Pool Shock, Explained: When, How Much & Which Product
Updated September 2026
Fast answer: Shocking is just adding enough chlorine to overwhelm whatever's in the water. For routine cleanup that means lifting free chlorine to about 10–12 ppm. For algae it's a lot higher, and the number is tied to your CYA. Shock after heavy use, storms and long sunny heatwaves, or any time combined chlorine reads above 0.5 ppm. Do it at dusk with the pump running, and once your stabilizer is where you want it, use liquid chlorine or cal-hypo instead of dichlor.
Shock is a dose, not a product
There's nothing special in the bag. Any chlorine pushed high enough shocks a pool, and the retail packets labeled "shock" are cal-hypo or dichlor in single-dose packaging with a louder label. What matters is the concentration you hit: around 10 ppm for routine oxidation, and free chlorine at 30–40% of your CYA reading when you're killing algae.
You know the smell of a badly kept pool. Burnt hair, red eyes, everyone blaming "too much chlorine." It's the opposite. That's combined chlorine, chlorine that's latched onto sweat and other swimmer waste, where it irritates everybody and sanitizes almost nothing. A shock breaks those compounds apart. If your kit shows combined chlorine over 0.5 ppm, the pool wants a shock whatever the calendar says.
Liquid, cal-hypo or dichlor
Liquid chlorine is the cleanest shock you can buy. It works fast and leaves nothing behind but a little salt, so it suits routine shocks and algae kills equally well. Cal-hypo packs more chlorine per pound and stores easily, but every dose leaves calcium in the water. Dichlor leaves stabilizer, nearly a point of CYA for every point of chlorine.
Here's what that looks like over a summer. Say you run a 15,000-gallon pool and shock every Sunday, taking free chlorine from 2 up to 10. With 10% liquid that's about 1.2 gallons a week. With cal-hypo it's roughly a pound and a half, and each dose leaves around 6 ppm of calcium hardness behind. Twenty Sundays later you've added more than 100 ppm. On soft fill water, nobody notices. In Phoenix, where the hose might already deliver 300, you're buying scale. Run the same season on dichlor and every one of those shocks drags in about 7 ppm of CYA. Ten of them and a pool that opened at 40 is past 100, and now chlorine barely does its job. That's why dichlor is an early-season product, for when you're building stabilizer on purpose.
Chlorine-free shock (MPS) has its uses, but routine sanitizing isn't one. It oxidizes swimmer waste and kills nothing. Whatever product you choose, our calculator scales the dose to your exact gallons. Add half to two-thirds of it, let it circulate, and retest before topping up.
Timing, and the rule people get backwards
Shock at dusk. Sunlight destroys unstabilized chlorine within hours, so a 9am shock has lost half its punch by lunch while an evening shock gets the whole night to work. Keep the pump on, always. Shock sitting in a dead corner sanitizes nothing and can bleach a liner.
Pre-dissolve granular products in a bucket of pool water for vinyl and fiberglass. Pour liquid slowly around the perimeter, in front of the returns if you can. And never mix products. Cal-hypo meeting trichlor in a bucket or a skimmer can start a fire, and that's not a figure of speech.
How you know it worked
Retest the next morning. After a routine shock you're looking for combined chlorine back at or near zero and free chlorine heading down toward normal. Swim once it's under 5 ppm, usually the next day. If free chlorine collapsed overnight instead of easing down, something in the water is still eating it, and you're past routine. Treat it as the start of algae and go to the kill-level doses in our shock tables.
FAQ
How often should I shock my pool?+
Skip the fixed schedule and shock on triggers: combined chlorine over 0.5 ppm, after a party or a storm, at the first sign of algae, and during long heatwaves. A covered, well-balanced pool may need it once a month. A busy summer pool, weekly.
How long after shocking can you swim?+
When free chlorine drops back below about 5 ppm, which is usually 8 to 24 hours after a routine shock. Test rather than guess. After a heavy algae shock it can take several days.
Can I shock a pool during the day?+
You can, you'll just waste a lot of it. UV breaks down unstabilized chlorine within hours. Shocking at dusk gives the dose a full night to work, and it's the easiest efficiency win in pool care.
How do I shock a saltwater pool?+
Add liquid chlorine directly. The cell's boost or superchlorinate mode works too, but it takes a day or more to raise chlorine by a few ppm, which is too slow for a real shock. Pouring the chlorine in gets you to target in an hour.