Free Chlorine Level Low - How To Raise It?

Pool Free Chlorine Low - How To Raise It?

Everyone knows that chlorine is essential for keeping your pool clean, but did you know there are actually multiple types of chlorine?

Free chlorine is the one primarily responsible for sanitising your pool, making it crucial that its levels are kept in check. If your free chlorine is low, you should take steps to raise it as soon as you can.

In this article we will discuss why it’s bad for your free chlorine to be low, the ideal concentration of free chlorine, causes of low free chlorine levels, and how to increase free chlorine in your pool.

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Low Free Chlorine: Why Is It Bad?

Free chlorine in your pool is the only type of chlorine that is available to attack any contaminants, especially living ones such as bacteria and algae.

Other types of chlorine are also present in your pool, but these forms have already bonded with other contaminants, leaving them inactive for sanitation purposes.

Free chlorine (ie. chlorine in the form of hypochlorous acid or hypochlorite) is the sanitising agent in your pool. Without enough of it, microorganisms in your pool’s water will begin to flourish and reproduce, creating unsanitary conditions and leading to algae outbreaks.

If you do have low chlorine, you will notice that your ORP level drops accordingly, which is another sign of an unsanitary pool.

Ideal Concentrations Of Free Chlorine For Swimming Pools

Free chlorine in swimming pools should be kept between 2-4 parts per million (ppm). Above these levels, chlorine can cause irritation to swimmers and lead to other problems, while below these levels, unsanitary conditions will occur that make the pool unsuitable for swimming.

Causes Of Low Free Chlorine In Pools

Not Adding Enough Chlorine

The primary cause of low free chlorine in pools is not adding enough chlorine relative to your pool’s chlorine demand.

Even pools of the same volume can have quite different chlorine demands.

All organic contaminants that are introduced to your pool use up free chlorine. The more swimmers use your pool and the more leaves, twigs and grass clippings that blow into your pool, the greater your chlorine demand.

It’s critical that you maintain enough chlorine to keep free chlorine within 2-4 ppm. This will mean adding more chlorine for some pools than others.

Use the chlorine calculator to work out how much to add to your pool.

Insufficient Cyanuric Acid

Free chlorine degrades in sunlight naturally, and this happens especially quickly if there is not enough stabiliser present.

Essentially, a stabiliser is an essential component of your pool’s chemistry that prevents your free chlorine from degrading rapidly in the sunlight. If you are having a hard time maintaining normal amounts of free chlorine in your pool despite adding chlorine regularly, you may have low stabiliser.

If you are unfamiliar with stabiliser, also known as cyanuric acid, you can learn everything you need to know about the important role that stabiliser has on maintaining free chlorine molecules in our article here.

How To Increase Free Chlorine In Your Pool

#1 - Choose A Chlorine Product

When choosing what type of chlorine to add to your pool, it’s important that you think about cyanuric acid at the same time.

Whilst many will tell you to use stabilised forms of chlorine in your swimming pool because they contain cyanuric acid which limits UV degradation of your chlorine, I actually don’t recommend this.

Long term use of stabilised chlorine leads to excessively high cyanuric acid, which ultimately renders your chlorine ineffective. Add stabiliser (cyanuric acid) separately to chlorine for easier, independent control of these parameters.

I recommend either sodium hypochlorite (liquid) or calcium hypochlorite (solid). These are both good choices that will lift your chlorine without impacting your cyanuric acid concentration.

#2 - Test Your Free Chlorine Levels

Immediately before attempting to add any chlorine to your pool, it is important to test your free chlorine levels. Free chlorine can be measured using test strips or liquid test kits, just be sure that you are measuring for free chlorine rather than combined or total chlorine.

#3 - Calculate An Ideal Dose For Your Pool

To calculate the correct dose of chlorine for your pool, you will need to know your pool’s current concentration of free chlorine as well as the volume of your pool in litres.

Dosing information is typically included when purchasing your chlorine product, but you can also use our handy pool chlorine calculator.

Before using this calculator, you should locate the concentration of available chlorine in the product you have chosen, usually found on the packaging.

#4 - Add The Chlorine

After calculating your ideal dose of chlorine, add the product to your pool.

Granulated chlorine can typically be added directly to your pool’s water without premixing. To do so, broadcast this product across the surface of your pool while the pump is running.

Avoid dumping large amounts of granules in one location, as these could settle on the bottom of the pool before dissolving.

#5 - Retest Your Free Chlorine Levels

After adding your choice of chlorine product, run your pump for 12-24 hours before retesting your pool’s water. The longer end of this waiting period should be observed if using pucks or tablets, unless one of these was pre-crushed for faster dissolution.

Ideally, your pool’s free chlorine levels will now fall within the ideal range of 2-4ppm. If your chlorine levels are still low, you may need to add more chlorine product.


If your pool chlorine is low, you should take steps to add more free chlorine. By choosing the right product and calculating an ideal dose for your pool, you can return your pool to ideal chlorine levels before problems develop.

Do you have any questions about why free chlorine is low in your pool or what you can do to raise it? Leave us a comment down below, we’d love to help you out!

Louis from Pool Advisor


A chemical engineer by trade, Louis is committed to debunking myths in the pool industry by explaining the underlying chemistry and making it accessible to all.