How to shock a pool: step-by-step (2026)

How to shock a pool: step-by-step guide (2026)

Shocking means raising free chlorine to roughly 10 times your CYA level so it kills algae, bacteria, and combined chloramines. For most pools that is 1 pound of cal-hypo per 10,000 gallons or a gallon of 12.5 percent liquid chlorine per 10,000 gallons, added at dusk with the pump running.

2 min read · Updated May 23, 2026

When to shock

Shock when one of these is true:

Free chlorine is at or near zero
The pool smells strongly like chlorine (that smell is combined chloramines, not free chlorine)
The water is cloudy and not clearing with normal chlorine
After heavy rain, a storm, or a big party
Anytime algae is visible

You do not need to shock weekly as a habit. That is an old recommendation that wastes money and over-stabilizes the water if you use cal-hypo or dichlor.

What to use

Shock type	Pros	Cons
Liquid chlorine (12.5%)	Fast, no CYA added, no calcium added	Heavy to carry, short shelf life
Cal-hypo (calcium hypochlorite)	Strong, cheap	Adds calcium, can cloud water
Dichlor	Easy to dose	Adds CYA every time
Non-chlorine (MPS)	Swim sooner	Will not kill algae

For most owners, liquid chlorine is the right answer. For vinyl liner pools, always pre-dissolve cal-hypo in a bucket first.

Step-by-step

Test your water. You need to know your CYA level.
Calculate the target. SLAM (shock level and maintain) is roughly 10x CYA. So 30 ppm CYA = 30 ppm free chlorine target.
Balance pH first. Get pH between 7.2 and 7.4 before you shock. High pH cuts chlorine effectiveness.
Add the shock at dusk with the pump running.
Brush the walls and floor.
Run the pump for 24 hours minimum.
Retest in the morning. Repeat until chlorine holds overnight (loses less than 1 ppm) and water is clear.

How much for my pool size

For a 20,000 gallon pool with CYA around 30 ppm:

Liquid chlorine (12.5 percent): about 2 gallons
Cal-hypo (65 to 73 percent): about 2 pounds
Dichlor: about 1.5 pounds (only if you have room for more CYA)

Use a calculator like Pool Math (the Trouble Free Pool app) for exact doses.

Common shocking mistakes

Shocking with high CYA. If your CYA is 100, you need 100 ppm chlorine to actually shock. At that point, drain water first.
Shocking at noon. UV burns off most of it before it works.
Shocking and immediately adding tablets. Tablets keep raising CYA and you fight the same battle next week.
Adding shock through the skimmer with a tablet feeder downstream. You will damage the feeder.

When to call a pro

If you cannot hold chlorine overnight after three rounds of shock, or if the water has a yellow or pink tint that will not clear, you may have mustard algae, pink slime, or a high phosphate problem. A pool pro can drop in the right algaecide and save you days.

Frequently asked questions

Can I swim after shocking?

Wait until free chlorine drops back below 5 ppm. With the pump running that is usually 8 to 24 hours. Test before anyone gets in.

How often should I shock my pool?

Only when needed: zero chlorine, visible algae, after storms, after parties, or if the pool smells strongly like chlorine. A well-balanced pool with a working chlorine source does not need a weekly shock.

What is the difference between liquid chlorine and bleach?

Pool liquid chlorine is 10 to 12.5 percent sodium hypochlorite. Household bleach is 6 to 8.25 percent. Same chemical, different concentration. Plain unscented household bleach works in a pinch, you just need more of it.

Why is my pool still cloudy after shocking?

Three usual reasons: pH is too high, your filter is dirty or undersized, or you did not run the pump long enough. Clean the filter, check pH, run the pump 24 hours.

Should I shock at night?

Yes. Sunlight destroys chlorine fast. Adding shock at dusk gives it all night to work before UV cuts in.

Can I shock a saltwater pool?

Yes, the same way. The salt cell makes chlorine for daily use, but it cannot keep up with a real demand. Shock with liquid chlorine or cal-hypo just like any other pool.

Do I need to brush after shocking?

Yes. Brushing the walls and floor knocks dead algae into suspension so the filter can grab it. Skipping this step is the most common reason a shocked pool stays cloudy.