Shock your pool by adding 1 pound of calcium hypochlorite (cal-hypo) per 10,000 gallons of water, directly to the pool at dusk. This burns off chloramines, kills algae, and resets your free chlorine. Do it at least once a week during swim season, and any time the water looks off.
What You Need To Know
Shocking is just raising your chlorine high enough and fast enough to oxidize everything in the water that regular chlorine can’t touch — dead chlorine byproducts, algae, sunscreen residue, and anything swimmers brought in. Without regular shocking, your chlorine gets “used up” and stops protecting the water even when the test strip looks fine.
When to shock:
- After a pool party or heavy use (lots of swimmers)
- After heavy rain or wind drops debris into the water
- When the water smells strongly of chlorine (see Pro Tip below)
- When water looks dull, hazy, or faintly green
- When opening the pool for the season
- Every 1-2 weeks as standard maintenance through summer
What you need:
- Pool shock (cal-hypo is the standard choice — more on types below)
- Rubber gloves and eye protection
- A clean 5-gallon bucket
- A test kit or test strips
How to do it:
- Test your water first. Check free chlorine, pH, and if you haven’t in a while, cyanuric acid (CYA). Adjust pH to 7.2-7.4 before shocking — high pH kills chlorine effectiveness.
- Wait until dusk. UV from sunlight destroys free chlorine fast. Shocking in the afternoon is half-wasted before it even gets to work.
- Pre-dissolve cal-hypo in a bucket of water first — add shock to water, not water to shock. Never dump it straight into the skimmer.
- Walk the perimeter of the pool and pour slowly while the pump is running.
- Run the pump for at least 8 hours overnight.
- Test in the morning. Free chlorine should be back in the 1-3 ppm range before anyone swims.
Pro Tip: That strong “chlorine smell” at a pool? That’s not clean water — it’s chloramines, which are spent chlorine molecules bonded to ammonia from sweat and urine. A well-shocked pool has almost no smell at all. When your pool reeks, that’s the signal it needs shock, not that it’s over-chlorinated.
Cost: A 25-pound bucket of cal-hypo runs $50-70 and covers a full season for most pools. Per treatment, you’re looking at $0.50-$1.00 for a 15,000-gallon pool. See the full pool chemical cost breakdown if you’re budgeting for the season.
Deep Dive
Types of Pool Shock
Not all shock is the same. Here’s what’s actually on the shelf and when to use each:
Calcium hypochlorite (cal-hypo) — 65-78% active chlorine, cheapest per pound, the workhorse for most pool owners. It leaves a small amount of calcium behind, which can raise calcium hardness over time if you use a lot. Always pre-dissolve before adding. This is what I use for standard weekly shocking.
Sodium dichloro (dichlor) — Stabilized, dissolves fast, and can go straight into the water. The downside: every pound adds CYA to your pool. If you’re using dichlor as your primary sanitizer AND your shock, your stabilizer levels will creep up all season. Fine for occasional use, not ideal as a weekly shock if CYA is already high.
Potassium monopersulfate (non-chlorine shock) — This is an oxidizer, not a chlorine source. It burns off organic waste and chloramines, and swimmers can get back in after just 15 minutes. Good for mid-week maintenance or if someone needs to swim soon. It won’t raise your free chlorine reading, though, so it can’t substitute when you actually need a chlorine boost.
Sodium hypochlorite (liquid shock) — Basically pool-grade bleach at 10-12% concentration. Fast-acting, no residue, but heavier to haul and shorter shelf life. Useful if you want to avoid any calcium or CYA additions.
Breakpoint Chlorination: Why “A Little Shocking” Doesn’t Work
There’s a threshold called breakpoint chlorination. To actually destroy chloramines instead of just adding more, you need to bring free chlorine to 10 times the combined chlorine level. Below that point, you’re adding chloramines rather than eliminating them.
This is why half-measures backfire. If you eyeball it and throw in half the recommended dose, you can end up with more chloramines than you started with. Use the full dose — or, if you’re dealing with a serious algae bloom, use 2-3x the normal amount.
The CYA Problem
Cyanuric acid protects chlorine from sunlight, which is useful — but it also buffers chlorine and slows down how hard it hits. When CYA is above 80 ppm, shocking becomes significantly less effective. The chlorine is there, but it can’t react fast enough to oxidize contaminants.
If you’ve shocked multiple times and nothing seems to change, check your CYA before adding more chemicals. The fix at that point is a partial water drain and refill — not more shock. See the cyanuric acid guide for the full explanation and how to bring levels down.
pH Matters More Than Most People Think
At pH 7.8, only about 20% of your chlorine is in the active “hypochlorous acid” form. At pH 7.2, it’s closer to 60%. Same amount of chlorine in the water, three times the punch. Always bring pH to 7.2-7.4 before you shock. If you skip this step and wonder why the shock didn’t work, pH is usually the culprit.
For the full picture on pH adjustment, see Understanding pH and How to Adjust It.
Shocked But Still Green?
If you shocked and the pool is still green the next day, check these in order:
- pH was too high when you shocked — the chlorine barely worked
- CYA is above 80 ppm — chlorine was too buffered to act aggressively
- You underdosed — algae blooms need 2-3 pounds per 10,000 gallons, not the standard 1 pound
- The filter isn’t running long enough to clear dead algae
For a full walkthrough, see Green Pool Water: How to Fix It.
FAQ
How long after shocking can I swim?
Wait until free chlorine drops below 3 ppm. If you shock at dusk with the pump running, that’s usually 8-24 hours. Always test before anyone gets in — don’t just guess based on time.
Can you over-shock a pool?
Yes, but it’s not dangerous. Very high chlorine (above 10 ppm) will bleach swimsuits and irritate eyes and skin. It’ll come down on its own in 24-48 hours, faster if the sun’s out. The bigger risk is wasting money by overdoing it regularly.
Do I shock a saltwater pool?
Yes. Salt chlorinators produce chlorine continuously but don’t produce the high chlorine burst needed to break down chloramines and kill algae. You still need to shock a saltwater pool — use the same cal-hypo method, about once a month or when the water looks off. See how saltwater pools work for more context.