Pool-chemical safety: Never mix shock with other pool chemicals, including other kinds of chlorine. Clorox’s Shock Plus label says not to mix it with other shock products or pool chemicals “as violent reactions could occur,” and CDC’s chemical-use poster says to pre-dissolve a pool chemical only when the label tells you to, then add the chemical to water and never water to the chemical.

To shock a pool, test and balance the water, find the dose for your pool size on the shock’s own label, add it in the evening with the pump running, and keep everyone out until free chlorine tests back at 1–4 ppm. Every shock label below gives its own amount per gallon, so the label on your bag or jug is the only dose to use.

What “shocking” means

EPA treats “shock” as a label claim. Its guidance says “shock treatment and super-chlorination are terms usually used to describe claims to kill or control visible algae growth,” so a product sold as shock for algae or bacteria has to be EPA-registered.

The MAHC annex describes a shock dose as “high concentrations of CHLORINE” used “when BATHERS are not present,” and says “periodic shock dosing can be an effective tool to maintain microbial quality of water and to minimize build-up of biofilms and inorganic chloramines.” Pentair’s IntelliChlor manual gives a working definition: the water “can be considered super chlorinated” when free chlorine “reaches ten times the amount of combined chlorine.”

Step 1: Test and balance first

Test free chlorine, combined chlorine, pH and alkalinity before you shock. HTH’s Shock Advanced directions say to “test pool water and adjust total alkalinity and pH before application.” Clorox’s Shock Plus label says to adjust pH to 7.2–7.6, and In The Swim gives the same pH range for its cal hypo.

If you use stabilizer, test it too. Dichlor shock adds cyanuric acid (CYA); cal hypo and liquid chlorine don’t. The pool chemistry chart lists the published ranges, and the pool shock comparison helps you pick a shock for your CYA level.

Step 2: Read the dose for your gallons

Know your pool’s volume in gallons, then find the matching amount on the label. These are the makers’ own directions:

Shock Label dose Key label notes
HTH Shock Advanced (cal hypo) 1 lb bag per 13,500 gal Use the whole bag; not in a floater or feeder
In The Swim 68% Cal-Hypo 1 bag per 10,000 gal, for 5–10 ppm Vinyl pools pre-dissolve in a bucket of water
Clorox Shock Plus (dichlor) 1 lb per 12,000 gal; 13.4 oz bag per 10,000 gal Don’t pre-dissolve; not through skimmer or feeder
HTH Liquid Chlorine (10%; search) Per the container HTH’s chart is for maintenance dosing

Clorox’s EPA-accepted master label (registration 67262-27) pairs each package size with its own gallon figure, so check which size you have. HTH and In The Swim say to use the entire bag once it is opened.

Don’t apply one brand’s number to another product. The active ingredient and strength differ: HTH lists 56.44% calcium hypochlorite, In The Swim 68%, and Clorox’s label 58.2% sodium dichloro-s-triazinetrione.

Step 3: Pick the time and set up the equipment

Shock in the evening. HTH and In The Swim both direct adding their cal hypo during evening hours with the filter pump running. HTH says to add its liquid chlorine “in the evening to minimize chlorine loss from sunlight,” and one version of Clorox’s label says “use at night to prevent sunlight degradation.”

Keep the pump running. Clorox’s label says to operate the pump and filter continuously for 4 to 8 hours after adding Shock Plus.

Take shock out of feeders. Clorox’s label says not to add Shock Plus “through skimmer or chemical feeding device as other chemicals may be present which could result in a violent reaction.” HTH says not to use Shock Advanced in a floater or feeder. If you run a tablet feeder, Pentair’s Rainbow 320 guide says to “turn off Feeder OR remove tablets until the shock or Algae treatment is complete and all granules have dissolved,” warning that granules mixing in the feeder could cause “FIRE AND/OR EXPLOSION.”

Step 4: Add the shock

  • Granules: HTH says to broadcast evenly over a wide area, “preferably in the deepest part,” and to brush any granules that settle, because “undissolved granules may bleach or damage pool surfaces.” Clorox says to broadcast in the deepest area and not to pre-dissolve.
  • Vinyl pools with cal hypo: In The Swim says “vinyl pools should pre-dissolve Calcium Hypochlorite into a bucket of water.” Add the granules to the water, never water to the granules.
  • Liquid: HTH says to pour slowly close to the water’s surface, “preferably near a return inlet,” with the pump running.

