Cyanuric acid calculator
Stabiliser is sunscreen for chlorine. It is also cumulative, invisible in a chlorine test, and the single most common reason a pool that reads 3 ppm still turns green in July.
Stabiliser protects chlorine from sunlight and then, past a point, holds it hostage. WHO caps cyanuric acid at 100 mg/L where chlorinated isocyanurates are used.
Replace water
12,833gal
That is 58% of the pool. Nothing else takes CYA down: it does not gas off, burn off or filter out. Drain in stages if the water table is high or the shell is fibreglass.
Cyanuric acid after the correction
50ppm
Where it comes from
Almost nobody adds stabiliser on purpose twice. It arrives in the tablets: trichlor and dichlor are chlorinated isocyanurates, so every ppm of chlorine they deliver leaves cyanuric acid behind. A pool chlorinated entirely on tablets picks up roughly 0.6 ppm of CYA for every ppm of chlorine, week after week, and nothing removes it.
The dilution maths
This is the one correction with no chemistry in it at all. Replace a third of the water and you remove a third of the stabiliser. To go from 120 ppm to 50 ppm you must replace 58% of the pool — for a 22,000 gallon pool that is nearly 12,800 gallons down the drain and back off the meter.
| From | To | Replace | 22,000 gal pool |
|---|---|---|---|
| 80 ppm | 50 ppm | 38% | 8,250 gal |
| 120 ppm | 50 ppm | 58% | 12,833 gal |
| 150 ppm | 50 ppm | 67% | 14,667 gal |
| 200 ppm | 50 ppm | 75% | 16,500 gal |
Do not drain a pool blind
A high water table can float an empty fibreglass or vinyl pool out of the ground, and a drained plaster shell can crack in heat. Drain in stages with the hose running, or hire a reverse-osmosis truck where one is available.
Dosage notes
Granular stabiliser takes up to a week to fully dissolve and read true. Add it through a sock in the skimmer, run the pump, and do not re-test for seven days — techs who test on day two read a false low, add more, and land at 90 ppm by accident. If you are already over the ceiling, stop the tablets first; there is no point diluting a pool that is still being fed.