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Get My Free PlanIn pool water, free chlorine is the active sanitizer available to kill contaminants. Total chlorine is free chlorine (FC) plus combined chlorine (CC or chloramines) that has already reacted with contaminants and is no longer working as effectively.
In other words, both free chlorine and combined chlorine make up total chlorine levels.
FC + CC = TC
For pool owners, understanding free chlorine vs total chlorine is one of the basics that keeps water clean, balanced, and safe for swimmers. The key is knowing the difference between free, combined, and total chlorine, how chlorine actually sanitizes the water, and why regular testing and balancing matter.
Tired of decoding test results? The Pool Care Handbook tells you exactly what to do with every number.
It’s easy to confuse these terms until you understand what each one means and how each type of chlorine is created.
When you add chlorine to your pool in any form (liquid, granular, or tablet), it dissolves and forms hypochlorous acid (HOCI). This acid further dissociates in the water, combines with oxygen, and forms hypochlorite (CIO). This is an oxidizing ion and the active ingredient in household bleach.
Together, these chemicals form free chlorine. Free chlorine refers to the available disinfectant form in pool water. Chlorine works through a chemical reaction between hypochlorous acid and the hypochlorite ion.
Exactly as its name suggests, this is the chlorine that’s free or available to sanitize your pool. It acts as a sanitizer that can kill bacteria, viruses, pathogens, and other pathogens. This is what you test for when you’re balancing your pool water chemistry.
When you add chlorine, you’re increasing the amount of free chlorine in the water, and aiming for between 1 and 3 parts per million (ppm), the ideal level being 3 ppm. Maintaining a pH of 7.4 to 7.6 improves its effectiveness.
Once the free chlorine begins to react with organic matter and other contaminants in the water, it can drop and become combined chlorine.
In order to sanitize your pool water, free chlorine reacts to disinfect and break down contaminants such as nitrogen and ammonia. Then it combines with what it attacks and becomes combined chlorine. This is also how chloramines are formed.
Some chlorine does remain in the water, but combined chlorine is made of chloramine compounds and has less ability to rid the water of contaminants than free chlorine. Combined chlorine should stay below 0.5 ppm, since higher levels can cause a strong chlorine smell or skin irritation.
Simply put, this is the sum of free chlorine and combined chlorine in your pool water. If total and free chlorine levels are equal, combined chlorine is effectively zero. Meaning none of the chlorine has been used up yet.
If the total chlorine level is higher than the free chlorine level, the difference between the two is the combined chlorine level.
In order for your pool to be properly sanitized, the free chlorine level must remain higher than the combined chlorine level. This is why it’s so important to test your pool water regularly.
Chlorine test kits commonly use the DPD method to measure chlorine in a water sample. Electronic chlorine testers are another accurate method for measuring free and total chlorine. This is easy to do with test strips or a liquid test kit.
This complete water testing kit helps keep your pool or hot tub clean and safe to swim in, with easy-to-follow color instructions and all the supplies you need in one box.
To find your combined chlorine, subtract your free chlorine from your total chlorine. The formula is simple: CC = TC minus FC.
Most test kits give you a free chlorine number and a total chlorine number. They usually don’t give you a combined chlorine number on its own. So you have to do the quick math yourself.
For example, if your total chlorine reads 5 ppm and your free chlorine reads 3 ppm, then your combined chlorine is 2 ppm. That’s way too high, and it means it’s time to shock. And because your free chlorine is already in range, I would use a non-chlorine shock to oxidize (get rid of) the combined chlorine.
However, if your total chlorine is 3 ppm and your free chlorine is also 3 ppm, then your combined chlorine is 0 ppm. That means none of your chlorine has been used up, and your water is in great shape. Also, you don’t need to shock your pool.
If your free chlorine reads higher than your total chlorine, it’s a testing error, not a real result. Free chlorine can never be more than total chlorine.
Remember, free chlorine plus combined chlorine equals total chlorine. So free chlorine can only be equal to or lower than total chlorine, never higher.
When you see this, don’t panic. First, run the test again with a fresh sample. Then double-check that you’re reading the free chlorine and total chlorine steps in the right order.
If it keeps happening, your test strips or reagents may be old or worn out. Replace them and test again.
If you have little or no free chlorine but a high total chlorine reading, your water is dirty, and your chlorine is used up. In other words, all your chlorine has turned into combined chlorine, and your pool is not sanitized.
The fix is to shock the pool. Shocking adds enough chlorine to burn through the combined chlorine and bring your free chlorine back up. In this case, I recommend using an unstabilized chlorine shock like calcium hypochlorite (cal-hypo) or liquid chlorine (sodium hypochlorite).
