How to Lower Cyanuric Acid (CYA) in a Pool
Cyanuric acid above 50 ppm locks your chlorine up so it cannot sanitize, and no chemical product reliably reverses it. The only dependable fix is dilution: drain part of the pool and refill with fresh water. This guide walks through the drain math, how to drain safely, and how to stop CYA from climbing back up.
The only reliable way to lower cyanuric acid is a partial drain and refill. Drain percentage = (current CYA minus target CYA) / current CYA x 100. A 20,000 gallon pool at 90 ppm CYA needs about a 56% drain, roughly 11,100 gallons, to reach 40 ppm. Never drain a pool completely; work in stages and switch to unstabilized chlorine afterward.
The short answer: only dilution works
Cyanuric acid (CYA, or stabilizer) shields chlorine from sunlight, which is why every outdoor pool needs some: 30 to 50 ppm is the target. But CYA never leaves the water on its own. It does not evaporate, sunlight does not burn it off, and backwashing removes only a little. Every trichlor tablet and dichlor shock dose adds more, so CYA climbs all season until chlorine stops working even though the test reads plenty of it. That stuck state is often called chlorine lock.
No chemical reliably destroys CYA in pool water. Products that claim to do it get mixed results at best. The dependable fix is a partial drain and refill, and the math is simple:
Drain percentage = (current CYA - target CYA) / current CYA x 100
Worked example 1: 20,000 gallons, CYA 90 down to 40
The drop is 50 ppm (90 minus 40).
(90 - 40) / 90 x 100 = 55.6%, call it 56%.
56% of 20,000 gallons is about 11,100 gallons to drain and replace. That is more than half the pool, which tells you something important: at 90 ppm, one partial drain is barely enough. Draining in two stages of about 30% each is safer for the pool structure and lands you at roughly the same place after both refills.
Worked example 2: 15,000 gallons, CYA 70 down to 40
The drop is 30 ppm (70 minus 40).
(70 - 40) / 70 x 100 = 42.9%, call it 43%.
43% of 15,000 gallons is about 6,430 gallons. A single well-planned partial drain handles this one.
How to drain safely
Never drain a pool completely unless a professional tells you to. Vinyl liners shrink and tear when the water leaves, fiberglass shells can shift, and any pool in a high water table area can literally float out of the ground (hydrostatic pop). Work in stages of 25 to 35% at a time, refilling between stages. Drain to a proper location: many areas restrict where pool water can go, so check local rules and dechlorinate first if required. After refilling, circulate for several hours and retest CYA before redosing anything.
Stop CYA from climbing back up
The drain only buys time if you keep feeding the pool stabilized chlorine. Trichlor tablets add about 0.6 ppm of CYA for every 1 ppm of chlorine they deliver, and dichlor is similar. After your drain, switch daily chlorination to unstabilized 10% liquid chlorine, which adds zero CYA, and reserve dichlor for rare occasions. Backwash and splashout remove small amounts of CYA over time, which now works in your favor instead of fighting a losing battle.
Raising CYA: the math for the other direction
If your CYA is low instead, stabilizer granules are the fix: about 16 ounces (1 pound) of stabilizer per 12 ppm rise per 10,000 gallons. A 20,000 gallon pool going from 20 to 40 ppm needs (20 / 12) x 16 x 2 = about 53 ounces, or 3.3 pounds. Put the granules in a sock in the skimmer or dissolve them in a bucket first; they take 24 to 48 hours to fully register on a test, so wait before retesting.
Skip the arithmetic
Enter your CYA now and your target into our free pool chemical calculator to get your exact drain percentage, plus dosing for every other level.
Lowering CYA: common questions
Is there a chemical that lowers cyanuric acid?
No reliable one. Some products claim to reduce CYA, but results are inconsistent, and pool professionals still treat partial drain and refill as the dependable fix. Dilution is the only method that always works.
How much do I need to drain to lower CYA from 100 to 50?
50%. The formula is (100 - 50) / 100 x 100 = 50%. For large drops like this, drain in two or three stages of 25 to 35% each rather than all at once, refilling between stages.
Why is my CYA so high?
Trichlor tablets and dichlor granules are stabilized chlorine: every dose adds cyanuric acid along with the chlorine. A season of tablet chlorination can push CYA past 100 ppm. Switching to unstabilized liquid chlorine stops the climb.
Can I swim with high CYA?
You can, but the chlorine is largely locked up and cannot sanitize properly, so the water is not as safe as the test suggests. Shocking also fails at very high CYA. Bring CYA into the 30 to 50 ppm band so chlorine can actually work.
How do I add stabilizer correctly?
Put the granules in a sock in the skimmer basket or dissolve them in a bucket of pool water first, then add with the pump running. CYA dissolves slowly: wait 24 to 48 hours before retesting so the reading is accurate.