Does Chlorine Go Bad? Explaining Liquid Chlorine’s Strength Degradation

Of all the different types of chlorine, liquid chlorine, or bleach, is (in my opinion) the best form to use. It’s less expensive, and it is unstabilized, meaning it does not contain cyanuric acid (CYA).

However, liquid chlorine has a quick shelf life. In fact, it loses strength the second it leaves the manufacturing facility.

Understanding why liquid chlorine loses strength, knowing how to store it, and knowing what date it was bottled are all very useful pieces of information to keep it as strong as possible.

Why Liquid Chlorine Breaks Down

The simple fact is that bleach is a liquid (obviously), and liquids break down faster. The actual chlorine molecules float in water, allowing them to move, collide, and “react” with each other more easily than if they were in a solid, locked state.

Ironically, the stronger the liquid chlorine solution, the faster it dies. A 12.5% bleach degrades much faster than 10% because the higher density of molecules leads to more frequent internal reactions.

Estimates suggest that it can lose about 50% strength in 6 months and up to 90% in a year.

Besides time, two other items can speed up this natural breakdown:

• High Temperature. Heat provides energy to chlorine molecules, causing them to react faster.
• UV Rays. As we know, sunlight is the natural enemy of chlorine. This is why CYA is so important to help shield chlorine from UV rays when it’s in the pool.

Although there’s no way to completely slow down its degradation, there are some ways to make sure you are getting the most out of each bottle.

Know When Your Chlorine Was Manufactured

The good news is that every box or bottle of liquid chlorine will show the date it was manufactured.

The date itself will be a Julian date code. It’s a pretty common code found on many products to help simplify production dates.

This code uses a YYDDD format:

• YY is the last two digits of the year (e.g., 25 for 2025
• DDD is the numerical day of the year (e.g., 001 for January 1 and 365 for December 31)

In the example above, 24234 shows that this was bottled on the 234th day of 2024, or August 21, 2024.

Check the bottles before you buy and always buy the most recent date. Stores will often rotate the oldest stock to the front.

To make it easier, use our decoder below to find the manufacturing date:

Check How the Chlorine is Stored When You Buy It

Ideally, the chlorine you buy should be indoors in a cool, well-ventilated area. However, that isn’t always the case. Most big box stores store their chlorine in pallets in the garden section, which is outside.

This can degrade the chlorine much faster beyond just the manufacturing date. If the chlorine is exposed to the sun for extended periods, my advice would be to try to find a different location.

Keeping it Fresh!

It is extremely common to buy liquid chlorine in bulk due to its frequent use. To get the most out of each bottle, store it in a place that is cool, dry, and ventilated.

Also, there is nothing wrong with using older chlorine. You aren’t damaging anything by pouring in liquid chlorine that is one or two years old.

Just know that it isn’t nearly as strong as when you first bought it.