How to Shock a Pool
Shocking a pool clears combined chlorine, algae, and contaminants. Here's when to shock, which shock to use, and how to do it safely.
Updated June 2026
Quick answer
Shock a pool by balancing pH first, then adding the right dose of cal-hypo or liquid chlorine to circulating water — at dusk, so the sun doesn't waste it. Shock after heavy use, storms, or whenever the water smells of chlorine or looks dull, and wait until free chlorine falls back to 1–3 ppm before swimming.
When to shock
- On a regular cadence — roughly weekly in swimming season.
- After heavy use, a party, or a rainstorm that dumps in contaminants.
- When the water smells strongly of chlorine — that odor is combined chlorine (chloramines), a sign you need to shock, not that there's too much.
- At the first hint of algae or when water turns dull or cloudy.
Which shock to use
Cal-hypo (calcium hypochlorite) is a strong, economical granular shock — it adds a little calcium, so keep an eye on hardness. Liquid chlorine (sodium hypochlorite) shocks without adding calcium or stabilizer and is easy to pour. Avoid relying on stabilized shocks (dichlor) outdoors if your CYA is already high, since they push it higher.
How to shock, step by step
- 1
Test and balance pH
Shock works best with pH around 7.2–7.6. Adjust before adding it.
- 2
Shock at dusk
Sunlight burns through free chlorine fast — evening shocking lets it work overnight.
- 3
Measure for your volume
Dose to your pool's gallons and the product's strength; more isn't better.
- 4
Add to circulating water
With the pump running, add the shock (pre-dissolve granular cal-hypo if the label says to) and follow the directions.
- 5
Keep the pump running
Circulate for several hours, ideally overnight, to distribute and work through the contaminant load.
- 6
Re-test before swimming
Wait until free chlorine drops back to 1–3 ppm.
Never mix different shock products together or add them through the skimmer with other chemicals present. Aquavail calculates the shock dose for your pool and tells you when it's safe to swim again.
Common questions
How long after shocking a pool can you swim?
Wait until free chlorine falls back to the normal 1–3 ppm range — typically a few hours to overnight after a full shock, depending on the dose and sunlight. Always re-test rather than going by time alone.
Should you shock a pool at night or during the day?
At night, or at dusk. Sunlight rapidly breaks down unstabilized chlorine, so shocking in the evening lets it work overnight instead of being burned off — you get far more out of the same dose.
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