Types of Pool Shock
Not all shock is the same. The right choice depends on your pool's current chemistry, your water source, and what you're trying to fix. Here's what's on the shelf and how each one works.
Cal-Hypo (Calcium Hypochlorite) — 65–73% available chlorine
The most effective shock for most situations. Adds calcium to the water (relevant for soft-water pools — less so for hard-water areas). Available in granular form. Must be pre-dissolved in a bucket of water before adding to the pool — never add directly to pool surface.
Best for: heavy algae treatment, green pools, breakpoint chlorination, any time you need maximum killing power.
Liquid Chlorine (Sodium Hypochlorite) — 10–12.5% available chlorine
Easier to handle, doesn't add calcium, and works immediately without pre-dissolving. Good for maintenance shocks, quick top-ups, and pools with already-high calcium hardness. Check the label — bleach loses potency over time.
Best for: routine maintenance, weekly shock, pools with elevated calcium hardness.
Dichlor Shock — 56–62% available chlorine
Fast-dissolving and stabilized — adds CYA with each dose. This is useful when CYA is low, but it becomes a problem if CYA is already high. Use sparingly to avoid CYA creep. Once CYA gets above 80 ppm, chlorine stops working effectively.
Best for: early-season shock when CYA is low, or when quick dissolution matters.
Non-Chlorine Shock (MPS / Potassium Monopersulfate)
Oxidizes organic waste but does NOT raise free chlorine. Useful for weekly oxidation in salt pools or bromine systems, and for restoring water clarity after heavy use without adding chlorine. Cannot clear algae on its own — it is not a substitute for chlorine shock when algae is involved.
Best for: weekly oxidation in salt/bromine pools, oxidizing organics without spiking chlorine levels.
🔢 Want PoolDiag to calculate the exact dose for your pool volume and current FC reading?
Calculate My Dose →Dose Chart by Situation
Pool volume matters. All doses below are expressed per 10,000 gallons — multiply by your pool's volume in units of 10,000. (A 20,000-gallon pool doubles every dose.)
| Situation | Cal-Hypo Dose | Target FC | Notes |
|---|---|---|---|
| Weekly maintenance shock Routine | 1 lb per 10K gal | Raise FC 5–10 ppm | Add at dusk, run pump overnight |
| After heavy rain or pool party Moderate | 1–2 lbs per 10K gal | Raise FC 5–10 ppm | Test first; shock based on actual FC deficit |
| Cloudy water (organic load) Moderate | 1–2 lbs per 10K gal | Raise FC 10+ ppm | Run filter 24 hrs; add clarifier after 12 hrs if needed |
| Light green water (early algae) Moderate | 2 lbs per 10K gal | Raise FC 10+ ppm | Brush first; add at dusk; retest in 24 hrs |
| Dark green or cloudy green (heavy algae) Heavy | 3 lbs per 10K gal | Raise FC 15–20 ppm | Triple shock; brush thoroughly; run filter 24/7; backwash every 12 hrs |
| Black algae Heavy | 3+ lbs per 10K gal | Raise FC 20+ ppm | Use a stainless steel brush on affected spots; repeat treatment every 24 hrs |
| Breakpoint chlorination (CC above 0.5 ppm) Breakpoint | 10× the combined chlorine (CC) level in ppm | Must exceed breakpoint | Example: 1 ppm CC in a 10K gal pool = 1.67 lbs cal-hypo needed |
🧪 Not sure if your problem is chloramines, algae, or organic load? PoolDiag diagnoses it based on your specific symptoms.
Diagnose My Pool →How to Shock a Pool — Step by Step
Test water first
Know your current free chlorine (FC), combined chlorine (CC), and pH before starting. You need a real number — guessing your FC deficit leads to under-dosing or over-dosing.
Balance pH to 7.2–7.4 before shocking
Chlorine efficiency drops dramatically as pH rises. At pH 8.0, chlorine is only about 20% as effective as it is at pH 7.2. Lower pH first with muriatic acid, then shock.
Brush the pool
Break up algae colonies on walls, floor, steps, and behind the ladder before chemical contact. Brushing physically disrupts the algae structure and allows the shock to penetrate.
Pre-dissolve cal-hypo in a bucket of water
Fill a 5-gallon bucket with pool water. Add cal-hypo slowly — 1 lb at a time — while stirring. Never add water to the chemical; add the chemical to the water. Never mix different chemicals in the same bucket.
Add shock at dusk or at night
UV light destroys free chlorine rapidly in direct sunlight. Shocking during the day can burn off up to 90% of your chlorine before it has a chance to work. Always shock at dusk or after dark.
Pour slowly around the perimeter
Walk the bucket around the edge of the pool as you pour. Don't dump it all in one spot — concentrated shock can bleach pool surfaces and create hot spots that don't treat the whole pool evenly.
Run the pump and filter overnight
Circulation distributes the shock evenly and the filter captures dead algae and particles. Keep the pump running throughout the treatment period.
Retest in the morning — repeat if needed
Check FC. If you're treating algae, FC should still be elevated. If it's dropped to near zero and the pool isn't clear, the algae consumed all of it — you need another dose. Repeat until the water clears and FC holds at a normal level.
Common Shocking Mistakes
Shocking during the day
UV destroys free chlorine rapidly in direct sunlight — you can lose up to 90% before it does its job. Always shock at dusk or after dark.
Not pre-dissolving cal-hypo
Granules sitting on pool surfaces will bleach and etch your vinyl liner or plaster. Pre-dissolve in a bucket every time. This is non-negotiable with vinyl liner pools.
Shocking with high pH
Chlorine is largely inactive above pH 7.8. If your pH is at 8.0, most of the chlorine you add exists as hypochlorite ion — which barely sanitizes. Lower pH to 7.2–7.4 first, then shock.
Not shocking to breakpoint
A partial shock that doesn't destroy all combined chlorine is wasted effort. You must get FC to at least 10× the CC reading to break through the chloramine barrier. Half-measures mean the pool still smells and the problem persists.
Swimming too soon
Wait until FC drops below 5 ppm before swimming. High chlorine levels irritate eyes and skin, and are hard on swimwear. Retest before anyone gets in.
🔍 Need help figuring out why your pool keeps needing repeated shocks? PoolDiag identifies recurring chemistry problems.
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