Cyanuric Acid (Pool Stabilizer): How Much You Need and Why It Matters

If your chlorine reading looks fine but the pool keeps clouding over or turning green, stabilizer is one of the first things to look at. Cyanuric acid (CYA) solves one real problem and quietly creates another, and most pool advice only mentions the first.

What it does

Sunlight breaks chlorine down quickly. Without stabilizer, an outdoor pool can lose most of its free chlorine within a few hours of strong sun. CYA forms a loose, reversible bond with chlorine that protects it from UV light, so a dose lasts through the day instead of burning off by lunchtime.

The catch

Chlorine that’s bound to CYA isn’t actively killing anything. At any moment, only a small unbound fraction is doing the fast work, and as it’s used up, more is released from the reserve. More CYA means a bigger reserve but a smaller active share. So the free chlorine number you need depends on your CYA number.

A widely used homeowner rule keeps free chlorine at no less than about 7.5% of CYA, and raises it to about 40% of CYA to shock:

These come from the same rule our chlorine and shock calculators use: about 7.5% and 40% of CYA, with a 10 ppm floor for shocking. They’re guidance, not regulation. For public pools, CDC’s 2018 Model Aquatic Health Code caps CYA at 90 ppm and requires at least 2 ppm free chlorine when CYA is used. Later code work has looked at limits based on the CYA-to-chlorine ratio instead, so pool operators should check the edition their state follows.

Look at the right-hand column. At 90 ppm CYA, clearing algae means holding chlorine above 30 ppm. That’s why high stabilizer makes problems so hard to fix.

How much is right

• Outdoor pools chlorinated with liquid chlorine or cal-hypo: 30–50 ppm is a common target.
• Salt chlorine generator pools: often kept higher, around 60–80 ppm, because the cell makes chlorine steadily and the extra sun protection helps it keep up. Check your generator’s manual.
• Indoor pools: little or none.

Why it creeps up

Two of the most popular chlorine products contain CYA. The amounts follow directly from their chemistry:

• Trichlor (tablets, pucks) adds about 0.6 ppm of CYA for each 1 ppm of chlorine.
• Dichlor (granules) adds about 0.9 ppm of CYA for each 1 ppm of chlorine.

Here’s what that looks like over a summer. If tablets supply 2 ppm of chlorine a day, they also add about 1.2 ppm of CYA a day, which is roughly 36 ppm in a month. A pool that opens at 30 ppm can pass 100 by August without anyone adding a bag of stabilizer. Liquid chlorine and cal-hypo add none.

Raising it

Granular stabilizer is close to pure cyanuric acid, so the math is simple: about 13 ounces raises 10,000 gallons by 10 ppm. A new 20,000-gallon pool going from 0 to 40 ppm needs about 6.7 pounds.

It dissolves slowly. Put it in a sock or mesh bag in front of a return jet, don’t backwash for about a week, and retest after 5–7 days. Testing the next day and adding more because the reading is low is a common way to overshoot.

CYA (stabilizer) calculatorWork out how much stabilizer to add, or how much water to replace.

Lowering it

Replacing water is the method that reliably works. If your fill water has no CYA, the share to replace is (current − target) ÷ current. To bring 90 ppm down to 45, replace half the water.

CYA tests work by cloudiness and are hard to read precisely, so if a result surprises you, retest or ask a pool store to confirm before draining.