How Much Chlorine to Shock a Pool

"Shock" isn't a special chemical, it's just a much bigger dose of ordinary chlorine, added all at once to push your free chlorine level high enough to overwhelm whatever is using it up: chloramines, algae, or a heavy load of bather waste. The question of how much depends less on a flat number and more on one specific reading: combined chlorine. Get that number first, and the dose math is straightforward.

The breakpoint chlorination formula

When free chlorine reacts with ammonia and nitrogen compounds from sweat, sunscreen, and urine, it forms chloramines, the combined chlorine you see on a good test kit. Chloramines are weak sanitizers and they're the actual source of that sharp "pool smell" people usually blame on too much chlorine. Breakpoint chlorination is the process of adding enough free chlorine at once to break those chloramine bonds apart rather than just adding a little more to them. The standard rule of thumb pool professionals use: your shock dose, in ppm of free chlorine, should be roughly 10 times your combined chlorine reading. If you don't know your combined chlorine number, or it's reading at or near zero, a standard minimum shock dose of about 10 ppm free chlorine rise is the commonly used default.

Those product amounts are estimates built on standard, widely-published dosing figures: roughly 1 gallon of 12.5% liquid chlorine (sodium hypochlorite) per 10,000 gallons raises free chlorine by about 10 ppm, and roughly 1 lb of 65% cal-hypo granular shock per 10,000 gallons does about the same. Scale both up or down for your actual pool size and the exact concentration printed on your product's label; a weaker 10% liquid chlorine or a 73% cal-hypo will need adjusting. If you're not sure of your exact volume, Skimmer's pool volume calculator gives a solid estimate from your pool's shape and dimensions, and the shock calculator does the full dosing math for you once you have a combined chlorine reading and know which product you're using.

When and how to shock a pool

• Combined chlorine reads above 0.5 ppm, or the water has a noticeably stronger "chlorine" smell than usual, since that smell is chloramines, not excess chlorine.
• Visible algae, whether green, mustard/yellow, or black, or water that's gone cloudy or hazy.
• After heavy bather load: a pool party, a hot holiday weekend, or several days of frequent swimming.
• Opening the pool for the season, especially after it's been covered or sitting still over winter.
• After a heavy rainstorm, which dilutes chlorine and can wash in organic debris and contaminants.
• As routine maintenance: many maintenance schedules build in a shock roughly every one to two weeks as cheap insurance against chloramine buildup, even without an obvious trigger.

• Test combined chlorine first if you have a kit that measures it. That reading decides your dose, a guess doesn't.
• Know your actual pool volume. Gallons that are off by 20% mean your dose is off by roughly the same amount.
• Add the dose at dusk or after dark. Unstabilized chlorine like liquid chlorine and cal-hypo isn't protected by cyanuric acid and burns off quickly in direct sun.
• Pour liquid chlorine slowly around the pool's perimeter, or pre-dissolve cal-hypo granules in a bucket of water before adding; never pour concentrated granules straight into the skimmer.
• Run the pump and filter for several hours afterward, many pool owners just let it run overnight, so the dose circulates fully instead of pooling in one spot.
• Retest before swimming. Wait until free chlorine has settled back down into the normal range rather than estimating by the clock alone.

A few safety basics while you're at it

• Add chemicals to water, not water to chemicals, and always add one product at a time rather than mixing concentrates together first.
• Wear gloves and eye protection when handling concentrated liquid chlorine or cal-hypo granules, and dose in a well-ventilated spot.
• Never mix liquid chlorine and cal-hypo directly with each other, or with other pool chemicals like acid or algaecide; add them separately with time in between.
• Store leftover product in its original, tightly closed container, somewhere cool, dry, and out of direct sun, away from other chemicals.
• Keep kids and pets off the pool deck while you're dosing, and out of the water until a retest shows free chlorine back in the normal range.