Pool Stabilizer (CYA) Calculator
Check the label; granular is usually close to 100%.
Used for draining; tap water is normally 0.
Stabilizer to add
3.76 lb
Add 3.76 lb of stabilizer (cyanuric acid) to raise CYA in 15,000 gallons from 10 to 40 ppm.
- CYA increase
- 30 ppm
- Rate
- 13.4 oz per 10,000 gal per 10 ppm
Granular stabilizer dissolves slowly: wait 3–7 days before retesting CYA, or the reading will be low.
- Never mix pool chemicals with each other, wet or dry.
- Always add chemicals to water, never water to chemicals.
- Follow the manufacturer's label. It overrides any calculator, including this one.
How it's calculated
Raising CYA is a mass balance; lowering it is a dilution, because cyanuric acid stays in the water until the water leaves.
The dilution formula comes from mixing: after replacing a fraction f of the water, CYA becomes current × (1 − f) + fill × f. Solving for f gives the formula above. With tap water at 0 ppm it simplifies to 1 − target ÷ current.
Worked examples
Raising: a 15,000 gallon pool at 10 ppm, target 40 ppm, 100% stabilizer:
- Increase: 30 ppm × 56,781 L = 1,703,435 mg.
- ÷ 28,350 mg per oz = 60.1 oz, or 3.76 lb (1,703 g).
Lowering: a 20,000 gallon pool at 100 ppm that should be at 50 ppm, refilled with tap water:
- f = (100 − 50) ÷ (100 − 0) = 0.50.
- Drain and refill 10,000 gallons, half the pool.
If you cannot drain that much at once, two partial drains of 29% each reach the same result, because each round removes the same share of what is left.
Why CYA matters
Stabilizer shields chlorine from sunlight: without it, most chlorine is gone within a few hours of direct sun. But CYA also binds part of the chlorine, so more stabilizer means you need more free chlorine for the same sanitizing power. Keep it in range and set your chlorine level from it with the chlorine calculator.
Stabilizer by pool size
| Pool size | Per +10 ppm | New pool 0 → 40 ppm |
|---|---|---|
| 10,000 gal | 13.4 oz | 3.34 lb |
| 15,000 gal | 1.25 lb | 5.01 lb |
| 20,000 gal | 1.67 lb | 6.68 lb |
| 25,000 gal | 2.09 lb | 8.35 lb |
| 30,000 gal | 2.5 lb | 10 lb |
Share of water to replace to lower CYA
| From → to | Replace |
|---|---|
| 60 → 40 ppm | 33% |
| 80 → 50 ppm | 38% |
| 100 → 50 ppm | 50% |
| 100 → 70 ppm | 30% |
| 150 → 60 ppm | 60% |
Assumes fill water with no CYA. Granular stabilizer is cyanuric acid at close to 100% purity; check the label.
Frequently asked questions
How much stabilizer do I add to raise CYA by 10 ppm?
About 13.4 oz of stabilizer per 10,000 gallons raises CYA by 10 ppm. A new 20,000 gallon pool going from 0 to 40 ppm needs about 6.68 lb.
What should my CYA level be?
For a pool chlorinated with liquid chlorine or tablets, 30–50 ppm (chlorine). Saltwater pools usually run higher, 60–80 ppm, because the cell produces chlorine continuously. Indoor pools need little or none.
How do I lower CYA in my pool?
Cyanuric acid does not break down or evaporate, so the reliable way to lower it is to drain part of the water and refill with fresh water. Products sold to reduce CYA are enzyme-based and give inconsistent results. The calculator tells you what fraction to replace.
Why does my CYA keep going up?
Trichlor tablets and dichlor powder add stabilizer every time you use them: about 0.6 ppm of CYA per 1 ppm of chlorine from trichlor and 0.9 ppm from dichlor. Over a season that adds up. Switching to liquid chlorine stops the climb.
How long does stabilizer take to dissolve?
Granular stabilizer dissolves slowly, often taking several days. Put it in a sock or mesh bag in the skimmer basket or in front of a return, avoid backwashing for about 48 hours, and wait up to a week before trusting a CYA test.
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