What’s The Difference Between Total and Free Chlorine?

If you’ve had a pool for any amount of time, you’ve probably heard the term “free chlorine.” And in case you haven’t guessed it, there’s no magical store that gives away chlorine free of charge (but wouldn’t that be awesome?). . 

So, what does it mean, and why do you need to know?

Well, we hate to break it to ya, but if you don’t understand the difference between total and free chlorine, your pool proooooobably isn’t all that clean. So, let’s break it down one thing at a time.

What is chlorine?

The Shasta County, California chemistry fact sheet describes chlorine like this: “The most common chemical used in the treatment of swimming pool water is chlorine.  It not only eliminates bacteria and algae by disinfecting (killing) action, it also oxidizes (chemically destroys) other materials such as dirt and chloramines.”

Chlorine for pools comes in a couple of different forms — liquid and granular. But each of these are available in different compounds.

Solid chlorine, which is what they call powder, or granular chlorine, can be purchased as calcium hypochlorite or lithium hypochlorite. And liquid chlorine is always sodium hypochlorite.

That doesn’t mean a whole lot to the average pool owner except to know that calcium hypochlorite is the most popular because it has more available chlorine than the others (65-75%). This is what is almost always used to shock a pool.

Free Chlorine vs Total Chlorine: What’s the Difference?

To make things even more confusing, there are actually three types of chlorine — total chlorine, free chlorine, and combined chlorine. So, the formula to find total chlorine is this:

Free chlorine + combined chlorine = total chlorine

We know…that doesn’t clear things up a whole lot, but let’s take a look at what all this means.

Free Chlorine

Free chlorine is an important factor in sanitizing your pool because it’s referring to the amount of chlorine available to fight contaminants in the water. In other words, you can still have chlorine in your pool that has been used up and is no longer effective.

The way free chlorine is formed is by dissolving into hypochlorous acid, combining with oxygen, and forming hypochlorite. This substance is what fights bacteria and other contaminants.

The ideal level for free chlorine is 3 ppm (parts per million). Very much over or under that can cause some problems.

Combined Chlorine

During the sanitization process, when the chlorine is binding with the contaminants, it becomes what is known as combined chlorine. If you do have combined chlorine, it means it’s breaking down some bad stuff.

If you have any combined chlorine in your swimming pool water at all, it can start to build up and result in chloramines. This is what you’re smelling if you get a whiff of that super strong bleachy smell and it means you probably don’t have enough free chlorine to do its job right.

Total Chlorine

Total chlorine is the total of combined and free chlorine. Some testing kits will only test for total chlorine because ideally, you shouldn’t have any combined chlorine. 

But it’s way more effective to use a test that checks for combined and free chlorine because that will give you a clearer picture of how clean your water is.

Benefits of Chlorine

The CDC (Center for Disease Control) lists the following as the benefits of chlorination:

• Proven reduction of most bacteria and viruses in water

• Residual protection against recontamination

• Ease-of-use and acceptability

• Proven reduction of diarrheal disease incidence

• Scalability and low cost

There are some alternatives on the market, but as of today, there are none that prove to be quite as effective as chlorine. It’s known to get rid of almost all bacteria and viruses that can live in the water.

Additionally, it’s one of the cheapest ways to keep water clean and sanitized on the market.

How to Test for Free Chlorine vs Total Chlorine in Your Pool

Like we said earlier, some kits will only test for total chlorine. Those kits assume you have zero combined chlorine in the water. But other kits only test for free chlorine since this is the one that really matters.

In a pinch, we’d recommend using a kit that only tests for free chlorine over one that only tests for total chlorine because it will give you a little more information. 

But ideally, you should have a kit that tests for it all. This way, you can tell if you need to adjust something to make sure the chlorine can do its job better.

For example, if you notice a large amount of combined chlorine (over 0.5 ppm), it could be time to drain your pool a little and start with some fresh water. Or if you notice your free chlorine is regularly depleted, you might need to add some CYA (cyanuric acid) to stabilize it.

There are really only three ways to test these chemicals — a drop test kit, a strip test kit, or a professional analysis.

Drop test kits are usually a little more accurate than test strips, but I know pool owners who only ever use test kits and get by just fine. That’s really a matter of personal preference and trial and error.

To have your water professionally analyzed, you can take a sample to your local pool store. They usually have more sophisticated tools than the average pool owner and know a little more about it.

This method can get expensive, depending on how much your particular store charges, so we wouldn’t recommend doing it all the time. However, if you’ve had a chemical balancing problem for a while, it’s not a bad idea to get a second opinion.

Free Chlorine vs Total Chlorine FAQs

Should total chlorine be higher than free chlorine?

If your total chlorine measures higher than your free chlorine, you have a real problem. That means you have too much combined chlorine, which could lead to chlorine lock and ineffective sanitizing.

Your total chlorine should actually measure the exact same as your free chlorine. This means you have enough chlorine available to kill germs.

What should free chlorine level be?

Your free chlorine level should ideally be 3 parts per million. But a safe range is between 1 and 3 ppm.

What happens if free chlorine is low?

When your free chlorine levels get too low (especially below 1 ppm), microorganisms start to multiply faster because there’s nothing there to fight it. Additionally, nuisances like algae start to bloom and spread quickly.

Both of these things can be big problems if they get out of hand.

How do you increase free chlorine?

The only way to increase your free chlorine is by adding some more using whatever method you usually use. 

If you have a chlorine feeder or floater, you’ll need to add some to that. Or if you usually pour it directly into your skimmer, you’ll need to add another dose. 

The important thing is to add it gradually and test the water again each time so that you don’t get too much.

If it doesn’t seem to be helping after a couple of doses, though, it’s probably time to shock your pool. Some people call this super-chlorinating because you’re actually just adding a huge dose of chlorine all at once to sort of kill everything off and restore the balance. 

This needs to be done at least twice a month to keep things in balance, but sometimes you’ll have to do it more often when things get out of whack.

Conclusion

Now that you know everything there is to know about free chlorine and total chlorine, hopefully you have a better understanding of how it all works. Armed with this information and a little diligence, you’re likely to have the cleanest swimming pool on the block!