Pool Shock Amount Calculator
How much cal-hypo, dichlor or liquid chlorine to shock your pool, from its volume and the free-chlorine rise you need.
How it works
Available Cl needed (mg) = ΔFC(ppm) × liters · Product = needed ÷ active fraction · liters = gallons × 3.785
One part per million equals one milligram of free chlorine per liter, so the total chlorine you must add is the desired rise times the pool volume in liters. Dividing by the product’s available-chlorine fraction converts that to actual product — cal-hypo is about 65% available, dichlor 56%, and liquid chlorine 12.5%. Shock at dusk so sunlight does not burn it off before it works, and always add product to water, never water to product.
Worked example
To raise free chlorine from 1 to 12 ppm (Δ11) in 15,000 gallons: 15,000 × 3.785 = 56,775 L, × 11 = 624,525 mg of chlorine. With 65% cal-hypo that is 624,525 ÷ 0.65 ÷ 1,000 ≈ 961 g, about 34 oz or 2.1 lb.
Frequently asked questions
How high should I shock?
A common target is 10–12 ppm free chlorine, but if you use a stabilizer (CYA) the effective shock level is about 10× your CYA reading. High CYA ties up chlorine, so you need more to reach the same sanitizing power.
Cal-hypo, dichlor or liquid — which should I use?
Cal-hypo is strong and cheap but adds calcium; dichlor adds stabilizer (CYA) which builds up over time; liquid chlorine adds nothing extra but is bulky and short-lived. Choose based on what your water chemistry can spare.
How long before I can swim?
Wait until free chlorine drops back to roughly 1–3 ppm, usually the next day. Test before swimming — going in at shock levels can irritate skin, eyes and swimwear.
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Estimate only — always test your water and follow the product label; handle pool chemicals with care and never mix them.