How long after shocking a pool can you swim?

The short answer is on the bag. Every chlorine shock label carries a re-entry rule, and it's a chlorine reading, not a clock.

So the real question is how to know when you've hit it.

What the labels say

Three shock labels, three slightly different limits:

Shock type	Label says don't swim until free chlorine is
Cal-hypo (HTH Pool Shock)	1 to 4 ppm
Liquid chlorine (12.5% sodium hypochlorite)	Below 4.0 ppm
Dichlor (Clor Mor Dy-Shock)	3.0 ppm or lower

That's why there's no single number for every pool. Read your product's label and use its limit. If you've lost the bag, 3 ppm is the most cautious of the three.

You may see 10 ppm mentioned online. That's the top of the range some states allow in public pools during normal operation (California allows up to 10 ppm, for example). It isn't permission to jump in right after a shock when your label says 4.

Why "wait 8 hours" isn't reliable

Ask around and you'll hear 8 hours, 24 hours, even 48. Swim University says wait at least 8 hours; Leslie's says 24 to 48. Both also tell you to test first, and that's the part that matters.

How fast chlorine falls depends on how much you added, how sunny it is and how much algae or waste is in the water eating it. A light shock on a cloudy evening might be swimmable by morning. A heavy algae treatment might take days. Test, don't count hours.

How to test after a shock

Use a drop-count kit if you have one. The CDC says DPD kits are more accurate than strips.

One trap: very high chlorine can bleach a standard DPD color test so it reads falsely low or zero, the CDC warns. If you shocked hard and the test says 0, don't believe it and don't swim. Wait, then retest, or use an FAS-DPD drop-count kit, which counts drops instead of matching colors.

Check pH while you're at it. The CDC says to keep it between 7.0 and 7.8.

Non-chlorine shock is different

Non-chlorine shock is potassium monopersulfate, often sold as MPS. Product directions allow swimming soon after: Leslie's Fresh 'N Clear says you can swim in as little as 15 minutes, and BioGuard Oxysheen says 15 minutes after application.

Two catches. A Pool & Hot Tub Alliance tech note points out that MPS isn't a sanitizer or an algaecide, so it won't clear a green pool on its own and you still need chlorine in the water. Leftover MPS also shows up as combined chlorine on a DPD test. Test before you add it, or add it in the evening and test the next morning.

Shock safely, at dusk

Add shock straight into the water, never water onto shock, and never mix it with other chemicals, including a different type of shock. The HTH and dichlor labels both warn that mixing can cause fire or explosion. Keep everyone out of the pool while you add it, and wear goggles and gloves.

Unstabilized shock (cal-hypo and liquid chlorine) is best added in the evening so the sun doesn't burn it off before it works. Swim University recommends this, and it also means the long wait happens while everyone's asleep. Dichlor carries its own stabilizer, so daytime is fine for it.

To work out how much to add, use the shock calculator. For a 15,000-gallon pool, raising free chlorine from 1 to 10 ppm takes about 1.35 gal of 10% liquid chlorine. Every extra ppm you add is more you'll wait for to come back down.

Before anyone gets in

Test free chlorine and check it against your label, usually 4 ppm or lower. Check pH. Make sure the water is clear enough to see the bottom. Then swim. Write the reading on your test log so you know how long your pool usually takes next time.