Taking care of your pool doesn’t have to be a chore… or a chemistry experiment. But chlorine levels in your pool should be measured and tracked on an ongoing basis. This simple task will keep your pool clean, disinfected, and sparkling all year long.

You don’t need to be a trained chemist for this, we promise. We’ll break down the 3 chlorine states—free chlorine, combined chlorine, and total chlorine—and what they can tell you about the cleanliness of your pool.

Need a quick answer? Free chlorine is unused and ready to sanitize. Combined chlorine has already been used up but is still present in the water. Total chlorine is the sum of both.

What is Chlorine Really and Why Should You Care?

Chlorine kills bacteria through a simple chemical reaction. When you add chlorine solution into your pool, it breaks down into many different chemicals, including hypochlorous acid (HOCl) and hypochlorite ion (OCl-). 

Both of these chemicals kill microorganisms and bacteria, then combine with other chemicals (like ammonia), rendering them oxidized and harmless.

Chlorine can have some unpleasant side effects, however. It’s a strong chemical with a very strong odor that can sometimes cause swimmers to have itchy skin after a dip, or to notice some color fading on their brand-new bikini. Ugh, what a bummer.  

Types of Chlorine, Demystified

You may have heard terms like “free chlorine” or “combined chlorine” before. Let’s clear that up what exactly they mean, shall we?

  • Free Available Chlorine (FAC): This type of chlorine is what you test for in your pool. Free chlorine is unused and available, like invisible magic dust floating around, just waiting to sanitize your swimming pool water.
  • Combined Available Chlorine (CAC): This is the used-up chlorine that has already done its job disinfecting your pool. It’s tired, overworked, and spent — but it still matters… and is still lurking in your pool. Waiting.
  • Total Chlorine (TC): This is just the sum of both: free chlorine + combined chlorine = total chlorine.

Easy, right? Now that we’ve gotten the definitions down, let’s talk about why these terms matter and how to test for them in your glorious backyard oasis. 

How to Test fOR Combined, Total AND FREE Chlorine in Your Pool

Ideally, you should be testing every day, but you can usually get away with doing it every other day. Frequent testing is especially important if your pool is uncovered and exposed to rain, wind, leaves, grass clippings, and sunlight.

Testing Methods

There are the 3 main ways to test the chlorine level in your pool:

  • Chlorine Test Strips: These are usually made of plastic with chemical pads adhered to the sticks. The chemical pads are designed to react with water and change color to give a good estimate of the combined, total and free chlorine levels in your pool.
  • Chlorine Test Kits: Chlorine test kits come in all shapes and sizes but all rely on the same process of titration. Simply add drops of a reagent to measure the amount of chlorine in the sample and compare the color to the instruction sheet.
  • Electronic/Digital Chlorine Tester: These are devices that measure the chlorine levels in your pool without you having to compare colors manually.

Some Quick & Painless Pool Math

Any form of chlorine added to water has the same chemical reaction: It forms HOCl (hypochlorous acid) and OCl- (hypochlorite ion). These two together = free chlorine, which is the active sanitizer and oxidizer that is doing the job of killing bacteria/germs and removing organics from your pool.

However, when free chlorine has done its job and is depleted, you’re left with combined chlorine. Combined chlorine, also called chloramines, are smelly and irritating. They’re the nasty buggers that make your skin itchy and cause your new swimsuit to fade.

free chlorine to combined chlorine

Chlorine in a combined state is pretty ineffective (compared to free chlorine) and high levels of it in your pool are bad news. They generally mean your pool chemistry is out-of-whack and needs some attention… fast.

Free chlorine, combined chlorine, and total chlorine can be easily understood with the following equation:

FAC + CAC = TC

For example, if your free chlorine (FAC) levels and total chlorine (TC) levels are the same, then there’s no combined (or used chlorine) in your water and there’s no need to add any chemicals today. Yay!

Safe Levels of FAC: According to the American Chemistry Council, the recommended level of free chlorine should be between 2 and 4 ppm (parts per million) and to never let it fall below 1 ppm. I always aim for 3-4 ppm, just to be on the safe side.

Safe Levels of CAC: It’s best to keep the combined available chlorine (CAC) levels in your pool at less than 0.2 ppm. Some health departments in the USA set a maximum level at 0.4 ppm. If your pool exceeds that, the health department can get involved. You might even notice the “pool smell” of chloramines at any level above 0.2 ppm of combined chlorine.

Testing Tip: If your free chlorine is higher than your combined chlorine, your pool is sanitized and in good shape. Frequent testing and monitoring will keep it that way!

Conclusion

As you learn more about pool maintenance, you’ll come to appreciate the importance of chlorine (and particularly free chlorine) in contributing to safe, clean water. Now that you’re familiar with the terminology, you’re ready to start testing your pool for a clean and sanitized pool all year long.