Shock is a level to reach, not a bag to tip in

Most pool shock comes with a dose on the bag. One pound per 10,000 gallons, or words to that effect, and the implication is that tipping in the recommended amount is the job.

It is not a dose. It is a level, and the level you need depends on what is already in the water.

Breakpoint is a threshold, not a quantity

The reason to shock is usually combined chlorine, which is chlorine that has already reacted with sweat, urine, sunscreen and whatever else arrived in the pool. Combined chlorine is a poor sanitiser and it is what people smell and blame on too much chlorine.

Getting rid of it takes breakpoint chlorination, where the free chlorine level is pushed high enough to oxidise those compounds completely. The important word is completely. Below the threshold you partially react, which produces more combined chlorine rather than less, and the pool smells worse afterwards.

The threshold moves with stabiliser

Here is the part no bag mentions. Cyanuric acid holds much of your chlorine in reserve, so the free chlorine needed to cross breakpoint rises as stabiliser does.

A commonly used operational figure puts the shock target at around 40% of the cyanuric acid level. At 50 ppm stabiliser, that means taking free chlorine to about 20 ppm, which is far beyond what one bag of anything does to a typical pool. At 100 ppm stabiliser, the target is 40 ppm of chlorine, which is a serious quantity of product and a strong hint that the stabiliser itself is the real problem.

So the honest sequence is to test cyanuric acid, work out the target, then calculate the dose that gets you there from where you are now. The pool chemical calculator does that last step for the product you actually own, which matters because a 10% liquid and a 73% cal-hypo are wildly different amounts for the same rise.

Pick the product for the side effect

All shock raises chlorine. What separates the options is what else they leave behind.

Cal-hypo adds calcium, which is fine in soft water and a problem in hard water where scaling is already a risk. Dichlor adds cyanuric acid, which is the last thing an over-stabilised pool needs. Liquid chlorine adds neither and is the usual choice for a pool that is already carrying enough stabiliser, at the cost of being bulky to store and losing strength on the shelf.

What to do after dosing

Run the pump. Shock needs circulation to distribute, and a dose tipped into a still pool sits in a layer where it does little except bleach whatever is beneath it.

Dose after sundown. Sunlight destroys free chlorine quickly, and a shock level is a lot of chlorine to hand to UV before it has had time to work.

Then wait and retest rather than swimming. The pool is safe to swim when free chlorine has fallen back to its normal range, which usually takes a day or so depending on sun and temperature.

If it comes straight back

A pool that needs shocking repeatedly is telling you the maintenance level is too low for its stabiliser, not that it needs more shock.

The same applies to algae that returns within a week. The shock killed it, the maintenance chlorine failed to keep it from returning, and the fix is a higher daily target or a lower stabiliser level rather than another bag at the weekend. Treating a chronic problem with repeated shock is expensive, hard on equipment, and never actually resolves it.

When shock is the wrong tool entirely

Cloudy water with normal chemistry is usually filtration rather than sanitation. A filter that needs cleaning or a pump running too few hours a day will not be improved by chlorine, and the shock gets credited when the cloudiness clears for unrelated reasons.

Metal staining is another. Shocking a pool with iron or copper in the water can set the stain rather than lift it, which is a frustrating way to turn a cosmetic problem into a permanent one.