Pool Shock, Explained: When, How Much & Which Product
Updated September 2026
Quick answer: Shocking means raising free chlorine high enough to oxidize contaminants and kill anything growing — routinely 10–12 ppm, or much higher for algae. Do it after heavy use, storms, strong sun weeks, or whenever combined chlorine passes 0.5 ppm. Add it at dusk (sun burns off unstabilized shock), with the pump running, and use liquid chlorine or cal-hypo — not stabilized dichlor — once your CYA is where it belongs.
What shock actually is
Shock isn't a special product — it's a dose size. Any chlorine source pushed high enough shocks a pool; the retail bags labeled 'shock' are just cal-hypo or dichlor in single-dose packaging. What matters is hitting the target concentration for your situation: ~10 ppm for routine oxidation, chlorine at 30–40% of your CYA level for an algae kill.
The 'burnt hair and red eyes' smell of a badly kept pool is combined chlorine — chlorine locked up with sweat and other bather waste, irritating and nearly useless as sanitizer. Shocking breaks those compounds apart. If your test kit shows combined chlorine above 0.5 ppm, the pool is asking for a shock regardless of the calendar.
Choosing the product
Liquid chlorine is the cleanest shock: fast, no added calcium or stabilizer, ideal for routine and algae shocks alike. Cal-hypo is stronger per pound and cheap, but every shock adds calcium — fine for soft-water fills, a slow problem in hard-water regions. Dichlor adds nearly a point of CYA per point of chlorine, which makes it the right choice only early season when you're building stabilizer anyway.
Skip 'chlorine-free shock' (MPS) for routine use unless you have a specific reason — it oxidizes bather waste but kills nothing. Whatever you use, the dosing chart on our calculator scales it to your exact gallons; add ½–⅔, let it circulate, and retest before topping up.
Timing and technique
Shock at dusk. Sunlight destroys unstabilized chlorine within hours, so a morning shock wastes half its power by noon; an evening shock works all night. Pump on, always — shock sitting in a dead corner sanitizes nothing and can bleach liners.
Pre-dissolve granular products in a bucket for vinyl and fiberglass pools, pour liquid slowly around the perimeter, and never mix products — cal-hypo meeting trichlor in a bucket or skimmer can ignite. Swim again when chlorine falls back under 5 ppm, typically the next day for a routine shock.
FAQ
How often should I shock my pool?+
There's no fixed schedule — shock on triggers: combined chlorine over 0.5 ppm, after parties or storms, after any algae sighting, and during sustained heatwaves. A well-balanced covered pool may need it monthly; a busy summer pool weekly.
How long after shocking can you swim?+
When free chlorine drops back below ~5 ppm — usually 8–24 hours after a routine shock. Test, don't guess; after a heavy algae shock it can take several days.
Can I shock a pool during the day?+
You can, but you'll waste much of it: UV destroys unstabilized chlorine within hours. Dusk shocking gives the dose a full night to work — it's the single easiest efficiency win in pool care.