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What's the Difference Between Total Chlorine and Free Chlorine?
Even new pool owners will know that keeping track of your swimming pool’s chlorine levels is essential to its overall health and cleanliness. When you test these levels, you’ll come across different chlorine readings that tell you slightly different things about the state of the chlorine in your water.
Whether you use chlorine tablets or granules to keep your pool clean, we recommend getting your head around what is meant by free, total and combined chlorine to ensure your pool stays properly sanitised.
This is the main chlorine reading you’ll check when testing your water. Free chlorine refers to the chlorine in your pool that hasn’t come into contact with any contaminants yet. Essentially, it is the amount of chlorine in your water that is available to actively sanitise it.
When you add chlorine to your water, it dissolves to become hypochlorous acid. This combines with oxygen to form the oxidising ion hypochlorite. Hypochlorous acid plus hypochlorite equals free chlorine.
So, when you add new doses of chlorine to your pool, you are increasing the free chlorine levels. The ideal range for free chlorine is 1-3 ppm.
When free chlorine comes into contact with dirt or bacteria and does its job to break down any contaminants, it becomes combined chlorine. This process of breaking down contaminants and sanitising your water also creates waste products known as chloramines, which is what gives that harsh chlorine smell.
High levels of combined chlorine mean there’s not much active chlorine left to sanitise your pool – free chlorine levels will need to be increased again.
Unsurprisingly, total chlorine refers to the sum of the free chlorine levels and combined chlorine levels in your pool.
Free chlorine (FC) + combined chlorine (CC) = total chlorine (TC)
If your free and total chlorine levels are the same, it means there is no combined chlorine (i.e. your chlorine hasn’t come into contact with contaminants and been used up yet).
If the total chlorine level is higher than the free chlorine reading, then there is combined chlorine present – the difference between the free and total chlorine levels will give you the amount of combined chlorine.
When the free chlorine level is lower than the total level, it’s an indication that you’ll need to add more chlorine tablets, granules or shock to make sure it stays in the ideal range for keeping water clean.
Whether you think of it as increasing the free chlorine or lowering the total/combined chlorine, this is the main adjustment you’ll need to make when it comes to your pool chlorine.
The answer here is pretty simple – add more chlorine. If your free chlorine reading is below 1 ppm, your water isn’t being sanitised properly, and you’ll need to take action.
If the levels remain low when adding a dose of your usual chlorine tablets or granules, it’s a good idea to shock your pool. Chlorine shock introduces a much higher dose of chlorine into the water to increase the amount of free chlorine rapidly and eliminate the buildup of chloramines, essentially resetting your pool’s chlorine levels.
You can find out more about shock dosing in our guide on how to use swimming pool shock treatments.
You should check on your free chlorine levels around two or three times a week or daily if your pool experiences particularly heavy use. There are several methods for testing the chemical levels in your pool water, including liquid test kits, tablets and test strips.
Aquachek Yellow Free Chlorine test strips offer an easy and convenient way to check your water. You just have to dip the strips in the water, and you’ll get the results immediately.
Whatever chlorine products you need, we stock a range of sanitisers and cleaning chemicals to keep your pool sparkling.
