Total and Free Chlorine: What’s the Difference?

Checking your chlorine levels can get confusing due to the different names used for different types of chlorine readings in the pool. Total chlorine? Free chlorine? What does it all mean?

My handy guide will help you understand everything you need to know about chlorine levels, including free, combined, and total chlorine. I’ll also discuss their differences and how they’re all connected, including the formula for total chlorine which is highlighted below.

Types of Chlorine Readings

There are three types of chlorine readings in your swimming pool: free, combined, and total. Every kind of chlorine works differently in the pool water, so it’s often helpful to understand how each of them works.

What Is Free Chlorine?

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

To break the process down chemically: chlorine forms hypochlorous acid (HOCI) when it dissolves in your pool. It then combines with oxygen and forms hypochlorite (CIO), an oxidizing ion (and an ingredient in bleach). Together, HOCI and CIO form free chlorine.

Essentially, you use free chlorine to kill bacteria and microorganisms that can potentially harm you or other pool patrons. This fact is why it is the most important chlorine type to monitor.

I would recommend testing the free chlorine levels daily. Add the chemicals needed to bring the free chlorine to the correct range if the levels are off. Keep your free chlorine levels between 2-4 ppm (parts per million).

What Is Combined Chlorine?

When chlorine comes into contact with organic material (oil, sweat, or even urine) and oxidizes nitrogen and ammonia, it creates combined chlorine or chloramines.

So if you test your water and notice combined chlorine levels, it is a sign that the chlorine is breaking down contaminants in the water. If your swimming pool is outside, the combined chlorine level will hover near zero as long as your pool gets direct sunlight and you maintain your free chlorine level.

Combined chlorine 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. Combined chlorine is also where the harsh chlorine smell comes from. Combined chlorine can often cause allergies, asthma, and severe health problems.

Combined chlorine doesn’t have as much disinfecting power. Free chlorine is much more reliable. You’ll have to add nearly ten times more free chlorine to eliminate it (AKA, overdosing, or super chlorination).

The preferred range of combined chlorine should be under 0.5 ppm. If it’s over 0.5 ppm, you need to shock your pool.

What Is Total Chlorine?

Total chlorine is simple. It is merely the sum of the combined and free chlorine levels in the pool water. The equation for calculating the total chlorine is:

Free Chlorine (FC) + Combined Chlorine (CC) = Total Chlorine (TC)

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 calculate the combined chlorine level by finding the difference.

How to Check Free, Combined, and Total Chlorine Levels

Testing the chlorine in your swimming pool is an absolute must. Otherwise, there’s no guarantee it’s sanitized or clean. You should test for free, combined, and total chlorine to ensure your water is safe to swim in.

Ideally, you should test the water daily with a liquid test kit because they are the most accurate. You can also use test strips or a digital or electronic chlorine tester. With a liquid dropper testing kit, you will get an accurate measurement for all three types of chlorine and determine what chemicals you need to add to stabilize the levels.

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Download my free, printable pool maintenance checklist to help you accomplish regular pool care tasks for any type of swimming pool.

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The Bottom Line

The main goal is always for your swimming pool to be performing at its best, with clean and gorgeous water. 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 functioning at its best. 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, you’ll be able to balance your swimming pool with more confidence.

Questions? Let me know.

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