CYA / Stabilizer Calculator
Calculate how much cyanuric acid (stabilizer/conditioner) to add to protect your chlorine from UV sunlight.
Safety
CYA above 80 ppm causes "chlorine lock" — chlorine stops sanitizing effectively. The only way to lower CYA is to drain and refill. Dissolve granular CYA in a sock placed in front of a return jet, or pre-dissolve in warm water. Wait 48–72 hours before retesting.
To raise cyanuric acid, add about 13 oz (0.8 lb) of granular stabilizer per 10,000 gallons for every 10 ppm you need. Aim for 30–50 ppm in a chlorine pool, 60–80 in a salt pool; it dissolves slowly, so wait 48–72 hours before retesting. No chemical lowers CYA — only a partial drain and refill does. Enter your pool's numbers above for the exact amount.
Stabilizer Protects Chlorine — Until There Is Too Much
Cyanuric acid — stabilizer, conditioner, CYA — is sunscreen for chlorine. With none in the water, ultraviolet light burns off most of your free chlorine within a couple of hours of full sun, and an outdoor pool becomes almost impossible to keep sanitized. Add some and chlorine holds through the day. The catch is that the same shielding that protects chlorine also weakens it, so there is a right amount and the band is narrow: 30–50 ppm for a chlorine pool, 60–80 ppm for a salt pool whose cell runs at a steady low output.
The number that matters is FC as a share of CYA
This is the single idea that makes stabilizer make sense. A free-chlorine reading means nothing on its own — it only means something relative to the CYA holding it back. The working minimum is about 7.5% of your CYA level: below that ratio, algae gains ground no matter what the FC number says. So a pool at 30 ppm CYA is safe around 2–3 ppm FC, while a pool at 80 ppm CYA needs 6 ppm just to stand still. The generic "keep chlorine at 1–3 ppm" advice is what gets high-CYA pools into trouble — at 80 ppm CYA, 1–3 ppm is effectively unsanitized.
| CYA level | Minimum FC 7.5% of CYA |
Shock FC 40% of CYA |
|---|---|---|
| 20 ppm | 1.5 ppm | 10 ppm |
| 30 ppm | 2.3 ppm | 12 ppm |
| 40 ppm | 3.0 ppm | 16 ppm |
| 50 ppm | 3.8 ppm | 20 ppm |
| 60 ppm | 4.5 ppm | 24 ppm |
| 70 ppm | 5.3 ppm | 28 ppm |
| 80 ppm | 6.0 ppm | 32 ppm |
Shock target is the higher of 10 ppm or 40% of CYA — the same rule the shock calculator uses. This is exactly why high CYA is expensive: every shock has to reach a level that climbs with it.
Why it creeps up: your tablets
If your CYA keeps climbing and you never add stabilizer on purpose, your chlorine is adding it for you. Trichlor tablets are about 55% cyanuric acid by weight, and a pool floating a tab or two a week picks up roughly 25–30 ppm of CYA a month on top of the chlorine. Dichlor is the granular version of the same trap, adding about 9 ppm of CYA for every 10 ppm of chlorine it delivers. Cal-hypo and liquid chlorine add none — so a pool that started the season at 40 ppm and ran on tabs can finish it at 80 or 100 with nobody having touched the stabilizer bag.
The ceiling, and "chlorine lock"
There is a hard top to this. Once CYA passes roughly 90–100 ppm, chlorine is so heavily buffered that keeping a working FC ratio becomes impractical, and CDC guidance treats water above about 90 ppm as effectively unsanitized — which is why many public-pool codes cap it there. "Chlorine lock" is the loose name pool stores give this: the chlorine is present on the test but too weak to do its job, because the FC-to-CYA ratio has collapsed. It is not a chemical bond you break with a product; it is a ratio problem you fix by lowering CYA.
Raising it: cyanuric acid, slowly
13 oz of granular cyanuric acid per 10,000 gallons raises CYA about 10 ppm. It dissolves stubbornly — allow 48 to 72 hours before the reading catches up. The clean method is a sock: put the granules in a tube sock or nylon, hang it in front of a return jet or in the skimmer basket, and squeeze it occasionally. Do not broadcast it over the floor, where it sits undissolved and reads low, and above all do not re-dose because the test has not moved yet. That is the fastest route to an accidental overshoot you then have to drain out.
Cyanuric Acid Dose Chart
How much granular cyanuric acid raises CYA by a given amount, at two common pool sizes.
| CYA change | Cyanuric acid 10k gal |
Cyanuric acid 20k gal |
|---|---|---|
| 10 ppm | 13 oz (0.8 lb) | 26 oz (1.6 lb) |
| 20 ppm | 26 oz (1.6 lb) | 52 oz (3.3 lb) |
| 30 ppm | 39 oz (2.4 lb) | 78 oz (4.9 lb) |
| 40 ppm | 52 oz (3.3 lb) | 104 oz (6.5 lb) |
| 50 ppm | 65 oz (4.1 lb) | 130 oz (8.1 lb) |
16 oz = 1 lb. Dissolves over 48–72 hours — retest before adding more. Enter your own volume above for the exact figure.
Lowering it: only water comes out
Nothing removes cyanuric acid chemically at a residential scale. The reliable routes are dilution — a partial drain and refill — or reverse osmosis, which strips CYA out and returns the same water. The drain fraction is the same arithmetic as any dilution: the share you replace is the gap over the current level, so halving CYA means changing out about half the water.
| Current CYA | Target | Water to replace |
|---|---|---|
| 60 ppm | 40 ppm | 33% |
| 80 ppm | 40 ppm | 50% |
| 100 ppm | 40 ppm | 60% |
| 120 ppm | 40 ppm | 67% |
Assumes CYA-free fill water. The full method, including how to prevent the climb in the first place, is in how to lower CYA.
Once CYA is where you want it, set your chlorine to match with the chlorine calculator, and if the ratio idea is new, pool chemistry 101 walks through how FC, CYA, and pH fit together.