How the calculation works
13 oz of cyanuric acid per 10,000 gallons raises the level by about 10 ppm. Scale to your pool and the gap you need to close.
The number that matters more is what it commits you to afterwards. Free chlorine has to be held at roughly 7.5% of the CYA reading to stay effective, and shocking takes about 40% of it. Doubling your stabiliser doubles both.
Worked example
A 20,000 gallon pool at 20 ppm, aiming for 40 ppm. The gap is 20 ppm, so 13 × 2 × 2 = about 52 oz, near enough 3.25 lbs.
At 40 ppm CYA the pool then needs at least 3 ppm free chlorine held at all times, and 16 ppm to shock — up from 1.5 ppm and 8 ppm at the old level.
Assumptions
- Doses are calculated from the product strength you selected, not a generic chart.
- Pool volume is accurate — a volume that is out by a quarter puts every dose out by a quarter.
- Water is circulating while the chemical is added and for several hours afterwards.
- Granular cyanuric acid at close to full purity, added to circulating water.
Limitations
- Test kits vary. Strips are convenient but a drop-based kit is markedly more reliable for anything beyond a rough check.
- Products degrade. Liquid chlorine loses strength within weeks of manufacture, especially in heat, so an older jug delivers less than its label claims.
- These figures are estimates for a typical residential pool, not a prescription for your specific water.
- Trichlor tablets and dichlor granules add cyanuric acid every time they are used. A pool chlorinated only by tablets will climb past the useful range within a season with no stabiliser ever being added deliberately.
- Nothing removes cyanuric acid from water except replacing the water.
Questions people ask
What does cyanuric acid do?
It shields chlorine from sunlight. Without it, an outdoor pool can lose most of its free chlorine in a few hours of summer sun. With it, chlorine lasts through the day.
Can there be too much?
Yes. Above roughly 60 ppm chlorine becomes sluggish — the test shows plenty, but it acts slowly and algae gets a foothold anyway. The usual cause is months of trichlor tablets rather than anyone adding stabiliser on purpose.
How do I lower it?
Drain and refill. Removing a quarter of the water removes roughly a quarter of the cyanuric acid. There is no chemical that takes it out.
Do indoor pools need it?
No. With no sunlight there is nothing to protect chlorine from, and stabiliser only slows the chlorine down. Indoor pools are normally run at zero.