For most pools, chlorine is the driving force behind clean and clear water — but it’s actually free chlorine that makes the magic happen.
If you’re new to pool care or you just need a quick refresher, this is our beginner’s guide to free chlorine in a swimming pool, including what it is, how it works, how to maintain it, and everything in between.
What is Free Chlorine?
When you add chlorine to your water, it immediately converts into hypochlorous acid and hypochlorite ion, collectively known as free chlorine.
This free chlorine—also referred to as free available chlorine, FAC, or simply FC—is the chlorine in your pool that hasn’t been used up yet. It’s the active sanitizer or oxidizing form of chlorine.
Chlorine in this form is ready to go to battle against bacteria, viruses, and other nasty contaminants that find their way into your pool.
This is important because a swimming pool is under constant attack from outside influences.
This includes things like algae, insects, animals, rainwater, and dirt — as well as contamination brought in by swimmers, such as sweat, saliva, lotions, hair, urine, and yes…. even poop.
Overall, without free chlorine to disinfect the water, it soon becomes dirty, green, and unsafe to swim.
How is it Different from Combined or Total Chlorine?
Free chlorine doesn’t stay free for long.
During one of these “battles”, your free chlorine becomes spent as it combines with contaminants to form a new compound called combined chlorine (CC).
As such, if you stop adding chlorine to your water, your free chlorine level will always decrease as your combined chlorine level increases; like a chemical see-saw.
Finally, your free chlorine level along with your combined chlorine makes up your total chlorine (TC).
Quick reference:
- Free chlorine = FC
- Combined chlorine = CC
- Total chlorine = TC
What is the Ideal Free Chlorine Level?
A long-standing recommendation in the pool industry is to maintain a free chlorine level of between 1 and 3 parts per million (ppm).
This is generally enough for effective sanitation but it’s also not foolproof.
Some pools are exposed to higher levels of contamination, which means they have a higher chlorine demand, which means they require a higher free chlorine level — even sometimes above 3 ppm per day.
But it goes even beyond that.
Most residential pools rely on cyanuric acid, also known as chlorine stabilizer. This substance shields chlorine from UV light, preventing it from breaking down too quickly under direct sunlight.
While incredibly useful for preservation, cyanuric acid also suppresses chlorine, forcing you to add more chlorine to maintain the same sanitizing power.
Because of this, the ideal level should be based on your cyanuric acid level. You should maintain a free chlorine level that is 7.5% of cyanuric acid level if you have a traditional chlorine pool, or 5% for a saltwater pool.
For example, if your cyanuric acid level is 60 ppm, you would aim to maintain a free chlorine level of around 4.5 ppm in a traditional chlorine pool.
Note: We have a calculator you can use here to figure out your ideal FC level.
How Does Free Chlorine Influence Other Levels?
Adding chlorine to your water directly increases your free chlorine level, but there’s a whole more happening under the hood.
The most immediate change is your pH level.
Different types of chlorine have wildly different pH levels, so they can have a very noticeable impact on the pH of your pool water.
For example, most chlorine tablets will slowly lower the pH of your water as they dissolve because they themselves are acidic.
Your total alkalinity will also be lowered during this process as it tries to buffer your pH against any acid introduced to the water.
Finally, all stabilized chlorine products (including most tablets and granules) contain high levels of cyanuric acid, which raises your cyanuric acid level every time they’re added to your pool.
What Causes Free Chlorine to Change?
Your free chlorine level is always on the move, and understanding the forces at play will help you get a better grasp on your sanitizer.
What Raises Your Free Chlorine?
The only thing that raises your free chlorine level is adding more chlorine to the water, even if you’re doing it unknowingly.
For example, many products used to shock your water contain as much as 75% chlorine. This may not be immediately obvious without reading the contents on the packet, especially if you’re still new to pool maintenance.
While not as likely, it’s also possible you have some leftover chlorine tablet dissolving in a floater or possibly even your skimmer basket.
In general, though, you don’t have to worry about your free chlorine rising without cause if you know what you’re putting into your water.
What Lowers Your Free Chlorine?
Free chlorine is used up during the sanitation process, so any contaminants in the water will eventually consume free chlorine.
That means any time someone uses your pool, or it rains, or leaves fall in, or virtually anything else touches your water, your free chlorine level will take a hit.
Algae in particular is a big factor.
Algae spores are everywhere in the environment, so they’re always being introduced to your pool one way or another, adding a constant downward pull on your free chlorine.
