Free & Total Chlorine explained
Free Chlorine vs Total Chlorine
Understanding the difference keeps your pool safe and sparkling
Chlorine Explained in Plain English
Think of chlorine in your pool like workers at a job site:
Free Chlorine
Workers ready and available to do the job—killing bacteria, algae, and germs
Combined Chlorine
Workers who are exhausted—they've already done their job and can't do any more
Total Chlorine
The total number of workers on site = Free + Combined
What Does This Mean For You?
Free chlorine is what matters most. It's the chlorine that's actually protecting your pool right now. If your total chlorine is high but your free chlorine is low, you've got a problem—most of your chlorine has been "used up" and isn't working anymore.
💡 The Golden Rule
Your free chlorine and total chlorine readings should be very close together. If there's a big gap, you need to take action.
The Chemistry Behind Chlorine
When you add chlorine to your pool, it dissolves and forms hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻). Together, these make up your free chlorine—the active sanitising agents in your water.
How Chlorine Gets "Used Up"
When free chlorine encounters contaminants like sweat, urine, body oils, sunscreen, and organic debris, it reacts with them. This reaction produces chloramines—specifically monochloramine (NH₂Cl), dichloramine (NHCl₂), and trichloramine (NCl₃).
These chloramines are what we call combined chlorine. They have dramatically reduced sanitising power—roughly 25 to 100 times less effective than free chlorine—and they're responsible for that unpleasant "chlorine smell" and eye irritation.
Combined Chlorine = Total Chlorine − Free Chlorine
Ideal Chlorine Levels
Breakpoint Chlorination
When combined chlorine levels become problematic, you need to perform breakpoint chlorination—also known as shocking the pool. This involves raising free chlorine levels to approximately 10 times the combined chlorine reading. At this concentration, the free chlorine oxidises and destroys the chloramines, effectively "resetting" your water chemistry.
That "Chlorine Smell" Isn't What You Think
Here's something that surprises most people: a strong chlorine smell actually means there's NOT ENOUGH working chlorine in the pool.
The Reality: That pungent smell is caused by chloramines (combined chlorine)—the exhausted, ineffective form of chlorine. A properly sanitised pool with adequate free chlorine has almost no smell at all.
If you walk into an indoor pool centre and the chlorine smell hits you immediately, it's a sign that the water has high chloramine levels and likely needs treatment. The same applies to your backyard pool—if you can smell it strongly, it's time to shock it.
Lo-Chlor Solutions for Chloramine Problems
When combined chlorine builds up and your pool needs attention, these Lo-Chlor products are your go-to solutions:
Lo-Chlor Miraclear Clarifier
While not a direct chloramine treatment, Miraclear helps by removing the organic particles and contaminants that contribute to chloramine formation. Cleaner water means your chlorine works more efficiently.
Lo-Chlor Pool Algaecide
Algae creates significant chlorine demand. By preventing and treating algae with a quality algaecide, you reduce the workload on your chlorine, helping maintain higher free chlorine levels.
Lo-Chlor Multi Stain Remover
Organic staining and debris on pool surfaces create ongoing chlorine demand. Multi Stain Remover eliminates these problem areas, allowing your chlorine to focus on sanitisation rather than fighting surface contaminants.
Lo-Chlor Phosphate Remover
Phosphates are food for algae. By removing phosphates, you eliminate a major contributor to chlorine demand. Less algae growth means more free chlorine available for sanitisation.
💡 Pro Tip
The best way to deal with chloramines is to prevent them from building up in the first place. Regular shocking (every 1–2 weeks), good filtration, and keeping organic contamination low will keep your free chlorine levels healthy.
Fun Fact!
The distinctive "pool smell" that most people associate with chlorine is actually trichloramine (nitrogen trichloride)—and it takes just 0.02 ppm in the air to be detectable by the human nose. Interestingly, trichloramine is also what causes "swimmer's cough" and eye irritation in poorly maintained indoor pools. Olympic swimming venues use advanced UV and ozone treatment systems specifically to break down chloramines and keep that smell at bay—so the cleanest pools in the world are actually the ones that smell the least like a pool!
Quick Reference: What To Do
Need Help With Your Pool Chemistry?
Poolwise Padbury stocks the full range of Lo-Chlor products and professional testing equipment. Bring in a water sample and we'll help you get your chlorine levels balanced perfectly.
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