What is Free Chlorine?
Free chlorine (FC) is the amount of chlorine in your pool that's available to sanitize the water. It's the active, working chlorine that kills bacteria, viruses, and algae. When you test your pool water, this is the number that matters most for sanitization.
Free chlorine exists in two forms in water:
- Hypochlorous acid (HOCl) - The powerful sanitizer that does the heavy lifting
- Hypochlorite ion (OCl-) - Less effective but still contributes to sanitization
The ratio between these two forms depends on pH, which is why maintaining proper pH (7.2-7.8) is crucial for chlorine effectiveness.
What is Total Chlorine?
Total chlorine is the sum of all chlorine in your pool water, including:
- Free chlorine - Available for sanitization
- Combined chlorine - Chlorine that has already reacted with contaminants
The formula is simple:
Total Chlorine = Free Chlorine + Combined ChlorineWhat is Combined Chlorine?
Combined chlorine (CC), also called chloramines, is chlorine that has bonded with nitrogen compounds like ammonia, urea, and sweat. This is the "used up" chlorine that:
- Cannot sanitize - It's already reacted
- Causes the "chlorine smell" - Ironically, a strong chlorine smell means you need MORE chlorine
- Irritates eyes and skin - Chloramines are the real culprit, not free chlorine
- Indicates contamination - High CC means organic matter is present
Calculating Combined Chlorine
Combined Chlorine = Total Chlorine - Free ChlorineFor example:
- Total Chlorine: 3.0 ppm
- Free Chlorine: 2.5 ppm
- Combined Chlorine: 0.5 ppm
Ideal Levels for Pool Professionals
| Measurement | Ideal Range | Action Threshold |
|---|---|---|
| Free Chlorine (with CYA) | 3-5 ppm | Below 2 ppm |
| Free Chlorine (no CYA) | 1-3 ppm | Below 1 ppm |
| Combined Chlorine | 0-0.2 ppm | Above 0.5 ppm |
| Total Chlorine | Same as FC | n/a |
Key insight: If your Total Chlorine equals your Free Chlorine (or is very close), your pool has minimal combined chlorine—this is ideal.
When Combined Chlorine is High
If combined chlorine exceeds 0.5 ppm, you need to perform breakpoint chlorination. This means shocking the pool to a level that destroys all the chloramines.
Breakpoint Chlorination Formula
To reach breakpoint, raise free chlorine to approximately 10x the combined chlorine level. For example:
- Combined Chlorine: 1.0 ppm
- Shock target: 10+ ppm free chlorine
This is why understanding the difference matters—you can't effectively shock a pool without knowing your starting FC and CC levels.
Testing Tips for Pool Professionals
- Always test both FC and TC - Many test kits only show FC. Use a kit that measures both.
- Test in the shade - Direct sunlight can affect readings.
- Test before adding chemicals - Get baseline readings first.
- Test at the same depth - Take samples 12-18 inches below surface.
- Document everything - Track readings over time to spot trends.
Common Mistakes
Mistake 1: Only Testing Total Chlorine
Some basic test strips only show total chlorine. This can be misleading—you might think you have 3 ppm of working chlorine when half of it is actually combined chlorine.
Mistake 2: Adding More Chlorine When Pool Smells
The chlorine smell comes from combined chlorine, not free chlorine. Adding a little more chlorine won't help—you need to shock past breakpoint.
Mistake 3: Ignoring CYA in the Equation
With cyanuric acid (stabilizer), you need higher free chlorine levels. The CYA-FC relationship means a pool with 50 ppm CYA needs 4+ ppm FC to be effective.
Using PoolOps for Chemistry Tracking
PoolOps automatically calculates and tracks both free and combined chlorine at every service stop. The app:
- Alerts you when combined chlorine is high
- Calculates exact shock dosage needed
- Tracks chemistry trends over time
- Generates professional reports for customers