Total and Free Chlorine: What’s the Difference?

If you’re a homeowner who owns a pool, you’ve probably seen a few different types of chlorine readings in your pool. Maybe you’re a new pool owner, or perhaps you just want to learn more about chlorine readings.

Chlorine is a widespread chemical, even outside the realm of swimming pools. People all over the world rely on chlorine for clean drinking water, due to its capability to kill bacteria and microorganisms. It’s a very versatile chemical that’s also used to make products like paper, paint, or textiles. It could be used in vinyl flooring, water pipes, and window frames, too, given that it’s necessary to produce PVC. Chlorine can even be used when making medicines.

Still, whether we’re talking about drinking water or swimming pools, getting the correct amount of chlorine in the water is a necessity.

It’s fairly inexpensive as far as cleaning solutions go, and it’s simple to use. The level of protection pool owners get from using chlorine often isn’t possible with other similar products and chemicals.

Regardless, our handy guide will help you understand everything you need to know about chlorine levels — including free, combined, and total chlorine — as well as the differences between them, how they’re all connected, and why you as a pool owner should care.

Types of Chlorine Readings

To begin, there are three types of chlorine readings: free, combined, and total. Each type of chlorine works differently in your swimming pool water, so it’s often helpful to understand what each type does.

As you might’ve guessed, the total chlorine is the free chlorine and combined chlorine added together.

What Is Free Chlorine?

When you’re testing your swimming pool for healthy chlorine levels, you’ll usually be checking for free chlorine. This is the chlorine that’s still able to sanitize your pool water and hasn’t come into contact with any possible contaminants yet.

The way to complicated chemical definition: Chlorine forms hypochlorous acid (HOCI) when it dissolves in your pool. It then combines with oxygen and forms hypochlorite (CIO), which is an oxidizing ion (and an ingredient in bleach). Together, HOCI and CIO form free chlorine.

Essentially, free chlorine is in charge of killing bacteria and microorganisms that can potentially make you sick. It’s the most crucial kind of chlorine to monitor, just because free chlorine protects you from these microscopic dangers.

It’s very important to make sure you have enough free chlorine in your swimming pool. Otherwise, the bacteria can build up, making it even harder to eliminate.

Since free chlorine is so critical to your safety and well being, we’d recommend testing the free chlorine levels daily. If the levels are off, you should add the chemicals you need to get the free chlorine in the right range.

When you measure free chlorine, you’ll want to make sure the levels are between 2-4 ppm (parts per million). This will make sure your swimming pool water quality is up to par. It’s also important to note that these chemicals to level out your free chlorine are best used at night since the sun’s UV rays can cause the chlorine to break down quickly.

However, once this type of chlorine reacts with contaminants in your swimming pool, it transforms into what’s referred to as combined chlorine.

What Is Combined Chlorine?

In the process of sanitizing your swimming pool, you’ll come across combined chlorine. Combined chlorine is the level of chlorine that reflects how much has already been used to disinfect or clean your water. So, when chlorine interacts with organic material (think oil, sweat, or even urine) and oxidizes nitrogen and ammonia, it creates combined chlorine or chloramines.

So, if you do have combined chlorine levels, which you probably do have, it’s a numeric sign that there’s material in the water that the free chlorine is currently breaking down. However, if your swimming pool is outside, the combined chlorine level will hover near zero, as long as your pool gets direct sunlight, and you keep up with an excellent free chlorine level. If your swimming pool is both disinfected and clean, you should have zero combined chlorine.

This is the type of chlorine that irritates the skin and eyes of those who swim in your pool. It might even cause the color on your swim trunks to fade a little. This type of chlorine also, well, smells like chlorine. In fact, when you’re around combined chlorine often, it can cause allergies, asthma, or other severe health problems.

However, once combined chlorine is present, it doesn’t have as much disinfecting power. For example, free chlorine is more reliable in that it takes much more combined chlorine to do the benefits of free chlorine in a ratio of 25:1.

If you know the amount of combined chlorine in your swimming pool, think of it this way: You’ll have to add nearly 10 times more free chlorine to eliminate it (AKA, overdosing, or super chlorination).

The preferred range of combined chlorine should be about 0.5 or under. If it’s over 0.5, it’s probably time to give your swimming pool the shock treatment.

What Is Total Chlorine?

Luckily, total chlorine is an easy enough concept to grasp. It’s merely the sum — the total of combined and free chlorine levels in the pool water.

So, if the free and total chlorine levels are identical, that means there isn’t a presence of combined chlorine in your swimming pool. In other words, none of the chlorine has been utilized yet.

On the other hand, if the free chlorine level is lower than the total chlorine level, you can use that to calculate the combined chlorine level.

Why Is It Important to Check Free, Combined, and Total Chlorine Levels?

If you’re wondering why it’s essential to keep an eye on free, combined, and total chlorine levels, the answer is really quite simple. You’ll want to know if your swimming pool is properly sanitized, and you can only really know this for sure if the free chlorine level is higher than the combined chlorine level.

You can confirm this by testing the water, ideally with a liquid test kit, as they’re more accurate. Still, you can also use test strips or a digital or electronic chlorine tester.

Measuring the chlorine in your swimming pool is an absolute must. Otherwise, you won’t be able to enjoy the water as there’s no guarantee it’s sanitized or clean (and if there’s no chlorine, you can be sure that it’s not clean), or on the other side of the spectrum, you could have too much chlorine (in which case, head over to my article on how to lower chlorine levels).

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The Bottom Line

The goal with chlorine testing isn’t to make having your own swimming pool feel like a chore. The goal is to make sure your swimming pool is performing at its best, with clear, gorgeous water that’s also clean and disinfected. Don’t think of it as a chemistry experiment, but more as a way to get the most out of your swimming pool.

Measuring chlorine levels should be a simple, quick task to ensure your swimming pool is shipshape. If your combined chlorine is lower than your free chlorine levels, you know you’re in good shape, so you won’t have to worry.

Although it might sound confusing at first, don’t let the types of chlorine levels and swimming pool chemistry overwhelm you. Once you understand how free, combined, and total chlorine levels work individually, and how they work together, you’ll be able to balance your swimming pool with more confidence.

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