The FC–CYA ratio
Last reviewed August 11, 2026
Free chlorine doesn't have one correct number. What counts as enough depends on how much stabilizer is in the water.
Why
Cyanuric acid grabs onto chlorine to shield it from the sun. Chlorine that's being held is protected but not working. The more stabilizer, the bigger the share of your free chlorine that's sitting in reserve, and the more you need in total to keep the same amount on the job.
Your test kit measures all of the free chlorine, held and working together. It can't tell you the split. That's why the number alone doesn't tell you whether the pool is sanitized.
The chart
| CYA | Minimum FC | Target range | Notes |
|---|---|---|---|
| 0–30 | About 2 ppm | 3–5 ppm | Indoor and spa range |
| 30 | 2.3 ppm | 3–5 ppm | Standard outdoor floor |
| 40 | 3.0 ppm | 4–6 ppm | |
| 50 | 3.8 ppm | 4–7 ppm | Standard outdoor ceiling |
| 60 | 4.5 ppm | 5–7 ppm | Low end for salt |
| 70 | 5.3 ppm | 6–8 ppm | |
| 80 | 6.0 ppm | 7–9 ppm | High end for salt |
| 100 | 7.5 ppm | 8–12 ppm | Time to reset the CYA |
Source: the Trouble Free Pool / PoolForum FC–CYA chart, widely used across the trade.
What it looks like when it's off
- Chlorine reads in range but algae shows up anyway
- Chlorine drops fast between visits with normal use
- Stronger chlorine smell, which is combined chlorine, not free
- Shock works for a few days and then it's back
If you're seeing those on a pool whose chlorine keeps testing fine, check the stabilizer before anything else.
Salt pools
Salt pools often run CYA 60–80 on purpose. The cell makes chlorine all day, and the extra stabilizer holds onto it between cycles. That's a reasonable setup, but it moves the chlorine target up with it. A salt pool at CYA 70 holding 4 ppm isn't well run. It's short, and it looks fine on paper.
Two ways out
Either hold chlorine proportionally higher, or bring the stabilizer down. Lower CYA with matching chlorine is the easier pool to run. A high-CYA pool held at the floor has no margin, and one hot week or one pool party puts it under. See cyanuric acid for how to bring it down.
Logging chemistry on every stop? PoolSpartan keeps the readings, works out the LSI as you type them, and sends the customer a service report when you close the visit. Free to start. We only get paid when you do.
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