6+ Key Differences Between Free Chlorine and Total Chlorine

We all know chlorine. It’s that distinct smell that emanates from virtually all pools that you’ve ever visited throughout your life, be it from the local community pool where you and your childhood friends went to during summer, or to the pool in the hotel that your company rented out during a company retreat.

But what exactly is chlorine? Well, to put it briefly, chlorine is a powerful disinfectant used in a variety of other applications aside from pool use, such as water treatment, and is an ingredient in various cleaning solutions such as bleach.

It’s because of these potent disinfectant properties that it is an essential part in any swimming pool due to its crucial role in keeping them sanitary.

A pool can be used by a large number of people at any given time, so it’s absolutely essential to keep the water clean for human use. As numerous people swim about, scores of bacteria varieties would flourish if left unchecked. Thus, chlorine is used to sanitize the pool.

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But did you know that there’s more to chlorine than just dumping it into a pool and calling it a day? Today, we’re going to find out the key differences between two distinct classifications of chlorine: free chlorine, and total chlorine.

To understand what is the difference between free chlorine and total chlorine, first, we have to understand what free chlorine and total chlorine are.

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What Is Free Chlorine and Total Chlorine?

Free chlorine and total chlorine are essentially two separate measurements of the chlorine level in a swimming pool.

In order to understand both free chlorine and total chlorine, one must also understand the third variety of chlorine measurement: combined chlorine.

Combined chlorine is the measurement of chlorine that has already bonded with various bacteria, impurities, and other contaminants in the water. Hence the word “combined”.

In other words, combined chlorine is the amount of chlorine that has already been “used” in the pool. This is the “dirty” chlorine that can no longer hold any more molecules from contaminants. Consequently, the combined chlorine has to be zero in a freshly cleaned pool.

With this, one can deduce what free chlorine and total chlorine are, and what is the difference between free chlorine and total chlorine:

Free chlorine is the amount of chlorine that can still disinfect any contaminants in a pool

Essentially, free chlorine is the level of chlorine that is still “free” to do its job in sanitizing the pool. Chlorine works by binding its molecules to the various contaminants in the water, forming combined chlorine.

When this happens, the amount of free chlorine in a pool steadily drops the more contaminants are introduced into the pool. In short, free chlorine is the amount of clean chlorine that’s available to the pool in sanitizing the water.

Total chlorine is the grand total amount of chlorine, regardless if it’s free or combined

On the other hand, total chlorine is the sum total of both free chlorine, and combined chlorine. The total chlorine is a set amount, dependent on the size and depth of a pool.

However, its effectiveness would diminish over time, thanks to the lowering amounts of free chlorine, and the rising amounts of combined chlorine.

Total chlorine as a whole is a fixed number dependent on the volume of water a pool can hold. Consequently, a perfectly clean pool has to have an equal amount of free chlorine and total chlorine (ditto when using an alternative system).

Free chlorine is the overall level of chlorine that a pool has left to disinfect its waters with

Basically, free chlorine is a gauge on how much time a pool has left until it needs to be cleaned and replaced with a new batch of chlorine.

Looking back at the mechanics of how chlorine molecules work, it’s clear that the amount of free chlorine available to neutralize contaminants is finite, and that over time, the free chlorine will eventually run out.

In fact, the smell of free chlorine is actually odorless, and the “swimming pool smell” we associate with chlorine is actually combined chlorine, with the smell being a byproduct of the free chlorine combining with the various bodily wastes that swimmers discharge while in the water.

Total chlorine also shows how much chlorine is available or unavailable to sanitize pools

With free and combined chlorine essentially being the addends that result in the sum of total chlorine, total chlorine can be used to make a rough estimate on when pools need cleaning, and its water replaced.

Depending on the volume of the pool and the expected number of people to be swimming in it per day, the total chlorine level is an invaluable tool in scheduling when a pool needs to be cleaned with various chemicals.

With these factors, the total chlorine requirement of the pool already provides a recommended scheduled time when the chlorine needs to be replaced.

Free chlorine is one of the most useful measures on whether a pool is still sanitary or not

The level of free chlorine is one of the most straightforward ways of determining whether a pool is safe to swim in or not. Simply put, if the level of free chlorine is still high, then the pool is still safe.

Free chlorine is a simple yet effective way in determining this very crucial status, and all it takes is a simple colorimeter, which is a device used to measure the various chlorine levels in a swimming pool.

Total chlorine may also be a more vague measurement if taken by itself

On the other hand, total chlorine by itself may not be enough to fully and safely determine whether a pool is sanitary for personal use. Since total chlorine measures both free and combined chlorine, if you can only measure the total chlorine levels, then you’re only getting an incomplete picture. 

