What Time Of Day To Add Chlorine?

What's The Best Time Of Day To Add Chlorine To My Pool?

The concentration of chlorine in your pool decreases over time, due to reaction with organic material, plus breakdown from the sun’s UV rays, leaving it prone to unsanitary conditions.

Because of this, adding chlorine to your traditional chlorine pool is a regular part of maintenance.

Chlorine can technically be added to your pool during any time of the day, but did you know that there is actually an ideal time of day to add chlorine to your pool in order for it to be the most effective?

Read on to learn more about the best time of day to add chlorine to your pool, as well as the reasons why adding chlorine to your pool at this time of day is the most effective for sanitization and for your budget.

What Is The Best Time Of Day To Add Chlorine?

Technically speaking, the best time of day to add chlorine to your pool is in the evening, ideally shortly after sunset. This is because chlorine will break down in sunlight, even with the help of stabilising agents like cyanuric acid.

When you add chlorine to your pool after sunset, you are allowing your chlorine product to react with the algae, bacteria and other organic material in the pool for a good 12 hours before once again being exposed to sunlight.

This length of time may not seem like much, but it is plenty of time to allow your chlorine to effectively kill off billions of microorganisms that may be living in your pool’s water.

When chlorine neutralises a microorganism like bacteria or algae, it becomes ineffective. Because it also loses its effectiveness from exposure to sunlight, it’s important to allow time for your chlorine to react with harmful contaminants before it dissipates.

Does It Really Matter What Time I Add Chlorine?

In saying the above, the reality is that if your cyanuric acid concentration is within the recommended range (30 - 50 ppm), the loss of chlorine to UV breakdown is so small that it’s not worth worrying about it.

If morning or lunch time is the only chance you get to add chlorine to your pool - go for it! You will hardly notice any difference compared to adding it in the evening.

While the evening is technically the best time to add chlorine, in reality, it’s more important that you actually add it regularly, rather than spend too much time optimising exactly when you add it to your pool.

With that said, if you do have low cyanuric acid in your pool, correct your stabiliser level before adding chlorine during the day.

Use this cyanuric acid calculator to work out the correct dose of stabiliser.


Adding your chlorine to your pool in the late evening is technically the best way to get the most out of your sanitiser.

In practicality, it doesn’t really matter as long as you have sufficient stabiliser in your pool.

Use this chlorine calculator to work out the correct dose of chlorine for your pool.

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.