To shock a pool, raise free chlorine to breakpoint — roughly 10× your CYA reading — with cal-hypo or liquid chlorine, added at sunset with the pump running. Then wait until FC falls back to 1–3 ppm, usually 8–24 hours, before swimming. Test before and after; the dose depends on your volume and current FC.
What is Pool Shocking?
Pool shocking (also called superchlorination) is the process of adding a large dose of chlorine to your pool to reach breakpoint chlorination — the point where chloramines are destroyed and your free chlorine level is reset. Think of it as a deep clean for your water chemistry.
When chlorine kills bacteria and organic matter, it forms combined chlorine (chloramines). These are the compounds that actually smell like "chlorine" and irritate your eyes and skin. Shocking breaks them down through oxidation and restores your water to a clean, sanitized state. When you are clearing an actual algae bloom rather than just chloramines, the same idea is stretched into the SLAM method — hold that shock level for days until the water passes an overnight chlorine-loss test. How to SLAM a pool walks through that version step by step.
Key concept
Breakpoint chlorination = FC level of 10x your CYA (cyanuric acid) level. For a pool with CYA of 30 ppm, you need to reach FC of 30 ppm to hit breakpoint. Below this, you're wasting chlorine without actually destroying chloramines.
When Should You Shock Your Pool?
Shock your pool in any of these situations:
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Weekly maintenance
Weekly shocking prevents chloramine buildup. Best done at sunset — UV destroys chlorine.
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After heavy use
Pool party, lots of swimmers, or heavy rain — all introduce contaminants that consume chlorine.
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Algae or green water
Visible algae means your chlorine can't keep up. Shock hard, brush, filter, repeat.
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Strong chlorine smell
Ironically, a "chlorine smell" means you need MORE chlorine — you're smelling chloramines.
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Opening or closing the pool
Shock at season start to kill anything that grew over winter, and before closing to protect through the off-season.
Calculate Your Exact Shock Amount
Don't guess. Enter your pool volume and current chlorine level to get the exact amount of shock you need.
Open Shock CalculatorStep-by-Step: How to Shock Your Pool
Test your water
Measure Free Chlorine (FC), Combined Chlorine (CC), and CYA. You need all three to calculate the correct shock dose. If CC is above 0.5 ppm or you see algae, shocking is needed.
Calculate the dose
Use our Pool Shock Calculator to determine the exact amount. Target FC = CYA × 0.4 (or at least 10 ppm above current FC).
Wait until sunset
UV sunlight destroys unstabilized chlorine fast — up to 90% in 2 hours. Shocking at dusk gives the chlorine all night to work without UV loss.
Pre-dissolve (granular shock only)
Fill a 5-gallon bucket with pool water. Add shock to the bucket and stir until dissolved. Never add water to chemicals — always chemicals to water. Skip this step for liquid chlorine.
Pour around the perimeter
With the pump running, slowly pour the solution around the pool perimeter. For liquid chlorine, pour directly. Avoid splashing and keep away from skimmer.
Run the pump for 8+ hours
Keep the pump running overnight to circulate the chlorine throughout the entire pool. Brush walls and floor to dislodge any algae.
Test again in the morning
Check FC and CC levels. If CC is still above 0.5 ppm, repeat the shock process. If FC has dropped to normal range and CC is near zero — success.
Safety Precautions
Critical Safety Rules
Never mix chlorine with acid. This produces toxic chlorine gas that can cause serious injury or death. If you need to adjust pH, wait at least 30 minutes between chemicals.
Always add chemicals to water — never water to chemicals. Adding water to concentrated chemicals can cause violent reactions.
Wear protective gear: chemical-resistant gloves, safety goggles, and old clothes. Avoid breathing fumes.
Keep children and pets away until FC drops to 1-3 ppm (safe swimming range).
After Shocking — When Can You Swim?
Wait until Free Chlorine drops to 1-3 ppm before swimming. This typically takes 8-24 hours depending on how much shock you added, sunlight exposure, and water temperature. Always test — don't guess.
| Shock Type | Typical Wait Time |
|---|---|
| Cal-hypo (granular) | 8-24 hours |
| Liquid chlorine | 8-24 hours |
| Dichlor | 8-24 hours (also raises CYA) |
| Non-chlorine shock (MPS) | 15-30 minutes |