After you shock, test again. Once your free chlorine and total chlorine numbers are close to equal, your water is back on track.
To lower combined chlorine, shock your pool to break up the chloramines. Which shock you use depends on where your free chlorine level sits.
First, test your free chlorine. If your free chlorine is low along with high combined chlorine, use a cal-hypo or liquid chlorine. This raises your free chlorine and breaks up the combined chlorine at the same time.
This fast-acting, quick-dissolving swimming pool shock kills bacteria, controls algae, and destroys organic contaminants in pools.
Keep an eye on your cyanuric acid (CYA) level. This is a chlorine stabilizer. And if it gets too high (above 90 ppm), it limits the effectiveness of the chlorine. I bring this up because when you add a large dose of chlorine (a.k.a shocking), you want to use a unstablized chlorine (chlorine that does not contain cyanuric acid). So, avoid using dichlor shock in this case.
Next, if your free chlorine is already normal at around 3 ppm but your combined chlorine is still high, use a non-chlorine shock (oxidizer). This breaks up the chloramines without adding more free chlorine.
Finally, retest after the water has circulated. You want your combined chlorine back under 0.5 ppm.
That strong “chlorine” smell is not a sign of combined chlorine, also known as chloramines.
It feels backward, but a clean, well-balanced pool has almost no smell at all. The heavy chemical odor you notice at some pools is the chloramines building up.
The same goes for red, stinging eyes and irritated skin. Those are usually caused by combined chlorine, not by healthy free chlorine levels.
So if your pool smells strong, don’t hold back on chlorine. Test your water, then shock it to clear out the chloramines and get your free chlorine back up.
The most accurate way to test free and total chlorine is with a DPD liquid test kit or a digital tester, not cheap test strips. Both give you separate free chlorine and total chlorine readings so you can do the math.
Test strips are quick and fine for a fast glance. The problem is they aren’t precise enough to catch small but important gaps between free and total chlorine. A tiny difference in combined chlorine can be the line between clear water and a pool that’s about to turn.
A DPD liquid kit uses drops that change color, and it’s more accurate for spotting that gap. A digital tester takes the color guessing out of it and gives you a number on a screen.
As a baseline, test at least once a week. Then test more often during heavy use, hot weather, after a big pool party, or after a storm.
Your free chlorine should sit between 1 and 3 ppm, but the right target actually depends on your cyanuric acid (CYA) level. CYA is your chlorine stabilizer, and it protects chlorine from the sun.
Your free chlorine should be about 7.5% of your CYA level. So the higher your CYA, the more free chlorine you need to keep the water sanitized.
For example, if your CYA is 40 ppm, aim to keep your free chlorine around 3 ppm. If you only look at “1 to 3 ppm” and ignore your CYA, your chlorine may be too weak to do its job.
So test your CYA too, and keep it in a healthy range of 30 to 50 ppm for most pools. I built this handy free chlorine calculator that takes CYA into account.
Pick the type of chlorine you’re adding and enter your numbers. The dose updates as you type.
Get exact doses in The Pool Care App
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Salt water pools follow the same free vs total chlorine rules, but they usually run higher CYA. Salt pool makers often recommend keeping CYA between 60 and 80 ppm.
That’s because the sun is doubly hard on saltwater pools, breaking down both the salt and the chlorine it makes. With your salt system, you can also check the generator’s chlorine output, then confirm it with a real test of your free chlorine.
Combined chlorine builds up when your chlorine has more contaminants to fight than it can handle. The more junk in your water, the faster your free chlorine turns into combined chlorine.
Common causes include a heavy swimmer load, plus sunscreen, sweat, and body oils that swimmers bring in. Leaves, dirt, and other debris use up chlorine too.
Rain and yard runoff add contaminants and can dilute your chemistry after a storm. Low CYA is another big one, since it lets the sun burn off your free chlorine before it can work.
To stay ahead of it, keep your chlorine steady, clean out debris, and shock after parties or storms. Then test your water so you can catch combined chlorine before it climbs.
Most pool owners don’t struggle because pool care is hard. They struggle because nobody ever showed them a simple routine. That’s why I created The Pool Care App. Just enter your numbers to know exactly what they mean and what to do for your specific pool.
On top of that, The Pool Care Handbook gives you a simple weekly routine, the right range for every chemical, and step-by-step fixes when your numbers don’t look right. And it works for chlorine and saltwater pools.
Answer a few questions and we'll tell you what to test, what to add, and when.
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Matt is the founder of Swim University. He's been in the pool and spa industry since 1996. His mission is to make pool and hot tub care easy. Each year, he continues to help people with water chemistry, cleaning, troubleshooting, and more.
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