Free Chlorine: The Active Sanitizer in Your Pool Water

Quick Reference

What it is: Active chlorine available to sanitize your pool
Target level: Minimum FC ≈ 7.5% of your CYA level (round up)
Test frequency: Daily is ideal in-season, minimum few times per week
Key relationship: FC effectiveness depends on the FC/CYA ratio

What is Free Chlorine?

Free chlorine (FC) is the "available" or "active" chlorine in your pool water—the portion that's ready and able to kill bacteria, viruses, algae, and other contaminants. When you add chlorine to your pool, whether as liquid chlorine, granular shock, or through salt water chlorine generation, you're adding free chlorine.

Free chlorine exists as hypochlorous acid (HOCl) and hypochlorite ions (OCl-) in your water. In stabilized pools (with CYA present), the FC/CYA ratio is what determines sanitizing effectiveness, not just the FC number alone.

Free vs. Combined vs. Total Chlorine

Understanding the relationship between these three measurements is crucial for proper pool maintenance:

Free Chlorine (FC): Active sanitizer doing the work
Combined Chlorine (CC): "Used up" chlorine bound to contaminants
Total Chlorine (TC): FC + CC = the sum of both

For a complete explanation of how these interact and what to do when combined chlorine builds up, see our detailed Combined Chlorine article.

Target Free Chlorine Levels

Pro Tip: Your minimum FC should be approximately 7.5% of your CYA level. This ratio ensures adequate sanitizing power regardless of your stabilizer level.

The FC/CYA Rule: Minimum FC ≈ 7.5% of CYA (round up), with your target a bit higher so normal daily loss doesn't drop you below the minimum.

CYA Level (ppm)	Minimum FC (ppm)	Target FC Range (ppm)
30	2-3	3-4
50	4	4-6
70	6	6-8
80	6-7	7-9

Examples calculated from 7.5% of CYA, rounded for practical dosing.

Testing Free Chlorine

Accurate free chlorine testing is essential for proper pool maintenance:

Test Frequency

Daily testing is ideal during swim season, and more often with heavy use or weather events. If you're maintaining a stable routine, a few times per week can work—but don't "set and forget" your FC levels.

Test Strip Accuracy

Basic test strips are convenient for routine monitoring when everything is normal, but they have limitations:

Less accurate at higher chlorine levels (above 5 ppm)
Can show false readings in the presence of certain chemicals
May not distinguish well between free and total chlorine

When to Use DPD Testing

Use a DPD (N,N-diethyl-p-phenylenediamine) test kit for:

Troubleshooting water problems
Verifying shock treatment effectiveness
Testing after adding large amounts of chemicals
When test strips give inconsistent results

Pro Tip: If you're troubleshooting algae/cloudiness or running elevated FC, use a FAS-DPD drop test—it stays accurate at higher chlorine levels and gives reliable combined chlorine readings.

Maintaining Proper Free Chlorine

Daily Chlorine Loss

Pools typically lose 1-3 ppm of free chlorine daily due to:

UV breakdown from sunlight (biggest factor)
Oxidation of organic matter
Bather load and contaminants
pH and temperature effects

Chlorine Sources

Liquid chlorine (sodium hypochlorite): Most common, no byproducts
Salt water chlorine generators: Produces liquid chlorine in-situ
Cal-hypo (calcium hypochlorite): Adds calcium, good for low-calcium areas
Trichlor tablets: Adds stabilizer (CYA), use sparingly

Important: Never mix different types of chlorine products directly. Add them separately and allow circulation between additions.

When Free Chlorine Won't Stay Up

If you're constantly adding chlorine but levels keep dropping:

Check combined chlorine levels - May need to shock/SLAM
Test CYA levels - High CYA doesn't "break" chlorine, but it does require higher FC to get the same sanitizing punch. Past some point (often 80-100+ ppm), the required FC gets impractical, so lowering CYA via partial drain/refill is usually the cleanest fix.
Verify pH balance - High pH reduces chlorine effectiveness
Look for algae growth - Even microscopic algae consumes chlorine
Check for organic contamination - Heavy rains, debris, or high bather loads

Safety Considerations

For public pools, codes typically cap free chlorine at 10 ppm while open. For home pools, follow product labels and use the FC/CYA relationship—higher FC can be perfectly normal when CYA is higher.

If the water "burns" eyes or smells strongly "chlorine-y," suspect chloramines (combined chlorine) and/or pH issues, not just high FC. True irritation and "chlorine smell" are very often from combined chlorine, not normal free chlorine.

General safety practices:

Store liquid chlorine in cool, dark locations
Never add chlorine products directly to skimmer or while people are in the pool
Follow product label instructions for safe swimming times after chemical additions

For detailed information about the relationship between free, combined, and total chlorine, including step-by-step troubleshooting when combined chlorine becomes a problem, see our comprehensive Combined Chlorine guide.