chlorine weaker. Here is how to keep it in the useful range.
- Keep stabilizer (CYA) at 30–50 ppm and never above 100 ppm.
- About 13.5 oz (380 g) of stabilizer raises CYA by 10 ppm in 10,000 gallons.
- Only replacing water lowers it. Measure before every addition.
What stabilizer does, and the catch
Cyanuric acid (CYA) bonds loosely with chlorine and shields it from sunlight, so an outdoor pool keeps its chlorine for much longer. The catch is that the chlorine held by CYA is not doing the disinfecting. The CDC notes that CYA lengthens the time chlorine needs to kill germs, which is why it recommends stabilizer for pools but not for hot tubs.
The practical rule: the more stabilizer you have, the more free chlorine you need. Poolora sets the minimum at about 7.5% of your CYA.
| Stabilizer (CYA) | Minimum free chlorine | Target free chlorine |
|---|---|---|
| 20 ppm | 2 ppm | 3–5 ppm |
| 30 ppm | 2.25 ppm | 4–6 ppm |
| 40 ppm | 3 ppm | 5–7 ppm |
| 50 ppm | 3.75 ppm | 6–8 ppm |
| 60 ppm | 4.5 ppm | 7–9 ppm |
| 80 ppm | 6 ppm | 9–11 ppm |
The right range: 30–50 ppm
The Pool & Hot Tub Alliance and the National Plasterers Council recommend 30–50 ppm, and the Model Aquatic Health Code sets 100 ppm as the maximum. Indoor pools and hot tubs do not need it.
There is a second reason to stay low: your chlorine label. Labels we checked limit swimming to 3 or 4 ppm. With a 3 ppm label, the chlorine minimum only fits under the limit while CYA is 40 ppm or less; with a 4 ppm label, about 53 ppm. Above that, no chlorine level is both legal for swimming and strong enough. That is why Poolora caps the default stabilizer target at 40 or 50 ppm depending on your label.
How much stabilizer to add
Dry cyanuric acid is close to 100% pure, so the dose is a simple mass balance. Per 10,000 gallons:
| Raise CYA by | Dry stabilizer (100%) |
|---|---|
| 10 ppm | 13.5 oz 380 g |
| 20 ppm | 1.7 lb 755 g |
| 30 ppm | 2.5 lb 1.15 kg |
Worked example: a 15,000-gallon pool at 10 ppm going to 40 ppm needs a 30 ppm increase, which is 3.8 lb (60 oz · 1.7 kg) of 100% stabilizer.
Stabilizer dissolves slowly, often over several days. Follow the product directions (many say to dissolve it in a bucket of warm water or place it in the skimmer basket), and do not add a second dose until the first one has dissolved and you can measure it.
Tablets add stabilizer every week
Trichlor tablets and dichlor granules contain cyanuric acid. For every 10 ppm of chlorine they add, trichlor also adds about 6.1 ppm of CYA and dichlor about 9.1 ppm. A pool that uses 2.5 ppm of chlorine a day from trichlor tablets gains about 11 ppm of CYA every week, which is why tablet pools tend to end the summer with too much stabilizer.
If you use tablets, test CYA every couple of weeks, and consider switching to liquid chlorine once CYA reaches the top of your range.
Too much stabilizer: replace water
No common chemical removes cyanuric acid from a residential pool. The reliable fix is dilution: drain part of the water and refill with water that has no stabilizer. The share to replace is 1 − (target ÷ current):
| From | To | Water to replace |
|---|---|---|
| 60 ppm | 40 ppm | 33% |
| 80 ppm | 40 ppm | 50% |
| 90 ppm | 40 ppm | 56% |
| 100 ppm | 50 ppm | 50% |
Some pools should not be fully drained (vinyl liners, fiberglass shells and pools in high groundwater can be damaged). Replace water in steps if needed, and follow your builder's guidance. Test the fill water too: if it already contains stabilizer, you will need to replace more.
Algae and high stabilizer
Clearing algae takes free chlorine around 40% of your CYA, held until the water is clear. The labels we checked cap superchlorination at 10 ppm, which only covers CYA up to 25 ppm. If you have algae and CYA above that, lower stabilizer first, then shock within the label limit.
Common questions
Does a salt pool need stabilizer?
Yes, if it is outdoors. The generator makes chlorine, but without stabilizer the sun destroys it almost as fast.
Do indoor pools need stabilizer?
Usually not, because there is no sunlight to protect chlorine from, and stabilizer would only make chlorine work more slowly.
Will stabilizer go down on its own?
Not in any useful way. It leaves with the water: splash-out, backwashing and rain overflow. Replacing water is the only reliable way to lower it.
How accurate is a stabilizer test?
Stabilizer tests read cloudiness, so they are least precise at the very low and very high ends. If a reading will lead to draining water, test twice or have it confirmed.
Do it with your own products
Poolora turns your test into the exact amount of the products on your shelf, in cups and ounces, in the right order, and tells you when it's safe to swim. Free to start; every chemical, water balance and history are part of Pro.
Keep going
Sources
- CDC: Home pool and hot tub water treatment and testing (CYA lengthens kill time; pools only)
- Pool & Hot Tub Alliance (PHTA): cyanuric acid technical bulletin (30–50 ppm; never above 100)
- National Plasterers Council (NPC): recommended CYA 30–50 ppm
- Model Aquatic Health Code (MAHC) 5.7.3.1.3.1: CYA maximum 100 ppm
- U.S. EPA registered labels for sodium hypochlorite pool products (Reg. No. 70271-10 and 82357-1): swimming range and 5–10 ppm superchlorination
- O'Brien, J. E. et al. (1974), chlorine and cyanuric acid equilibria in water