Leave pH adjustments for later. HTH’s pH Down directions say to wait 4–6 hours after a shock treatment before adding it, “to avoid off-gassing.”

When it’s safe to swim

Test before anyone gets in. The labels give the same re-entry range:

  • HTH Shock Advanced: “Do not enter the pool until free available chlorine levels return to 1–4 ppm.”
  • In The Swim cal hypo: don’t enter until the free available chlorine residual is 1–4 ppm.
  • HTH Liquid Chlorine: don’t enter until chlorine is back to 1–4 ppm.
  • Clorox Shock Plus: wait at least 15 minutes, then “enter when chlorine level is 1-4 ppm.” The label adds: “Re-entry into treated swimming pools is prohibited above levels of 4 ppm chlorine due to risk of bodily injury.”

The MAHC annex explains the ceiling: “EPA has not approved any POOL product use above 4 ppm (mg/L) DPD-FC” when swimmers are present.

Watch for a false low reading. CDC warns that above 10 ppm a DPD test “might partially or totally bleach out, resulting in a false low or 0 ppm chlorine result,” so a zero right after shocking doesn’t prove the water is ready. The test kit list covers kits that read free chlorine.

How often to shock

  • HTH Shock Advanced: “shock the pool at least once per week.”
  • Clorox Shock Plus: “shock pool weekly and also after heavy use, high temperatures, or rain.”
  • HTH Liquid Chlorine: shock once a week as a preventive treatment, with more “when water appears hazy or dull, after heavy swimmer use, or following heavy rainstorms.”

For public pools, the MAHC lists superchlorination among the actions to take when combined chlorine exceeds 0.4 ppm. For when to shock and when to rely on tablets, see the shock versus tablets guide .

Shocking a salt pool

Salt chlorine generators have a built-in high-output mode:

  • Hayward AquaRite: the Super Chlorinate switch “electronically ‘super chlorinates’ (shocks) the water for 24 hours (filter pump must be on during this time).” Hayward says to put the switch back to AUTO afterward.
  • Pentair IntelliChlor: Boost Mode raises cell output “to 100% for the next 24 hours.”

The MAHC annex notes that in saltwater pools “a separate chlorinating product might be needed” for a high enough shock level. Pentair says that on initial startup, “it is best to super chlorinate using a shock treatment purchased at your local pool supplier,” then wait until free chlorine falls to 2–4 ppm before running the generator. Pentair also warns against using trichlor or dichlor with its generator, so check the shock’s chemistry. The salt chlorine generator list covers each system.

Sources

Facts were checked against these sources on October 9, 2026. This guide summarizes published guidance and labels; it is not based on hands-on testing.

FAQ

How much shock do I need for my pool?

Use the amount on your product’s label for your pool’s gallons. For example, HTH Shock Advanced says one 1 lb bag per 13,500 gallons, In The Swim’s 68% cal hypo says one bag per 10,000 gallons, and Clorox Shock Plus’s EPA label says 1 lb per 12,000 gallons. Don’t use one brand’s number for another.

When is it safe to swim after shocking a pool?

When a test shows free chlorine back in the 1 to 4 ppm range the labels give. Clorox Shock Plus also says to wait at least 15 minutes, and its label prohibits re-entry above 4 ppm chlorine.

Should I shock my pool at night?

The labels say so. HTH and In The Swim direct adding shock in the evening with the pump running, HTH says evening dosing of its liquid chlorine minimizes loss from sunlight, and one version of Clorox’s label says to use it at night.

How often should I shock my pool?

HTH says at least once a week, and Clorox’s label says weekly and also after heavy use, high temperatures or rain. HTH adds that liquid chlorine shocks may be needed when water looks hazy or dull.

Can I put shock in my skimmer or chlorine feeder?

No. Clorox’s label says not to apply Shock Plus through a skimmer or feeder because a violent reaction could result, and HTH says not to use Shock Advanced in a floater or feeder.