Let’s not forget about sunlight, either. Free chlorine degrades very quickly under UV light. In fact, without stabilizer, a pool will lose almost all of its free chlorine in around 2 hours.
Even with cyanuric acid to stabilize your chlorine, some of your chlorine is always susceptible to UV because of how stabilization works. While it strongly mitigates the impact sunlight has on your chlorine, it can’t prevent it entirely.
How Do You Test Free Chlorine?
There are two main ways to test your pool’s free chlorine or FC level.
The first, and most common, is using a test strip.
Pool test strips are typically thin white strips with 5 colored tabs to represent each of the primary pool parameters, which are pH, total alkalinity, cyanuric acid, calcium hardness, and free chlorine.
When a strip is dipped into pool water, it causes each individual tab to change color. These colors are compared against a color chart to determine the rough level for each parameter.
The second way is using a liquid test kit, otherwise called a drop kit.
As the name suggests, it uses drops of liquid (known as reagents) to color a sample of water taken from your pool, which is then compared against the provided color chart.
While both methods use a similar approach, test strips are cheaper and easier to use, while liquid test kits allow for a far more accurate reading.
How Do You Adjust Free Chlorine?
Now, let’s talk about how to raise or perhaps even lower your free chlorine when it’s too low or too high.
How Do You Raise Your Free Chlorine?
The easiest and fastest way to raise your free chlorine level is to add either granular or liquid chlorine to your water.
Granular chlorine is either dichlor, trichlor, or calcium hypochlorite, and is typically dissolved in a bucket before being poured around the perimeter of your pool.
Liquid chlorine is sodium hypochlorite, and this can actually be poured directly into your pool.
Either of these will raise your free chlorine level quickly, which is helpful when your residual chlorine level falls too low, or when you’re facing serious contamination in your water (storm, dead animal, poop, etc.).
Chlorine tablets are usually trichlor and are placed in a chlorine floater or feeder to dissolve slowly over time.
Tablets are better for maintaining your residual chlorine level, meaning they help keep the free chlorine stable over days or weeks rather than give you a sudden and dramatic increase.
How Do You Lower Your Free Chlorine?
Lowering your free chlorine level isn’t quite as straightforward. Fortunately, though, it’s not something you’re ever likely to worry about.
Assuming you have some cyanuric acid (CYA) in your water, you can safely swim at levels far exceeding your target free chlorine (FC) level.
Here’s a general guide for the acceptable limits:
| CYA level (ppm) | Max. FC level (ppm) |
| 20 | 10 |
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 70 | 28 |
| 80 | 31 |
| 90 | 35 |
| 100 | 39 |
However, if your free chlorine level is still too high to swim, it’s usually best to just wait it out.
Most pools consume around 2 to 3 parts per million of free chlorine per day, so it should only take a few days at most to get within safe levels for swimming.
For a more immediate solution, you can use a chlorine neutralizer like sodium thiosulfate, which will help to break your chlorine down within a few hours. This approach also doesn’t interfere with the rest of your water chemistry.
Lastly, you could drain and replace some of your water — but that means removing more than just chlorine so it’s rarely cost-effective to go this route.
How Do You Maintain Free Chlorine?
All pools lose free chlorine over time; that’s just chlorine doing its job.
As long as you continue to add more chlorine to the water, you can always replenish what you lose and maintain a relatively stable free chlorine level.
As mentioned earlier, tablets are ideal for this because they slowly release chlorine as they dissolve. You also have some control over how much chlorine is released over time by adjusting the number of tablets you use.
An alternative approach is to add granular or liquid chlorine on a daily basis, adding a day’s worth of chlorine loss with each top-up.
If you have an outdoor pool, cyanuric acid is non-negotiable for maintaining your free chlorine level. Most tablet and granular chlorine contain cyanuric acid, which stabilizes chlorine and protects it from breaking down under sunlight.
You can also add it separately if you’re using unstabilized chlorine such as liquid chlorine, which is often recommended as too much cyanuric acid will “lock up” the free chlorine and prevent it from being able to effectively sanitize.
Finally, preventing unnecessary contamination will reduce the impact on your free chlorine, such as regular brushing and vacuuming, showering before swimming, and covering the pool when it’s not being used.
The Bottom Line
Free chlorine is the active form of chlorine, and it plays arguably the most essential role in your water; keeping contaminants at bay.
Fortunately, this job goes largely unnoticed in a well-kept pool, so as long as you maintain an ideal free chlorine level, you can rest easy knowing most of the work is being taken care of under the surface.
