The Difference Between Total and Free Chlorine

by Matt Giovanisci | Last Updated: March 29, 2016

The chlorine levels in your pool are one of the most important measurements you must keep track of all the time. After all, it’s what sanitizes your pool.

There are different types of chlorine, and it confuses pool owners. What they should be testing for in their pools?

Pool experts throw around terms like “free chlorine” and “total chlorine.” This just leads to confusion. So, let’s stop and explain the differences between free and total chlorine. Because once you understand, you will then be able to better track the levels in your pool.

Combined, Total, and Free Chlorine

We’re focusing on total free chlorine here, but there are actually three types of chlorine If you click this link and make a purchase, we earn a commission at no additional cost to you. :

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Breaking Down the Differences in Total and Free Chlorine

To help explain the differences, take a look at this simple formula for chlorine:

FC + CC = TC

When you add chlorine to your pool, it reacts with the water to form hypochlorous acid and hypochlorite ion. These compounds together form what we call free chlorine.

Once this chlorine begins to react with the contaminants in the water, such as nitrogen and ammonia, it becomes combined chlorine.

In this state, the chlorine isn’t as effective at sanitizing compared to free chlorine. Your goal is to make sure your pool is sanitized. You want to make sure your free chlorine levels stay in check.

For example, if your free chlorine levels and total chlorine levels are the same, then there’s no combined (or used chlorine) in your water. If your total chlorine level is higher than the free chlorine level, the difference of the two are the combined chlorine levels.

To make sure your pool is sanitized, your free chlorine should remain higher than your combined chlorine.

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Time for Some Simple Pool Math

I know, you didn’t expect to have to do math when you bought your pool. But it’s critical to maintain high water quality.

These numbers will help you determine exactly how much chlorine to add for it to continue doing its job of destroying bacteria and any other contaminants in the water.

If you notice your combined chlorine levels going up, you know the time has come to rebalance the sanitizer in the water by adding extra chlorine to your pool. If the levels are low, then you may not need to do anything at all that day.

Did This Help Clear up The Difference Between Total and Free Chlorine?

While it can be confusing at first, understanding the differences between free and total chlorine can help you take care of your pool. Sanitation of your pool water is essential. It keeps the water clean and clear and ensures that the water is safe for swimming.

Now that you know what each of them actually are, you will be able to more easily recognize what you need to do in order to keep the sanitizer levels where they should be so your water stays clean, clear and safe all summer long. So break out your water testing kit and test your water for its chlorine levels.

Happy Swimming!

Matt Giovanisci, the founder of Swim University®, started in the pool and spa industry at age 13 and moved to bigger companies along the way, helping thousands of pool and hot tub owners every year. He wanted to share his knowledge and unique teaching style on a larger scale, so he launched Swim University® in 2007. Since then, he's worked to make pool and hot tub care easy for over 10 million homeowners. And each year, he continues to help more people with water chemistry, cleaning, and troubleshooting.

Frustrated by adding chemicals and trying to keep your pool clear all the time?

We cut out all the confusion of pool maintenance in this easy-to-read illustrated ebook and video course. It'll help you save $100 right away on pool care!

Click here to learn more
The Pool Care Handbook