CYA / stabiliser calculator
Cyanuric acid protects chlorine from sunlight — but too much of it makes your chlorine less effective, and you cannot filter it out. This calculator tells you how much to add to reach a target without overshooting.
How the calculation works
pounds of cyanuric acid = (target ppm − current ppm) × gallons ÷ 120,000
Same divisor convention as salt dosing. See the source register for its confidence level and what still needs checking.
Typical CYA targets
| Pool type | Commonly cited CYA range |
|---|---|
| Outdoor, chlorine-fed | 30–50 ppm |
| Salt water (chlorinator) | 60–80 ppm |
| Indoor pool | Generally not needed — there is no sunlight to protect against |
Why you can't filter CYA out
Cyanuric acid is not removed by normal filtration or backwashing in practice; the practical way to lower it is draining/replacing water. That makes CYA a one-way ratchet in practice: every time you use stabilised chlorine (dichlor, trichlor) or add stabiliser, the level creeps up. The standard way back down is to drain part of the pool and refill with fresh water.
That is why it pays to add less than you think. If you are between two amounts, add the smaller one and retest.
Related
- Pool volume calculator — the input everything else depends on
- Pool salt calculator
- Cyanuric acid in a pool — what it does and why it only goes up
- How to backwash a pool filter — backwashing and refilling is one of the few things that quietly lowers CYA
- Pool maintenance schedule