Raising CYA from 20 to 40 ppm in a 15,000 gallon pool takes 2.5 pounds of cyanuric acid (40.5 oz), which is one 4 lb bottle. Lowering it works the other way round: from 100 to 50 ppm you replace half the pool, 7,500 gallons, because nothing you add to the water removes cyanuric acid. Choose Raise or Lower above, enter your reading and a target, and the calculator gives the amount and what it does to your free chlorine target.
How the stabilizer amount is calculated
Cyanuric acid = ppm to add × gallons ÷ 10,000 × 13.5 oz per 10 ppm per 10,000 gal
Stabilizer is sold at about 99% purity, so 10 ppm in 10,000 gallons takes 13.5 oz. Swim University quotes 13 oz and the Indiana State Department of Health chemical adjustment guide also gives 13 oz; the difference is purity and rounding. For 20 ppm in 15,000 gallons that is 20 × 1.5 × 1.35 oz = 40.5 oz.
The drain figure follows from mixing. If you replace a share f of the water with fill water at a CYA of zero, the new level is (1 − f) × the old level.
Share to replace = (current − target) ÷ (current − CYA of fill water)
From 100 to 50 ppm with plain tap water: (100 − 50) ÷ 100 = 50%. Swim University's rule of thumb agrees: if CYA is 10 percent too high, replace about 10 percent of the water. A drain-and-refill in stages needs slightly more water, because the new water mixes with the old.
What CYA does to your free chlorine
Cyanuric acid shields chlorine from sunlight, but it also slows chlorine down. Free chlorine has to rise with it. With the Trouble Free Pool ratios we follow, the minimum is 7.5% of CYA, the target 11.5% and the SLAM level 40%.
| CYA | Free chlorine minimum | Target | SLAM level |
|---|---|---|---|
| 20 ppm | 1.5 ppm | 2.5 ppm | 8 ppm |
| 40 ppm | 3 ppm | 4.5 ppm | 16 ppm |
| 60 ppm | 4.5 ppm | 7 ppm | 24 ppm |
| 80 ppm | 6 ppm | 9 ppm | 32 ppm |
That is why more stabilizer is not better. Doubling CYA doubles the chlorine you must hold, and a SLAM becomes impractical near 80 ppm.
Target and maximum: where the authorities differ
PHTA and the Pentair and CircuPool salt cell manuals ask for 30 to 50 ppm. The Hayward AquaRite manual asks for 60 to 80 ppm in salt pools. Orenda recommends no more than 50 ppm in a private pool. The CDC Model Aquatic Health Code sets a limit of 90 ppm and ANSI/APSP-11 says 100 ppm. For a fecal incident the CDC needs CYA at 15 ppm or less to use its 20 ppm chlorine protocol. The presets above use 40 ppm for chlorine pools and follow the salt cell maker for salt pools, and the checks under the result show which limit your number meets.
How to add stabilizer
Cyanuric acid dissolves slowly. Swim University fills a sock with the granules and hangs it in front of a return jet or sets it in the skimmer basket with the pump running, squeezing it every 15 minutes or so until it is empty. Do not pour it into the skimmer pipe. CYA cannot be tested accurately until it has dissolved, so wait before you retest and decide on more. Follow your bottle if it says otherwise.
Check what you are already adding. Dichlor adds about 0.9 ppm of CYA per ppm of chlorine and trichlor about 0.6 ppm, so a pool chlorinated with tablets or granules often rises without any bottle. The pool chlorine calculator shows how much CYA each product leaves behind.
Lowering CYA
Draining is the only sure way. If your fill water carries CYA, say water from another pool, enter it in the fill water box. If fill water has as much CYA as your target, no amount of draining reaches it, and the tool says so. After a partial drain, retest, and rebalance calcium and alkalinity in the new water with the pool chemical calculator.
What to do next
If CYA is high and the pool is green, lower CYA before you use the pool shock calculator. Salt pool owners can pair it with the pool salt calculator, and the LSI calculator uses CYA to correct alkalinity. Start with your gallons from the pool volume calculator. All chemistry tools are in the pool chemistry hub.