What Is Pool Shocking?
"Shocking" means raising free chlorine to a level high enough to destroy contaminants that regular chlorination can't handle — chloramines, algae, bacteria, and organic waste. The target is called breakpoint chlorination.
When to Shock Your Pool
- Weekly during swim season as preventive maintenance
- After **heavy use** (pool party, high swimmer load)
- After a **rainstorm** (dilutes chlorine, adds contaminants)
- When you smell a strong **"chlorine smell"** (that's actually chloramines — you need MORE chlorine)
- When **combined chlorine** exceeds 0.5 ppm
- After finding any **algae growth**
- At **pool opening** in spring
Three Types of Pool Shock
1. Liquid Chlorine (Sodium Hypochlorite)
- Concentration: 10–12.5%
- Pros: No CYA added, fast-acting, inexpensive
- Cons: Heavy to carry, short shelf life, raises pH slightly
- Best for: Regular weekly shocking, algae treatment
2. Cal-Hypo (Calcium Hypochlorite)
- Concentration: 65–73%
- Pros: Strong and fast, long shelf life as granular
- Cons: Adds calcium to water, must pre-dissolve, raises pH
- Best for: Heavy shock treatments, pools with low calcium
3. Dichlor (Sodium Dichloro-s-Triazinetrione)
- Concentration: 56%
- Pros: pH neutral, dissolves easily, contains CYA
- Cons: Adds CYA (can cause buildup over time), expensive
- Best for: Spas, above-ground pools, occasional use
Shock Dosage Chart
Target: 10 ppm FC (routine shock)
| Pool Size | Liquid Chlorine (12.5%) | Cal-Hypo (73%) | Dichlor (56%) |
|---|---|---|---|
| 10,000 gal | 1 gallon | 13 oz | 17 oz |
| 15,000 gal | 1.5 gallons | 20 oz | 26 oz |
| 20,000 gal | 2 gallons | 26 oz | 34 oz |
| 25,000 gal | 2.5 gallons | 33 oz | 43 oz |
For algae treatment, double or triple these doses. Use our shock calculator for exact amounts based on your current readings.
Step-by-Step: How to Shock a Pool
- Test the water — Record current FC, CC (combined chlorine), and CYA levels
- Calculate your dose — Based on pool volume and target FC level
- Wait until evening — Sunlight destroys unstabilized chlorine. Shock after dusk for maximum effect
- Pre-dissolve granular shock — Mix cal-hypo or dichlor in a bucket of water first (never add dry chemicals directly to the pool)
- Pour along the perimeter — With the pump running, distribute evenly around the pool edge
- Run the pump for 8+ hours — Overnight is ideal
- Retest the next morning — If FC has dropped below 5 ppm and CC is still above 0.5 ppm, shock again
- Don't swim until FC drops below 5 ppm — Usually 24–48 hours
Breakpoint Chlorination Explained
Breakpoint is the FC level at which ALL combined chlorine is destroyed. The formula:
Breakpoint FC = Current CC x 10
Example: If combined chlorine is 1.5 ppm, you need to raise FC to at least 15 ppm to reach breakpoint. Anything less and you'll just make MORE chloramines temporarily.
Common Shocking Mistakes
- Shocking during the day — Unstabilized chlorine loses 90% effectiveness in 2 hours of sunlight
- Not pre-dissolving cal-hypo — Granules can bleach pool surfaces on contact
- Underdosing — Half a shock is worse than no shock (creates more chloramines without destroying them)
- Adding shock through the skimmer — Can damage equipment. Always broadcast around the perimeter
- Swimming too soon — Wait until FC drops below 5 ppm
Automate Your Chemistry Tracking
PoolOps logs every reading and flags when combined chlorine is high — so you know exactly which pools need shocking before you arrive.