How to Shock a Pool (and How Much Shock You Actually Need)

The bag says “one pound treats 10,000 gallons,” so you add a bag, the water looks a bit better for a day, and then the green comes back. That’s the most common way shocking goes wrong, and it usually has two causes: the dose didn’t account for stabilizer, and the chlorine level wasn’t held long enough.

Shocking means raising free chlorine well above normal and keeping it there until whatever is using up chlorine, whether algae, organic debris or chloramines, is gone.

When it’s worth doing

• Green, yellow or cloudy water, or walls that feel slimy
• Combined chlorine above 0.5 ppm, often noticed as a sharp “chlorine” smell
• Opening the pool in spring or getting it ready to close for winter
• After something gets into the water, such as an animal or heavy storm debris. (A fecal accident is different: CDC publishes a specific response with set chlorine levels and contact times, and you should follow it.)

If free chlorine stays in range and combined chlorine stays low, there’s no need to shock on a weekly schedule. Day-to-day top-ups are a job for the chlorine calculator instead.

Step 1: work out your shock level

How high to go depends on your cyanuric acid (CYA). If you don’t know your level, test it first; the CYA calculator shows what to do if it’s very low or very high. Stabilizer holds most of the chlorine in reserve, so a pool with more CYA needs more free chlorine to get the same killing power. A widely used target is about 40% of your CYA reading, and never less than 10 ppm. The CYA guide explains why.

For a chlorine smell without algae, the older breakpoint rule also works: add free chlorine equal to about 10 times your combined chlorine.

Step 2: choose a product

Here’s what each option does to a 15,000-gallon pool with CYA at 40, raising free chlorine from 2 ppm to its 16 ppm shock level:

The amounts come from our shock calculator, and they scale with pool size, so start from an accurate figure (the pool volume calculator works it out from a few measurements). The side effects follow from each product’s chemistry. Liquid chlorine is usually the cleanest choice. Cal-hypo is convenient, but in hard-water areas the extra calcium can cloud the water or leave scale.

Pool shock calculatorCalculate your shock level and dose from your CYA and product.

Step 3: shock, then hold it

1. Get pH to about 7.2–7.6 first. Chlorine works better at lower pH. The pH guide covers how.
2. Dose in the evening, with the pump running, in front of a return jet. Pre-dissolve dry products by adding them to a bucket of water, never the other way round.
3. Brush the walls, floor, steps and behind ladders.
4. Retest every few hours while you’re up, and top back up to the shock level each time. Algae keeps using chlorine; if the level sags, the algae recovers.
5. Keep the filter running and clean it when the pressure climbs. Dead algae clogs filters quickly.

Staying safe

Keep everyone out of the water while chlorine is at shock level. Never mix dry chlorine products with each other, and keep all chlorine away from acid, because the combination releases toxic gas. Store both out of the sun and out of children’s reach. Very high chlorine also bleaches many test reagents, so a FAS-DPD test (or a diluted sample) gives the most reliable reading during a shock.

When it still won’t clear

• CYA is too high. At 80 ppm and above, the shock level runs past 30 ppm, which is hard to reach and hold. Partly replacing the water often fixes the pool faster than buying more chlorine.
• The level wasn’t held, even for one night.
• Circulation or filtration is poor.
• It’s metal, not algae. Water that turns green or teal within minutes of adding chlorine, especially with well water or after a copper-based algaecide, is often oxidized copper. Chlorine won’t fix that. A metals test and a sequestrant product are the usual next steps.