Pool Cyanuric Acid (CYA / Stabilizer) Calculator

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, because most of the chlorine is bound to the stabilizer rather than free to work. This is why high-CYA pools struggle: at 5 ppm free chlorine and 50 ppm CYA, the genuinely active hypochlorous acid is only a few hundredths of a part per million.

What matters is not either number alone, it is the ratio between them. That is the insight behind the FC/CYA chart the Trouble Free Pool community publishes, and it is the single most useful thing to know about pool chemistry. Your free chlorine target should be roughly 11-13% of your CYA, all the time:

Read that table twice before you raise your CYA. Every point of stabilizer you add commits you to carrying more chlorine forever, and to a much higher number if the pool ever needs shocking. A pool at CYA 80 has to reach 32 ppm to clear an algae bloom, which is a lot of liquid chlorine and several days of work. A pool at CYA 30 needs 12.

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: 13 ounces of CYA per 10,000 gallons raises CYA by 10 ppm

That number is not a rule of thumb, it is straight arithmetic, because CYA is measured as the weight of cyanuric acid itself. Ten parts per million means 10 milligrams in every litre, and 10,000 gallons is 37,854 litres, so you need 378,500 mg, which is 379 grams, which is 13.3 ounces. A pound raises it by 12 ppm.

Get this one right on the first attempt. There is no chemical that removes CYA, so an overdose is corrected by draining part of your pool and paying to refill it. When in doubt, add three quarters of what the calculator says and retest.

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:

1. Partial drain and refill (most common). To go from 100 to 50, drain 50% and refill.
2. Reverse osmosis water treatment (mobile services exist, expensive).
3. 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 = 13 oz × (25/10) × (20,000/10,000) = 65 oz = 4.06 lb

In a sock in the skimmer basket, retest in 48-72 hours. Adjust if needed.

Test note. The CYA test is a turbidity test, not the FAS-DPD titration you use for chlorine. You mix sample water with a melamine reagent, which clouds it, and then pour it into a tube until a black dot on the bottom disappears. Where the water line sits when the dot goes is your reading.

That method has a floor. Standard kits do not read below about 30 ppm, so a result of “under 30” covers everything from zero to 29 and you cannot narrow it further without diluting the sample with distilled water and doubling the answer. It is also a judgement call about when a dot has vanished, which is why two people testing the same water can disagree by 10 ppm. Take a reading in bright shade with your back to the sun, and do not chase small differences.