What stabilizer does
Cyanuric acid (CYA), sold as pool stabilizer or conditioner, protects chlorine from sunlight. Ultraviolet light breaks down unprotected chlorine so quickly that an outdoor pool can lose most of it in a few hours on a sunny day. A moderate amount of CYA holds the chlorine in reserve so it lasts much longer.
The trade-off is that stabilized chlorine works more slowly. The CDC recommends a higher minimum of 2 ppm free chlorine in pools that use cyanuric acid (compared with 1 ppm without it), and advises not using cyanuric acid, or chlorine products that contain it, in hot tubs. The more stabilizer in the water, the more chlorine you need, so the goal is "enough, not more".
Many pool owners and equipment makers aim for roughly 30–50 ppm in outdoor chlorine pools, and some salt chlorinator makers specify a higher level for their cells. Indoor pools that get no direct sun often use little or none. Use the level your chlorinator maker, local health rules or pool professional recommends.
Stabilizer (g) = ppm increase × litres ÷ 1,000
Rule of thumb: 13.4 oz (379 g) raises 10,000 US gallons by 10 ppm
Share of water to replace to lower CYA = (current − target) ÷ current
Worked examples
- New 15,000-gallon pool, 0 → 40 ppm: 5.0 lb (2.27 kg) of granular cyanuric acid.
- Same pool, 20 → 40 ppm: 2.5 lb (1.14 kg).
- 50,000-litre pool, 0 → 40 ppm: 2 kg (4.4 lb).
- Too high, 100 → 50 ppm: replace half of the water with fresh water (stabilizer does not evaporate or break down quickly, and fill water normally has none).
How to add stabilizer
- Granular cyanuric acid dissolves slowly. A common method is to put it in a sock or mesh bag hung in front of a return jet, or in the skimmer basket if the product label allows, and let it dissolve over a day or two. Do not backwash or clean the filter for several days afterwards.
- Liquid stabilizers dissolve instantly but contain less CYA per kilogram; follow the label dose instead of this calculator.
- CYA tests are slow to settle. Wait about a week after adding before retesting, so you do not add a second dose to an undissolved first one.
Hidden stabilizer from chlorine tablets
Trichlor tablets and dichlor granules contain cyanuric acid. Every 1 ppm of chlorine from trichlor adds about 0.6 ppm CYA; from dichlor about 0.9 ppm. Over a summer of daily tablets, stabilizer can climb well past 80–100 ppm, at which point chlorine becomes sluggish and algae appear even when the test shows "enough" chlorine. If that happens, partial draining is the only practical fix, and switching to liquid chlorine or a salt chlorinator stops the build-up. See the pool chlorine calculator.
Common mistakes
- Adding stabilizer to a hot tub. The CDC recommends against it.
- Retesting too soon. Undissolved granules give a low reading.
- Not raising chlorine with CYA. High stabilizer with minimum chlorine is a common cause of algae.
Estimates for general information. Follow the product label and local health rules.
Stabilizer dose chart
Granular cyanuric acid needed to raise stabilizer by 10 ppm, 30 ppm and from 0 to 40 ppm.
| Pool size | +10 ppm | +30 ppm | 0 → 40 ppm |
|---|---|---|---|
| 5,000 gal | 6.7 oz (189 g) | 1.25 lb (568 g) | 1.67 lb (757 g) |
| 10,000 gal | 13.4 oz (379 g) | 2.5 lb (1.14 kg) | 3.34 lb (1.51 kg) |
| 15,000 gal | 1.25 lb (568 g) | 3.76 lb (1.7 kg) | 5.01 lb (2.27 kg) |
| 20,000 gal | 1.67 lb (757 g) | 5.01 lb (2.27 kg) | 6.68 lb (3.03 kg) |
| 25,000 gal | 2.09 lb (946 g) | 6.26 lb (2.84 kg) | 8.35 lb (3.79 kg) |
| 30,000 gal | 2.5 lb (1.14 kg) | 7.51 lb (3.41 kg) | 10.01 lb (4.54 kg) |
| 10,000 L | 3.5 oz (100 g) | 10.6 oz (300 g) | 14.1 oz (400 g) |
| 25,000 L | 8.8 oz (250 g) | 1.65 lb (750 g) | 2.2 lb (1 kg) |
| 50,000 L | 1.1 lb (500 g) | 3.31 lb (1.5 kg) | 4.41 lb (2 kg) |
| 75,000 L | 1.65 lb (750 g) | 4.96 lb (2.25 kg) | 6.61 lb (3 kg) |
Because stabilizer only leaves with water, the amount you add in spring usually lasts the season unless you drain, backwash heavily or use tablets that add more.
Frequently asked questions
How much stabilizer raises a pool by 10 ppm?
About 13.4 oz (379 g) of granular cyanuric acid per 10,000 US gallons, or 100 g per 10,000 litres.
How much stabilizer for a 15,000 gallon pool?
From 0 to 40 ppm: about 5 lb (2.27 kg). From 20 to 40 ppm: about 2.5 lb.
Should I use stabilizer in a hot tub?
The CDC recommends not using cyanuric acid or chlorine products containing it in hot tubs.
How do I lower cyanuric acid?
Replace part of the water. To halve CYA, replace about half of the water with fresh water.
How much stabilizer for a 50,000 litre pool?
About 2 kg of granular cyanuric acid raises 50,000 litres from 0 to 40 ppm; 500 g adds 10 ppm.
Sources
- CDC: Home pool and hot tub water treatment and testing (2 ppm minimum with cyanuric acid; no cyanuric acid in hot tubs).
- CDC Model Aquatic Health Code.
Last reviewed 28 October 2026. Results are estimates; see our disclaimer.