How Often Should I Shock My Pool?
By The Aqualitx team at STEMania · Updated
The bottle says "shock weekly," but a pool that holds its free chlorine at the right level every day rarely needs a shock at all. Shocking is a fix for a specific problem, not a routine chore. Here's when it's actually worth doing.
Shock when one of these is true
- Combined chlorine is above 0.5 ppm. Chloramines are building faster than your daily chlorine clears them. Shock to about 10 times the CC reading.
- You see algae — green, yellow or black spots, slick walls, or cloudy water that won't clear.
- After a big load: a pool party, a heavy storm that washed debris in, or a week of heat and heavy use.
- After an accident: fecal contamination calls for a high chlorine level held for a set time; follow your local health department's guidance.
- Opening the pool after winter.
When you don't need to
If free chlorine stays at or above 7.5% of your CYA every day and combined chlorine stays at or under 0.5 ppm, the pool is already doing what a shock would do. A weekly shock on top of that mostly burns money. Test two or three times a week and let the numbers decide.
Why some pools seem to need it more
- Low CYA: the sun burns chlorine off before it can work. See ideal CYA levels.
- Chlorine that sags between doses: tabs that ran out, or a salt cell turned down too low.
- Lots of swimmers or sunscreen, which create combined chlorine.
- Phosphates or nitrates feeding algae — see phosphates in pool water.
If you find yourself shocking every week to stay clear, fix the underlying cause instead — usually holding free chlorine higher day to day.
How much each time
For a routine shock, raise free chlorine by about 10 ppm: roughly 10 fl oz of 12.5% liquid chlorine, or 2 oz of 65% cal-hypo, per ppm per 10,000 gallons. For combined chlorine, aim for 10 times the CC reading. See how much shock does my pool need for the full method.