What is Free Chlorine?
May 26, 2021

Tl;dr: Free chlorine sanitizes and tackles all the nasty microorganisms in your pool, so you definitely want it around. We'll go into how to check your free chlorine levels, how its used, and why it's important to monitor your free chlorine levels.


Free chlorine is chlorine that hasn’t bonded with already-chlorinated water. It sanitizes your pool, ensuring that all the germs living in your water are kept at bay before you take a dip.

Free Chlorine FAQs

What is the purpose of free chlorine?

Free chlorine tackles all the nasty microorganisms in your pool, so you definitely want it around. Once it’s bonded with chlorinated water, it can’t take care of any more pool gunk — that’s why monitoring your free chlorine levels is critical to keeping your pool water sanitized. If you don’t have enough, your water won’t be clean. An easy way to remember what free chlorine does is to focus on the “free” part: free chlorine is free to fight off contaminants in your pool.

How to check your free chlorine

This pool DIY pro — doublewide6 Repairs LLC — is happy to tell us that he has most definitely not been down to the pool store this year. A helpful tutorial on how to check and adjust your free chlorine, but we have no idea why it’s longer than 6 minutes. It’s way easier to do than this video suggests.

This second video goes into detail with the Taylor Complete FAS-DPD Pool Water Test Kit K-2006. Bonus points if you stick around until the 3-minute mark to hear a very small child intro the pH test and proceed to give play-by-plays of everything his DIY pool dad is doing.

How is free chlorine used?

Free chlorine is used to reduce contaminants. But how does free chlorine interact with other chemicals? That’s where chemistry and your sanitation strategy come in. There are lots of ways to up the volume of chlorine in your pool (and it’s way easier than reducing chlorine). Here are just a few common options:

In short: free chlorine is added to the pool to cancel out combined chlorine. The Chemistry 101 explanation is that pool chlorine is composed of hypochlorite ions, which combined with ammonia, oxidize and neutralize contaminants. If learning about hypochlorites sounds like your kind of thing, go for it — here’s info all about them.

What is the difference between free and total chlorine?

Great question. Free chlorine is part of total chlorine. Let’s break it down: free chlorine plus combined chlorine equals total chlorine. Free chlorine is what’s free to sanitize your pool. Combined chlorine is the chlorine that’s already bonded with contaminants. You’ll often see simple test kits that only measure for total chlorine, which isn’t a great idea — you really need to know all three stats (free, combined, and total) to guarantee that your pool stays in the best shape.

What happens if free chlorine is low?

When free chlorine is low, bacteria and other contaminants go bananas. There’s nothing keeping them in check, so they multiply rapidly. Combined chlorine is also on the rise at this stage. It’s not easy to clear combined chlorine — you’re pretty much guaranteed to have to shock your pool to overcorrect. After all, the more bacteria in your water, the less safe it is for swimming. Your water can get green and cloudy, and pool gunk can even cause middle ear infections. Not good all around.

Do I have to worry about free chlorine?

The short answer is yes. Most alternative sanitation methods — copper ionizers, UV pools, salt pools — require some free chlorine. You can’t skip it! Here are the products we think do the trick.

The Stuff You Need

We don’t do affiliate links, FYI. Anything we’re recommending is something we’d use to monitor our pools, too. If we start doing affiliate marketing, we’ll always let you know first.

Solid test kit that supports free chlorine readings:

Taylor K2005 Swimming Pool Chlorine Bromine Alkalinity Hardness pH DP Test Kit | $55.91

Cheaper test strips that support free chlorine readings:

AquaChek 7-Way Pool and Spa Water Test strips (100 strip pack) | $17.29