Total chlorine does not show the difference between free chlorine and combined chlorine. Rather, it only shows the grand total, without discriminating between the two varieties.

While an experienced pool owner may use the total chlorine level as a rough guide on when to clean, it’s much more safe and accurate to also take the free chlorine and combined chlorine levels.

Frequently Asked Questions

We’ve gathered some common questions about “what is the difference between free chlorine and total chlorine.” Check the answers below.

What should I do if total chlorine is higher than free chlorine?

Since free chlorine and total chlorine should be equal in a perfectly clean pool, a higher total chlorine level compared to free chlorine is a sign of increasing combined chlorine levels in a pool.

As mentioned before, total chlorine is the grand total of both free chlorine (chlorine available for sanitation) and combined chlorine (chlorine already bonded to contaminants).

Thus, if the total chlorine starts overtaking the free chlorine, then perhaps it is time to clean and replace the pool and water.

Why is my total chlorine high but free chlorine low?

The larger in volume your pool is, the higher the total chlorine would have to be in order to effectively sanitize a large amount of water for long periods of time.

However, once the free chlorine starts to get low, it may be because the pool is already nearing its required cleaning period, and the free chlorine is decreasing and is starting to get overtaken by combined chlorine.

More FAQs

What is more important free chlorine or total chlorine?

The importance lies in having a higher level of free chlorine compared to total chlorine when testing pool or spa water, as it is crucial for effectively sanitizing the water. Free chlorine acts as the necessary defense mechanism to keep contaminants at bay.

How do you increase free chlorine and not total chlorine?

The answer to the question “How do you increase free chlorine and not total chlorine?” can be rephrased as follows: To raise the free chlorine level without affecting the total chlorine, one can use chlorine-based shock. This type of shock increases the free chlorine level rapidly, while non-chlorine-based shock reduces combined chlorine without impacting free chlorine levels. It is important to note that chlorine-based shock should only be used when a minimum increase of 10ppm in chlorine level is required.

Why do I have total chlorine but no free chlorine?

My total chlorine level is high while my free chlorine level is low due to the possibility of chlorine becoming locked or chemically bound up by an excessive amount of pool stabilizer in the water. This could happen if too much stabilizer was added or if the pool water has not been partially drained and replenished periodically.

How much free chlorine should be in a pool?

The appropriate amount of free chlorine in a pool is recommended to be at least 1 ppm by the CDC, with a pH level between 7.2 and 7.8. For hot tubs and spas, the recommended free chlorine concentration is at least 3 ppm. When used correctly, free chlorine has the ability to eliminate most germs within a few minutes.

What happens if you have too much free chlorine?

If you have an excessive amount of free chlorine, it can lead to lung irritation, damage to the skin and eyes, and trigger asthma. This not only poses a risk to your health, but it can also have negative effects on your pool. The elevated levels of chlorine cause a decrease in the pH of the pool water, resulting in increased acidity.

Does chlorine raise or lower pH?

Chlorine can affect the pH of water. When chlorine levels are too high, the pH of the water will decrease, potentially causing corrosion. On the other hand, liquid chlorine, which has a pH of 13, can raise the pH of the water.

Should free chlorine be lower than total chlorine?

The free chlorine level should be higher than the total chlorine level in order for your pool to be effectively sanitized. Regularly testing your pool water is crucial to ensure this balance is maintained, and this can be easily done using test strips or a liquid test kit.

What should total and free chlorine be in a pool?

The total and free chlorine levels in a pool should be maintained at specific concentrations. Chlorine is added to the water in order to eliminate germs, although it may not be immediately effective. When used correctly, free chlorine can effectively eliminate most germs within a matter of minutes. According to the CDC, it is recommended to maintain a pH level between 7.2 and 7.8, and ensure a free chlorine concentration of at least 1 ppm in pools, and at least 3 ppm in hot tubs and spas.

What is a good free chlorine level?

The good free chlorine level for swimming pools, according to ANSI 1 standards, should be between 1.0 and 10.0 parts per million (ppm), while bromine levels should be between 2.0 and 10.0 ppm. For swimming pools, ANSI recommends an ideal range of 2.0 to 4.0 ppm.

Should I add chlorine when I shock my pool?

You should not add chlorine when you shock your pool. It is recommended to first shock the pool and then wait for the chlorine levels to decrease below 5 PPM.

Does shock increase free chlorine or total chlorine?

The rephrased answer is: Shocking increases the amount of free chlorine in your pool or spa by releasing combined chlorine and off-gassing contaminants. Total chlorine is the combination of both free and combined chlorine.