Project Guide

How to Shock a Pool

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What is a Pool Shock?
A person opening a package of pool chemicals.

"Shocking” refers to the process of adding chlorine or non-chlorine chemicals to your pool in order to raise the "free chlorine” level. The goal is to raise it to a point where contaminants such as algae, chloramines and bacteria are destroyed. A pool that smells strongly of pool chlorine doesn't mean that it's clean—a clean pool will be almost order-free. Strong chlorine smell is a sign of improperly treated water. The odor actually comes from chloramines, also known as combined chlorine. Chloramines form when the chlorine in the pool mixes with the nitrogen in sweat, oils and urine. However, smell alone does not dictate when you should shock your pool. In general, you should shock your pool when:

  • Algae begins to grow in your pool
  • The free chlorine level of your pool measures zero
  • The chloramines or combined chlorine level rises above 0.5 ppm

Types of Chlorine
A person holding a pool water tester kit.

Before you shock the pool, it's vital to understand the difference between the types of chlorine. 

  • Free Chlorine (FC): The amount of chlorine actively disinfecting the water. Proper free chlorine levels are between 1 and 3 parts per million (ppm).
  • Combined Chlorine (CC): The chlorine that’s been used. It will still be in the water but has a lower sanitizing power. Keep the CC level to less than 0.2 ppm.
  • Total Chlorine (TC) is the sum of FC and CC in your pool.
  • Breakpoint Chlorination: When there's enough FC to shatter the molecular bonds of chloramine. 

How to Shock the Pool
A man pouring pool shook into a pool.

The goal is to raise the free chlorine level of the pool water to roughly 10 times the combined chlorine level of the pool water. Reaching this mark is the breakpoint chlorination. Shocking the pool should be done at dusk. The sun will burn off unstabilized chlorine. Shocking your pool at night makes sure the chemicals work properly.

Here's how to shock a pool:

  • Test the water. You will need to check the pH level in your pool. Water testing kits can measure the FC and TC of your water. To find the CC of your pool, you will need to perform a simple calculation—subtract the FC from the TC. This will let you know how much chlorine you need to add to the pool. 
  • Calculate the amount of shock you will need to mix (refer to the dosage amount indicated on the package of the shock chemical you plan to use.)
  • Prepare shock ahead of time. This will save you time and help prevent calculation mistakes. Pre-mix the pool shock in a five-gallon bucket.
  • Ensure the pool pump is running.
  • Pour the mixture into the water, around the edges of the pool.
  • Let the pump run for about six hours or more.
  • Test the water to ensure proper chlorine levels.
  • Don’t use the pool right away. Wait for the free chlorine levels to drop to 1 – 3 ppm.

Tip: You need to reach the breakpoint every time you shock your pool. Not hitting the breakpoint can result in even more chloramines in your pool.

Types of Pool Shock
Pool shock chemicals on a white background.

There are several types of products you can use to shock the pool. Typically, you will not use regular chlorine tablets as pool shock chlorine.

Calcium Hypochlorite: Also referred to as cal hypo, this chemical is one of the most inexpensive and convenient ways to shock your pool. It's usually sold in granular form. 

  • Needs to be dissolved before you add it to the pool.
  • Must be used after dusk.
  • Eight hours before you can safely swim again.
  • Adds about 0.8 ppm of calcium to your water for every ppm of FC added.

Lithium Hypochlorite: Unlike cal hypo, this chemical doesn't raise the calcium level of your pool water but does cost more.

  • Dissolves much more quickly
  • Can add it directly to the pool without dissolving it beforehand
  • Commercial versions contain 35% chlorine.
  • Must be used after dusk.
  • Eight hours before you can safely swim again.
  • Toxic to aquatic life.

Dichlor: Short for sodium dichloro-s-triazinetrione or dichloroisocyanuric acid. This chemical is the safest and easiest to use.

  • Contains between 50% and 60% chlorine.
  • Can be used for both regular chlorine doses and shock treatments.
  • Don’t have to dissolve it ahead of time.
  • Adds 0.9 ppm of cyanuric acid for every ppm of additional FC.
  • Must be used after dusk.
  • Eight hours before you can safely swim again.

Potassium peroxymonosulfate: An inexpensive non-chlorine shock. 

  • Add it directly to your pool water at any time.
  • It takes roughly 15 minutes before you can safely swim again.
  • Does not function as an algaecide.

Shocking Saltwater Pools
Kids jumping into a pool.

Saltwater pools need to be shocked too. Saltwater pools use regular table salt to create chlorine, by passing the slightly salty water over two electrically charged plates. How often you shock a saltwater pool is no different from other chlorine pools. Follow the basic steps as shocking a chlorine pool. Granular pool shock, such as Calcium Hypochlorite, can also be used in a saltwater pool. 

How Often to Shock Your Pool
A child in rain gear sitting at the edge of a pool.

Don’t wait for a bad smell or eye irritation before you shock your pool. It's recommended that you should do a pool shock once a week. The more you use the pool, the more often you need to shock it. Occasionally, you may need to perform an extra pool shock after:

  • Heavy pool use (like a pool party)
  • Severe rainstorm 
  • Major water level change
  • A bowel-related pool accident

A pool shock treatment is insurance against algae, bacteria and other contaminants infecting your pool water. Regularly shocking your pool will keep the water fresh and clear. Shop The Home Depot for everything you need to shock your pool and other pool cleaning supplies.