Pool Chlorine vs Shock: What’s The Difference?



The pool world is filled with chemistry terms that can make it seem a lot more complicated than it really is.

Chlorine and pool shock are great examples of that since the two substances are more alike than most pool owners realize.

Quick answer: Shock is just a higher concentration of chlorine designed to “reset the water” by removing stubborn or high levels of contamination. They’re essentially the same thing with slightly different use cases.

Shock = Lots of Chlorine

Pool shock, or shock treatment, is simply a high concentration of chlorine.

While you might find some products labeled as chlorine and others labeled as shock, they’re really both chlorine with different dosage requirements.

Basically, if you add enough chlorine to your water, you’re shocking your pool.

Where you might typically add 1-3 parts per million (ppm) of chlorine to your water as part of your daily maintenance routine, a shock treatment would contribute many times that — possibly up to 10x times the amount.

Note: There is a type of shock treatment that isn’t chlorine. We’ll get to that later.

Why Would You Need So Much Chlorine?

Where your normal/residual level of chlorine is usually enough to keep your water in check, high levels of contamination often call for a much heavier dose.

For example, when:

  • You find an animal in your pool (dead or alive)
  • Lots of leaves or other debris get into your water
  • There’s heavy rainfall and the pool is uncovered
  • Little Jimmy decides to pee in the water
  • You have a lot of people using the pool
  • You have a buildup of chloramines (used chlorine)
  • And the list goes on…

Scenarios like this lead to significant and/or unexpected levels of contamination that overwhelm your sanitizer, leaving your pool vulnerable to bacteria, algae, and other unfriendly visitors.

The goal of using a ton of chlorine (known as shocking) is to reach something called “breakpoint chlorination”, where all contaminants are oxidized and effectively removed from the water.

From a chemistry perspective, it’s like dropping a nuclear bomb on your water but without the fallout to follow.

Are There Any Other Differences?

In a sense, yes.

Even though chlorine and shock are one in the same, chlorine products marked as “shock treatment” tend to have certain characteristics.

These characteristics don’t make them any less chlorine, but they do make them more appropriate shock treatments.

1. Shock comes in a particular form

Since shocking requires such a high concentration of active sanitizer, it’s always in the form of granular or liquid chlorine because they dissolve and disperse more quickly throughout your pool.

Granular shock (dichlor, trichlor, or calcium hypochlorite) typically comes in 1 lb bags that are first dissolved in a bucket of water prior to adding it to the pool.

As a general rule, you would use 1 bag of granular shock per 10,000 gallons of water, though more severe issues may require a double or triple dose.

Liquid shock (sodium hypochlorite) typically comes in 1-gallon jugs, and can be poured directly into the water as needed since it comes pre-dissolved.

Tablets or pucks would never be considered shock treatments as they dissolve far too slowly to reach the required level for breakpoint chlorination.

2. Shock Generally Isn’t stabilized

Chlorine stabilizer, or cyanuric acid, protects free chlorine from breaking down under the sun’s UV light.

Without it, an outdoor pool can lose up to 90% of its chlorine in just 2 hours.

In fact, stabilizer is such an important chemical for residential pools that most chlorine products contain cyanuric acid right out of the box; giving it the name stabilized chlorine.

While this is incredibly useful for maintaining a residual sanitizer level, a shock treatment only needs to last a few hours (or left overnight) to get the job done.

This is why recommended chlorine shocks—such as calcium hypochlorite—come without cyanuric acid, or unstabilized, as it allows the chlorine level to drop back down with a little help from the sun.

Of course, this doesn’t discount the cyanuric acid already in your water, so that always has to be accounted for.

3. Shock is Often packaged differently

Yes, shock is just chlorine — but using it to maintain a residual chlorine level will likely present some challenges.

For example, when using a granular shock, the bags usually aren’t resealable as they’re intended to be single-use. While liquid shock comes in jugs with resealable caps, they’re often sold in larger quantities and aren’t suitable for long-term storage.

The packaging also won’t provide instructions for regular daily use as they’re meant to spike the chlorine level far beyond a residual level.

Overall, while it is possible to use shock in place of other chlorine products, it’s not nearly as convenient due to how shock is marketed and sold.

A Caveat: Non-Chlorine Shock

There’s always an exception to the rule, and this is it.

Non-chlorine shock, or monopersulfate (MPS), is the only shock treatment that isn’t chlorine.

While chlorine is both an oxidizer and a sanitizer, MPS is only an oxidizer, meaning it’s unable to kill living organics like bacteria, viruses, or algae.

Instead, it breaks down non-living organic material such as sweat, saliva, urine, etc. In doing so, it alleviates stress on your chlorine and allows your residual chlorine level to focus on more serious contamination in the water.

If chlorine shock is a nuclear bomb for your pool, non-chlorine shock is more like a tactical air strike.

Overall, despite being called shock, it works in a fundamentally different way from traditional chlorine shock and certainly doesn’t replace it.

The Bottom Line

When it comes to pool chlorine or shock, the main difference is in their strength and application.

They’re intended to be used together, and if done correctly, you’ll quickly find out why they’re a great one-two punch for restoring your water to a pristine state.

In short: they’re allies, not rivals.

Categories: Pool Care, Pool Chemistry