Chlorine is the workhorse of pool sanitation. But “chlorine” isn't one number — it's three: free chlorine (FC), combined chlorine (CC), and total chlorine (TC). Understanding the difference is the whole game.
The three chlorine numbers
| Number | What it means | Target |
|---|---|---|
| Free chlorine (FC) | Available to sanitize. This is the one that protects you. | 1–3 ppm (residential) |
| Combined chlorine (CC) | Chlorine that's already reacted with contaminants — mostly worthless. | <0.2 ppm |
| Total chlorine (TC) | FC + CC. The number on basic test strips. | FC + CC |
If you only measure total chlorine, you're flying blind. A pool can read 3 ppm total chlorine and still be unsafe because all of it is combined (chloramines).
How CYA changes everything
Here's the detail most homeowners miss: chlorine's killing power depends on the ratioof FC to CYA (cyanuric acid). The classic “1–3 ppm” rule assumes zero CYA. In a stabilized outdoor pool with 40 ppm CYA, you need more like 3–5 ppm FC to have the same sanitizing effect. This is known as the FC/CYA ratio and the rule of thumb is:
Keep minimum FC at roughly 7.5% of your CYA level. So 40 ppm CYA → 3 ppm FC minimum.
Breakpoint chlorination: why you shock
When combined chlorine climbs above 0.5 ppm, normal dosing won't burn it off. You have to hit breakpoint: add enough chlorine to reach roughly 10x the combined chlorine level, all at once. That's what “shocking” is — not “add one bag per week.”
Symptoms of low free chlorine
- Algae starting (green tint, slimy walls)
- Cloudy water that won't clear
- Strong “chlorine smell” (that's actually chloramines — ironic, right?)
- Irritated eyes and skin after swimming
Symptoms of too-high free chlorine
- Bleached swimsuits and hair
- Corroded equipment over time
- Burned plant life around the pool
Briefly hitting 10+ ppm during shocking is fine; living there long-term damages your equipment.
How to test chlorine properly
Use a FAS-DPD test kit (Taylor K-2006 or similar). It separates free from combined chlorine to the nearest 0.2 ppm. Regular OTO/DPD test strips only measure total chlorine — useful for a quick gut-check, not for real management.