What is Free Chlorine (FC) and How to Test it in a Pool?

All pool owners have an understanding of what chlorine is and how it helps fight the bad stuff in the pool’s water. The measurement used to determine how much chlorine is available to fight is known as free chlorine (FC).

Essentially, free chlorine is what breaks down and kills organic contaminants in the pool and it keeps the water balanced.

But then you start to learn that there are different types of chlorine that have different effects on the overall water balance.

Let’s dive deeper into the effects free chlorine has on water and what we can do to keep it balanced.

What is Free Chlorine?

Free chlorine (FC) is the overall level of sanitation in the water that is killing all the germs, algae, and other contaminants that may be living. It is simply the amount of chlorine that is “free” or available to do work that hasn’t been used yet.

When free chlorine reacts with those contaminants, that is known as combined chlorine, or chloramines. In simple terms, free chlorine keeps chloramines out of the water.

There should always be free chlorine present in your pool. Having none is essentially an open invitation for algae to have a pool party. How much to raise free chlorine is determined by your cyanuric acid (CYA) level, but at a minimum should be around 2 ppm at all times.

What is Total Chlorine?

As stated above, free chlorine measures the amount of chlorine available and combined chlorine tells us how much is being used to fight the bad stuff.

Total chlorine (TC) is a math equation that adds free chlorine and combined chlorine.

TC = FC + CC.

You might hear or read about free chlorine and total chlorine being used interchangeably. While they are different, as described above, in an ideal scenario that should be OK. After all, if CC should be 0, then FC = TC, right?

How to Test for Free Chlorine in the Pool

To keep everything properly balanced, testing your water is very important. The basic rule of thumb here is that if TC = FC, your water is balanced. If TC > FC, there are chloramines present, meaning something in the water that needs fighting.

Some testing methods may only show total chlorine. However, without free chlorine measurement, it is nearly impossible to determine if combined chlorine is present.

Using a liquid-based testing kit to test for chlorine is by far the best testing method. Most kits offer free chlorine and combined chlorine testing so you can be sure you are seeing the whole picture.

How Much Free Chlorine Should Be in the Pool?

While free chlorine levels should always be at a minimum of 2 ppm, the recommended amount that should be in the pool at any given time should be determined by the amount of cyanuric acid (CYA) in the pool.

CYA is a stabilizer that acts as chlorine’s buffer or “bodyguard”. It protects chlorine levels from UV ray degradation so it can last longer in the pool.

Very smart people like Richard Falk determined that pools need at least 7.5% of the CYA level to be FC in order to fight algae growth. So, with that calculation in mind, the following FC levels should be recommended based on CYA, using the following math equation:

CYA Level x 7.5% = FC Level + (plus 2 to 4 ppm to get a recommended range)

Ideally, you should be in the higher range of these estimates but never fall below the minimum (nearest whole number).

Check your targeted range using our handy calculator or refer to the guide below:

Take note of how unsustainable it is to keep FC that high above 70 ppm; that is why it’s recommended for CYA to be between 30 – 60 to keep chlorine realistically level at all times.

However, owners of a saltwater chlorine generator (SWG) can keep CYA levels a bit on the higher end. In fact, most manufacturers recommend CYA to be between 60 to 90 ppm.

Keeping it All in Check

The concept of free chlorine and combined chlorine is pretty simple when it’s all laid out in front of you. It also reinforces why free chlorine levels should be tested daily rather than weekly.

Depending on how you sanitize, it doesn’t take long for free chlorine levels to drop. After all, no one likes that chlorine smell!

Frequently Asked Questions

What is the Difference Between Free Chlorine and Total Chlorine?

Free chlorine measures the amount of sanitation in the water that is able to attack or neutralize contaminants. Combined chlorine tells us how much free chlorine is being used at the time. Total chlorine is simply the sum of free chlorine and combined chlorine. TC = FC + CC.

How do I raise Free Chlorine in my Pool?

Raising free chlorine levels is pretty simple. Simply adding more chlorine, or your sanitizer of choice will raise free chlorine. You can use any stabilized or unstabilized chlorine to raise free chlorine levels.

How do I Lower Free Chlorine in My Pool?

The easiest way to lower free chlorine is to stop adding sanitizer and wait a few days. Levels will naturally drop by UV ray degradation. If it needs to be lowered faster, drain some of the pool and refill it with fresh water.

What do I do if my Free Chlorine is Low?

If testing shows free chlorine levels are lower than the recommended levels, add the necessary amount of sanitizer to the pool and re-test. Leaving free chlorine low for an extended period of time is an open invitation for algae and other organisms to start growing in the water. Daily testing and adding of free chlorine should eliminate that possibility.

Why is my Pool Showing no Free Chlorine?

If testing reveals no free chlorine, it either means you haven’t added any in a while or it is actively fighting contaminants. To know for sure, test for combined chlorine. If it is above 0.5 ppm, there is an active organism in the pool that needs taken care of immediately. You need to add chlorine and potentially shock your pool depending on how bad the combined chlorine levels are.

Will Shock Raise My Free Chlorine Levels?

“Shocking” refers to adding chlorine higher than the recommended free chlorine levels to fight algae outbreaks. By that definition, yes, shock will raise free chlorine levels.