How Much Chlorine First Time

How Much Chlorine To Add To Pool For The First Time (When Opening)?

One of the most important things to do when setting up a pool is the first dose of chlorine you will need to prepare your water for swimming.

Depending on the volume of your pool, differing amounts of chlorine will be needed, but one thing is certain - your fill water will have low chlorine and you will need to add more.

If you are a new pool owner, you may also need help choosing the right type of chlorine, because not all chlorine is the same.

Read on to learn more about the ideal concentration of chlorine in pools, what type of chlorine to buy for your new pool, and how much chlorine you will need to achieve these levels when starting from fresh water.

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Ideal Chlorine Concentration In Pools

The ideal free chlorine concentration in your swimming pool is 2-4 parts per million (ppm), as this is typically enough to achieve effective sanitation. To test your water, you will need a test kit that is capable of measuring free chlorine.

The term free chlorine refers to chlorine particles that are not combined with other chemicals in your water, which allows them to maintain their active disinfectant properties.

What Type Of Chlorine To Add

If you are opening an outdoor pool, you will want to add cyanuric acid (aka stabiliser) in addition to your chlorine. Cyanuric acid helps protect your chlorine from being broken down by the Sun’s UV rays and becoming ineffective.

This can be done by either adding stabiliser separately, or by adding what is known as stabilised chlorine, which is chlorine with stabiliser built into it. Stabilised chlorine is identifiable by the chemical names dichlor (dichloroisocyanurate) and trichlor (trichloroisocyanuric acid).

It is important not to use chlorine products that are intended as shock treatments as the first chlorine dose for your pool, especially non-chlorine based shocks, as these will not provide you with an ongoing free chlorine level.

Use this guide to help decide whether liquid or granular chlorine is best for your pool.

How Much Chlorine To Add To Your Pool

Whether you use stabilised chlorine, or unstabilised chlorine and separately dose stabiliser, you will need to take note of the available chlorine concentration in your product. For solid or granular chlorine products, this is written on the packaging.

You can then use this pool chlorine calculator to work out the appropriate dose for your pool.

As an example, for a 50,000 L pool using a calcium hypochlorite product with an available chlorine concentration of 700 g/kg, you would need to dose 210 g to go from 0 ppm to 3 ppm.

Bear in mind that some chlorine tablets dissolve quite slowly and may take a while to register on your free chlorine tests.

Unfortunately, trial and error is a necessary part of achieving the correct startup levels of free chlorine in your pool. Part of this variance between pools is due to unpredictable amounts of microscopic debris and bacteria in your fill water.

Because chlorine is actually used up as it works to neutralise and remove these contaminants, pools of similar volume may require different amounts. The sooner you can add chlorine to your pool after filling it, the better.

Whether you are starting up a new pool, or simply restarting your pool after closing it for winter, you will need to add chlorine on opening. I hope this article has helped you figure out how much to add!

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.