What’s with all the different names for chlorine?
We’ll cover the differences between free chlorine, combined chlorine, and total chlorine, and what they mean for your pool.
Quick answer: Free chlorine is unused chlorine that’s ready to sanitize. Combined chlorine is used up chlorine that has already been combined with contaminants in the water. Total chlorine is the sum of both.
Free Chlorine vs Combined Chlorine
Chlorine isn’t just chlorine.
It exists in your pool water as one of two chemical states, which dictates how it behaves and also how effective it is as a sanitizing agent.
They are as follows:
1. Free Chlorine
Free chlorine (FC) is the unused, active, and ready-to-sanitize chlorine you typically measure when testing your residual chlorine level.
This is the most effective form of chlorine in terms of sanitizing and oxidizing power, and arguably the most important chemical parameter of any pool.
You generally keep your free chlorine level around 2-3 parts per million (ppm), but this should scale up with the amount of cyanuric acid in your water.
In short, it’s the “good stuff”.
2. Combined Chlorine
Combined chlorine (CC) is the used-up chlorine that’s already done its job, and, unlike free chlorine, you can’t directly test for it in a pool.
This spent chlorine is also called chloramine, which continues to build up in your pool as free chlorine is consumed. It offers little in the way of sanitation and can actually reduce the quality of your water if left unchecked.
You want to keep your combined chlorine level as close to zero as possible, but anything under 0.5 parts per million is generally acceptable.
In short, it’s the “bad stuff”.
What About Total Chlorine?
Total chlorine (TC) isn’t a chemical state of chlorine, it’s simply the sum of the two chemical states discussed above.
Basically, if you add free chlorine to combined chlorine, you get total chlorine (FC + CC = TC).
But since you can’t directly test for combined chlorine, it’s impossible to calculate your total chlorine level using this formula.
Instead, total chlorine is used as a way to measure combined chlorine — not the other way around as it’s often written.
Total chlorine is really just a means to an end, with the end being to find the combined chlorine level. From a pool maintenance perspective, it’s not a particularly useful metric by itself.
With that said, if you take the total chlorine and remove your free chlorine level, you’re left with your combined chlorine (TC – FC = CC).
Why Do These Levels Matter?
Testing your free chlorine tells you how much chlorine is being consumed on a daily basis, and ultimately how often you need to add chlorine to maintain your free chlorine level.
But if you don’t know your total chlorine, you don’t know your combined chlorine, which means you don’t have the full picture.
Specifically, you don’t know how much chlorine has already been spent fighting contaminants, and how much of that combined chlorine (chloramine) is still lurking in your water.
This matters because a buildup of combined chlorine will cause eye and skin irritation, give off a pungent “chlorine” smell, and even fade the color of swimwear — symptoms that are often wrongly attributed to high levels of chlorine or improper pH.
In the end, knowing your combined chlorine level gives you much better insight into the condition of your water.
For example, if your total chlorine is much higher than your free chlorine, you have too much combined chlorine (chloramine) in your water and you need to get rid of them through a process known as shocking.
If your total chlorine is the same as free chlorine, you have no combined chlorine in the water and you can sleep easy… for now.
How to Test Free, Combined, and Total Chlorine
Almost any pool test kit will allow you to measure free chlorine.
This includes both test strips and liquid test kits. Liquid testing kits will give you the most accurate results but you can often get by using test strips.
However, note that only some tests—and particularly only some test strips—allow you to test for total chlorine as well, so it’s important to buy a kit that includes both parameters.
Remember, there is no test for combined chlorine. A test that includes both parameters will allow you to calculate combined chlorine by subtracting free chlorine from total chlorine.
If you want specific instructions, we have a step-by-step guide on how to test your water here.
The Bottom Line
Free chlorine and combined chlorine are measures of the different states of chlorine (active and spent).
Total chlorine is a sum of those states, and it’s mostly used as a way to measure combined chlorine because you can’t directly test for it.
Overall, all three parameters play a crucial role in understanding the chemistry of your pool water.
