How much shock do I need per gallon of pool water?

For standard shocking to 10 ppm with 65% calcium hypochlorite: approximately 1.54 oz per 10,000 gallons, or 0.000154 oz per gallon. For breakpoint chlorination (destroying chloramines), you may need 20–30 ppm, doubling or tripling the dose. Always test your water first to determine how much free chlorine you actually need to add.

Should I shock my pool at night or during the day?

Always shock at dusk or at night. UV rays from sunlight destroy free chlorine rapidly — shocking during the day can lose 25–75% of the chlorine to UV degradation before it can work. Nighttime shocking allows the chlorine to circulate and work through the full overnight cycle, then naturally drop to safe levels by morning.

How long after shocking can I swim?

Wait until free chlorine drops to 1–3 ppm. With normal overnight shocking, this is typically 8–24 hours. Test before swimming — don't just wait a fixed time, as dose, pool size, weather, and circulation all affect how quickly chlorine depletes. Shock at night and test the next afternoon as a standard practice.

What is breakpoint chlorination?

Breakpoint chlorination is the chlorine level at which combined chlorine (chloramines — the cause of strong chlorine smell and eye irritation) is completely oxidized and destroyed. This requires free chlorine at approximately 10× the combined chlorine level. If combined chlorine is 2 ppm, you must shock to 20 ppm free chlorine to break down all chloramines.

How to Calculate Pool Shock Dosage: Volume, Chlorine Level, and Concentration

Learn how to calculate the correct amount of pool shock (calcium hypochlorite or sodium hypochlorite) needed for your pool volume to reach breakpoint chlorination.

Related Calculators

Pool Shock Calculator
Estimate the amount of chlorine product needed to raise free chlorine from the current level to a target level.

What Is Pool Shocking?

Pool shocking means raising free chlorine to 10–30 ppm briefly to destroy chloramines (combined chlorine), kill algae, and eliminate bacteria. "Breakpoint chlorination" requires reaching 10× the combined chlorine level to break the chloramine bond.

Breakpoint Chlorination Target: Target free chlorine (ppm) = 10 × Combined chlorine (ppm)

Where: Combined chlorine = Total chlorine − Free chlorine

If your test shows Total Cl = 3.0 ppm and Free Cl = 1.0 ppm: - Combined Cl = 3.0 − 1.0 = 2.0 ppm - Breakpoint target = 10 × 2.0 = 20 ppm free chlorine

Calcium Hypochlorite (Cal-Hypo) Dosing

Cal-hypo is the most common granular shock, typically 65–78% available chlorine.

Formula: Shock needed (oz) = Pool volume (gallons) × Target Δppm / (Product's % available Cl × 10,000)

For 65% Cal-Hypo: Shock (oz) = Pool volume (gallons) × Target Δppm / 6,500

Pool Volume	Raise to 10 ppm	Raise to 20 ppm
10,000 gal	1.54 oz (43g)	3.08 oz (87g)
15,000 gal	2.31 oz (65g)	4.62 oz (130g)
20,000 gal	3.08 oz (87g)	6.15 oz (174g)
30,000 gal	4.62 oz (130g)	9.23 oz (261g)
50,000 gal	7.69 oz (218g)	15.38 oz (436g)

*Based on 65% Cal-Hypo; 78% Cal-Hypo requires about 17% less product.*

Sodium Hypochlorite (Liquid Chlorine) Dosing

Standard liquid chlorine is 10–12.5% available chlorine.

Formula for 10% liquid chlorine: Fluid oz = Pool volume (gallons) × Target Δppm / 7,500

For a 20,000-gallon pool to add 10 ppm: = 20,000 × 10 / 7,500 = 26.7 fluid oz (about 790 mL)

Shocking Procedure

Test first: Measure current free and total chlorine
Calculate dose: Based on pool volume and combined chlorine
Pre-dissolve (granular): Mix in bucket of water before adding; never add dry granules directly to liner pools
Shock at dusk or night: UV destroys chlorine quickly; nighttime shocking is far more effective
Distribute evenly: Pour around perimeter while pump runs
Wait: Allow free chlorine to drop to 1–3 ppm before swimming (typically 8–24 hours)
Retest: Confirm chlorine and pH levels before use

Shock Schedule

Situation	Shock Frequency
Routine maintenance	Bi-weekly or weekly in summer
After heavy rain	Within 24 hours
After heavy pool use (parties)	Within 12–24 hours
Visible algae	Treat immediately, triple dose
Pool opening in spring	Always shock on open
Algae super-treatment	3× normal dose, brush walls

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Frequently Asked Questions

How much shock do I need per gallon of pool water?: For standard shocking to 10 ppm with 65% calcium hypochlorite: approximately 1.54 oz per 10,000 gallons, or 0.000154 oz per gallon. For breakpoint chlorination (destroying chloramines), you may need 20–30 ppm, doubling or tripling the dose. Always test your water first to determine how much free chlorine you actually need to add.
Should I shock my pool at night or during the day?: Always shock at dusk or at night. UV rays from sunlight destroy free chlorine rapidly — shocking during the day can lose 25–75% of the chlorine to UV degradation before it can work. Nighttime shocking allows the chlorine to circulate and work through the full overnight cycle, then naturally drop to safe levels by morning.
How long after shocking can I swim?: Wait until free chlorine drops to 1–3 ppm. With normal overnight shocking, this is typically 8–24 hours. Test before swimming — don't just wait a fixed time, as dose, pool size, weather, and circulation all affect how quickly chlorine depletes. Shock at night and test the next afternoon as a standard practice.
What is breakpoint chlorination?: Breakpoint chlorination is the chlorine level at which combined chlorine (chloramines — the cause of strong chlorine smell and eye irritation) is completely oxidized and destroyed. This requires free chlorine at approximately 10× the combined chlorine level. If combined chlorine is 2 ppm, you must shock to 20 ppm free chlorine to break down all chloramines.

Last updated 7/28/2026