Pool Cyanuric Acid (CYA / Stabilizer) Calculator
How much cyanuric acid stabilizer to add to bring pool CYA into the 30-50 ppm range (or 60-80 for saltwater chlorinator pools).
What CYA does. Cyanuric acid (CYA, “stabilizer”, “conditioner”) protects free chlorine from UV destruction. Without it, sunlight breaks down most of your chlorine within 4 hours. With CYA at 30-50 ppm, you get roughly 8x longer chlorine life in direct sun.
The trade-off. CYA also reduces chlorine’s effective sanitizing strength. This is why high CYA pools struggle — chlorine can be at 5 ppm but only 0.3 ppm is “active” for killing pathogens. The Trouble Free Pool community publishes the FC/CYA chart that everyone uses now: as CYA rises, your FC target also has to rise.
Target ranges:
- Indoor pools: 0 ppm CYA (no UV exposure, do not need it)
- Outdoor chlorine pools: 30-50 ppm
- Outdoor saltwater pools: 60-80 ppm (cells are gentler at higher CYA)
- Hot tubs / spas: 0-30 ppm (lower because of high temperatures and turnover)
To raise CYA: add granular cyanuric acid stabilizer.
- Reference dose: 1.2 lb CYA per 10,000 gallons raises CYA by ~10 ppm
CYA dissolves slowly. Most products say to put it in a sock and hang it in the skimmer. Forty-eight hours later you can test.
To lower CYA: there is no chemical solution. The only options are:
- Partial drain and refill (most common). To go from 100 to 50, drain 50% and refill.
- Reverse osmosis water treatment (mobile services exist, expensive).
- Specialized “CYA reducer” products — not consistently effective.
Why CYA creeps up. If you use trichlor tabs or dichlor granular, every dose adds CYA along with chlorine. Over a season this can push CYA from 30 to 80 ppm without you adding stabilizer directly. Switch to liquid chlorine or cal-hypo to stop the creep.
Worked example. 20,000 gal pool, current CYA 15, target 40 (chlorine pool).
- Raise needed = 25 ppm
- Stabilizer dose = 1.2 lb × (25/10) × (20,000/10,000) = 6 lb
In a sock in the skimmer basket, retest in 48-72 hours. Adjust if needed.
Test note. Most pool store CYA tests are turbidity-based and not very accurate at low values. The home Taylor turbidity test (FAS-DPD test kit) is good but only reads down to 30. If you can read it, you have at least 30. If it disappears immediately, you have under 20.