Free Chlorine vs. Combined and Total Chlorine: Differences, How to Test, and More

Wondering what the differences are between free chlorine vs total vs combined chlorine? Here’s why each matter, the differences, how to test, and more.

If you’ve ever been to your local pool, you already know the smell…

The distinctive odor of chlorine that permeates the air, latches onto your skin, and gets into your hair.

Chlorine helps keep pool water safe and suitable for swimming, reducing the advance of bacteria that can make water unsafe.

But when it comes to balancing the water in your swim pool, there is more to it than just the smell.

Fortunately, there is no lab coat or quiz required to understand free chlorine vs total chlorine, key differences, how to test properly for both, and more.

Let’s jump right in!

Chlorine – What is it and What Does It do?

Chlorine is the world’s most popular disinfectant for swimming pools for its relative safety and reliability.

Adding chlorine into the waters of your above-ground or inground pool does the heavy lifting of keeping bacteria, algae, and other microorganisms from taking root and making the water dirty and unsafe.

Chlorine is very inexpensive, isn’t as harmful compared to other solvents, and works for extended periods of time, reducing reapplications.

Chlorine is still a chemical, however, and is not without its downsides.

See also: How to Protect Your Hair from Chlorine

Namely that overpowering odor (that’s usually not technically chlorine, but chloramines, but we can get to that another time) and skin itchiness from over-exposure.

That said, the upside of chlorine in your swim pool far outweighs the relatively harmless benefits of exposure.

What are the Different Types of Chlorine?

There are three different chlorine measurements you need to be aware of.

They include:

Free chlorine

This is the chlorine that is unused and ready to be deployed to disinfect the pool.

Free chlorine is “free” to get after the bacteria and organisms in the pool.

When you drop chlorine tablets or liquid chlorine into your swimming pool, the tablets dissolve like a cheap t-shirt, creating hypochlorous acid.

Before long, it gets oxidized in the water making hypochlorite, kicking off disinfection mode, creating a clearer, cleaner pool.

When testing the pool water, look for a free chlorine level of 2-4ppm.

Combined chlorine

Combined chlorine is the chlorine in the water that has already put in a shift in the pool.

When chlorine combines to foreign matter—note the word combines—this makes it combined chlorine.

It’s done it’s job.

Ideally, when you test the pool water, and the pool has been thoroughly disinfected, you’ll get a combined chlorine level of zero.

Total chlorine

Is the sum of both free and chlorine totals, or the sum of all chlorines in the water.

Easiest math ever!

This number can be misleading, however, as some cheaper pool test kits don’t specify how much of the total is free versus combined chlorine.

Testing for Chlorine in the Pool

Clean, swimmable pool water requires that you are constantly testing the pool water, making sure that the pool chemistry is on point.

But don’t worry, this is actually easier than it sounds, and once you add it to your pool maintenance and cleaning routine, it’s very easy.

(Especially now that you understand the basic differences between the different types of chlorine!)

The best ways to test your pool water include:

Pool test strips.

Pool test strips are very inexpensive and can be found at most stores that carry outdoor recreation goods (as well as Amazon, obviously).

Draw a sample from the pool, submerge the strip, and after about 30 seconds you compare the colors on the strip to the color-coded guide on the bottle.

For most pool and spa owners, this is more than adequate for testing free vs combined chlorine, as well as a range of other things like phosphates, pH, and total alkalinity.

Digital pool testers.

If you want to get more precise with your pool chemistry, investing in a digital pool tester is a great idea.

Not only can they give you highly specific PPM levels for free, combined, and total chlorines, but the best digital pool testers on the market can also give you pH, bromine, alkalinity, calcium levels, and more. All on a nice digital readout that takes away the color-coded guessing that happens with test strips.

The only thing it won’t do is clean your pool of leaves after a windy day ?.

Liquid pool test kits.

Water testing kits, or liquid pool test kits, are a little bit more “hands on” when it comes to testing, using reagents to pump out pool chemistry details.

Testing requires a steady hand—adding too much reagent to the sample can accidentally skew results—but testing this way will give you a slightly more accurate reading compared to test strips.

Free vs Combined vs Total Chlorine – FAQs

Which is more important, free or total chlorine?

Trick question! They both are.

Free chlorine is important, because you want chlorine ready to step in and disinfect the water. Having enough free chlorine in the water will keep the water clean.

As combined chlorine levels climb, you will begin to notice the distinctive smell of chlorine—which, again, is actually chloramines, aka combined chlorine—and the water will begin to cloud.

Keep your eyes on both via regular testing, and you will be well on your way to enjoying clean pool water all summer long.

Will shocking the pool raise free chlorine levels?

Shocking the pool—and no, we aren’t talking about throwing a plugged-in toaster into the water—is all about temporarily blasting the free chlorine levels to disinfect the pool.

When shocking the pool, we are raising the free chlorine levels by a factor of ten times, called “breakpoint chlorination.”

Over time, and with improper pool chemistry, chloramines (combined chlorine) can build up, creating an ideal scenario for the growth of bacteria and algae. In other words, free chlorine has been largely used up and we need to superchlorinate in order to bring balance back to our pool chemistry.

Shocking the pool should be done regularly during periods of heavy use, and is typically recommended after heavy rainfall or other weather-related events that send a heap of organic matter and added water to the pool.

Again, testing regularly will help inform how often you should drop some pool shock into the water.

The Final Lap

There’s not much better than jumping into a cool, clean pool on a hot summer day.

And while a lot of the lingo and science behind proper pool chemistry can feel intimidating to the newcomer, it’s actually very straightforward once you’ve got a grasp of the basics of chlorinating the water.

So go on, test those chlorine levels with confidence, and enjoy that pool or spa knowing the water is clean and safe!

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