How to Shock a Pool
Shocking raises your free chlorine to a high level for a few hours to kill bacteria, algae, and chloramines. Here is when to do it, how much to use, and when it is safe to swim again.
When to shock
- •Free chlorine reads below 0.5 ppm
- •Water looks green, cloudy, or has a strong chlorine smell (that smell is chloramines — used-up chlorine — not excess)
- •After heavy rain or a large number of swimmers
- •At pool opening after winter
- •After a contamination incident
How much to use — the CYA rule
The correct shock dose depends on your stabiliser (CYA) level. CYA reduces chlorine's effectiveness, so the more CYA you have, the higher your chlorine needs to go to actually sanitise the water.
The target is roughly 40% of your CYA level, with a floor of 10 ppm:
Which product to use
Liquid chlorine (10%)
The easiest option. Pour slowly in front of a return jet with the pump running. Does not affect CYA levels.
Granular Cal-Hypo (65%)
More concentrated — a smaller weight achieves the same dose. Pre-dissolve in a bucket of clean water before adding. Does not affect CYA levels.
Always add Cal-Hypo granules to water — never pour water onto granules. Adding water to Cal-Hypo can cause a violent reaction. Stir gently.
Avoid trichlor tablets for shocking
Tablets are slow-release and add CYA with every dose — they cannot raise chlorine fast enough to shock, and using large quantities will push your CYA even higher.
How to apply
- 1Test your current free chlorine and CYA levels first. Scan your test strip →
- 2Calculate the dose based on pool volume and CYA. Use the PoolScan calculator →
- 3Apply at dusk or night — sunlight destroys chlorine. You'll lose far less overnight.
- 4Pour near a return jet with the pump running. Keep the pump on for at least 8 hours.
- 5Retest the next morning. FC should still be elevated.
- 6Wait until FC drops back to a safe swimming level. For pools with CYA below 50 ppm, below 5 ppm FC is a safe target. For higher CYA pools, FC can safely be higher — once it has fallen well below the shock target (roughly half), re-entry is generally safe. If in doubt, wait until the next evening and retest.
Safety
- ⚠Wear gloves and eye protection when handling chlorine products.
- ⚠Never mix different chlorine products together — this can cause fire or toxic gas.
- ⚠Add chemicals to water, not water to chemicals.
- ⚠Do not swim until FC has dropped well below the shock target. For low-CYA pools (under 50 ppm), below 5 ppm is a safe guideline.
PoolScan calculates your exact shock dose based on your pool volume and CYA level.
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