Pool Shock Calculator
Find how much shock your pool needs to hit a high free-chlorine target and break down chloramines. Enter your volume, current FC and the FC you want to reach, choose a product, and get the dose in pounds or fluid ounces.
Calculator
To shock (raise free chlorine) by 10.0 ppm in 20,000 gallons, add 2.28 lb of cal-hypo (73%).
Shocking a pool means raising free chlorine sharply — well past routine levels — to oxidize built-up combined chlorine (chloramines), kill algae and clear cloudy water. The chemistry is identical to routine chlorination; the only difference is the size of the jump, so shock uses the same ppm math at a larger Δppm.
Cal-hypo is the classic granular shock because it is concentrated and fast, but you can shock with liquid chlorine or dichlor too. This calculator sizes whichever product you choose for the target FC you enter.
Formula
Shock reuses the chlorine dosing rule at a higher target:
available chlorine (lb) = gallons × Δppm × 8.34×10-6
product (lb) = available chlorine (lb) ÷ strength
with Δppm = target FC − current FC. For 12.5% liquid the volumetric shortcut still applies: fl oz = (gallons ÷ 10,000) × Δppm × 10.7. A true breakpoint shock aims for roughly ten times your combined chlorine reading; if you do not have that reading, a common preset is to raise FC to about 10 ppm.
Worked example
For a 20,000-gallon pool at 0 ppm FC that you want to lift to 10 ppm with cal-hypo (73%):
- Δppm = 10 − 0 = 10 ppm.
- available chlorine = 20,000 × 10 × 8.34×10-6 = 1.668 lb.
- product = 1.668 ÷ 0.73 = 2.28 lb of 73% cal-hypo.
The same 10 ppm jump with 12.5% liquid would be (20,000 ÷ 10,000) × 10 × 10.7 = 214 fl oz (about 1.7 gallons).
How to shock a pool
Shock at dusk or after sunset so the sun does not burn off the chlorine before it works, and run the pump for at least one full turnover afterward. Pre-dissolve cal-hypo when the label allows and add it around the perimeter. Do not swim until free chlorine has fallen back into the normal range and the water tests safe.
If your pool has a high cyanuric acid level, the FC target for an effective shock climbs with it — this is why some pools seem to “resist” shocking. Very high CYA can mean the only real fix is partial dilution rather than ever more chlorine. For routine top-ups instead of a big jump, use the chlorine calculator.
Cloudy water for a few hours after shocking is normal as the chlorine oxidizes contaminants; let the filter run and it usually clears. If it does not clear, or free chlorine falls back to zero overnight, you likely have a persistent algae or organic load and should repeat the shock and check the filter. Always re-test before anyone swims, and remember that reducing sun exposure or using a cover helps the shock hold long enough to finish the job.
Frequently asked questions
How much shock for a 20,000-gallon pool?
To raise a 20,000-gallon pool by 10 ppm free chlorine you need about 2.3 lb of 73% cal-hypo, or roughly 214 fl oz of 12.5% liquid chlorine. Adjust the target ppm to match your combined-chlorine reading and the calculator re-sizes the dose.
How high should I raise free chlorine to shock?
A breakpoint shock aims for roughly ten times the combined chlorine reading. Without that number, many owners raise FC to about 10 ppm as a starting preset. Higher cyanuric acid pushes the effective target higher.
When should I shock the pool?
Shock after heavy use, rain, an algae bloom, or when your water smells strongly of chlorine (a sign of chloramines). Do it in the evening so sunlight does not waste the chlorine, and run the pump overnight.
How long after shocking can I swim?
Wait until free chlorine falls back to a normal swimming range — usually 1-4 ppm — and the water tests clear. That can take several hours to overnight depending on how much you added. Always re-test before anyone gets in.
Can I shock with liquid chlorine instead of cal-hypo?
Yes. Any chlorine source can shock; only the amount differs. Liquid chlorine adds no calcium or stabilizer, while cal-hypo is more concentrated and adds calcium hardness. Pick the product in the calculator and it converts the ppm target into that product's dose.