Ideal CYA (Stabilizer) Levels for an Outdoor Pool
By The Aqualitx team at STEMania · Updated
Cyanuric acid (CYA), sold as stabilizer or conditioner, is sunscreen for chlorine. Without it, direct sun can burn off most of a pool's free chlorine in a few hours. With too much of it, the chlorine that's left barely works. Getting CYA right is what makes every other chlorine number mean something.
What CYA should be
- Chlorine pools (liquid, cal-hypo or tabs): 30–50 ppm. Enough to protect chlorine from the sun without tying up too much of it.
- Salt-water pools: 60–80 ppm. The salt cell makes chlorine all day, so the pool can carry more stabilizer and lose less to the sun without falling behind.
- Indoor pools: little or none. There's no sun to protect against.
Below 30 ppm, outdoor chlorine disappears fast on a sunny day. If free chlorine is fine at dusk and near zero by the next afternoon, low CYA is the first suspect.
Why more isn't better
CYA protects chlorine by holding most of it in reserve, bound up where the sun can't break it down — but where it also can't kill anything quickly. The more CYA in the water, the more free chlorine you need for the same sanitizing power. A reliable floor is free chlorine of at least 7.5% of CYA: 3 ppm at 40 CYA, 4.5 ppm at 60, 7.5 ppm at 100. See free chlorine vs combined chlorine for how that shows up on a test.
Past about 70 ppm in a non-salt pool, those chlorine targets get expensive to hold, and algae gets a foothold the moment chlorine sags.
How to raise CYA
Granular cyanuric acid: about 13 oz per 10,000 gallons raises CYA by roughly 10 ppm. Taking a 15,000-gallon pool from 20 to 40 ppm is 2 × 13 × 1.5 ≈ 39 oz (about 2.4 lbs).
- Put the granules in a sock or mesh bag and hang it in front of a return jet, or put them in the skimmer basket with the pump running. Don't backwash or clean the filter for several days afterward.
- Don't broadcast it on a vinyl liner — undissolved granules can bleach it.
- Give it 5–7 days to dissolve before re-testing. A test the next day reads low and tempts a second dose you don't need.
Where CYA comes from without you adding it
Stabilized chlorine brings CYA in with every dose. Each 8 oz trichlor tab adds about 3 ppm of CYA to 10,000 gallons, and dichlor shock adds a similar share. A pool fed on tabs all summer can climb from 40 to 100 ppm without a single bag of stabilizer going in. That's the most common way pools end up over-stabilized.
If CYA is already near the top of its range, switch from tabs to unstabilized chlorine — liquid chlorine or cal-hypo — until it comes back down.
How to lower CYA
CYA doesn't break down, filter out, or get neutralized by any chemical you can buy. The one reliable way down is dilution: drain part of the water and refill with fresh. To find how much, divide the target by the current reading — going from 100 to 50 ppm means keeping half the water, so drain about half. Above about 100 ppm, a partial drain is usually cheaper than the chlorine it takes to keep up.
Testing CYA
CYA test strips are rough, often 20–30 ppm off. A turbidity test (the black-dot-in-a-tube kind) is better, and reading it outdoors with the sun behind you helps. If a reading jumps when nothing containing stabilizer went in, re-test before acting on it — CYA can't rise on its own.
The short version
- Target 30–50 ppm for chlorine pools, 60–80 for salt pools.
- Raise it with about 13 oz of stabilizer per 10,000 gallons per 10 ppm, and wait a week to re-test.
- Tabs and dichlor add CYA with every dose.
- The only way down is a partial drain and refill.