How long after shocking can you swim?
Shocking a 20,000 gallon pool from 1 ppm up to the 16 ppm algae-killing level takes 2 gal 51 fl oz of 12.5% liquid chlorine, and you can get back in once free chlorine falls under 4 ppm — usually by the next morning.
The wait is set by a test strip, not a clock. Chlorine burns off at its own pace depending on sun, temperature and how much the pool has to work through.
Test, don't count hours
Every fixed answer is a guess at how fast your pool burns off chlorine, and pools differ by a factor of three. Free chlorine at or below 4 ppm is the level health guidance uses for swimming, and a test strip reads it in seconds. If your stabilizer is high, aim instead for about 7.5% of your CYA reading: 3 ppm at CYA 40, 6 ppm at CYA 80.
What usually happens
A pool shocked at dusk with liquid chlorine is normally back under 4 ppm by mid-morning, somewhere between 8 and 24 hours. Full sun strips 2 to 4 ppm a day on its own, so an uncovered pool clears faster than a covered one. Green water is the exception: if you are shocking daily to kill algae, the pool is meant to stay at shock level until it clears.
Non-chlorine shock is different
Potassium monopersulfate, sold as non-chlorine or oxidizing shock, does not raise free chlorine at all, and most labels clear you to swim 15 minutes after it dissolves. It burns off the sweat and sunscreen that dull your water, but it kills nothing, so it is no substitute when something is growing.
Getting in early
Water at 10 to 20 ppm will not injure an adult, but it stings eyes and takes the color out of swimwear. The real hazard is undissolved granular shock on the floor, which bleaches a vinyl liner wherever it settles. Brush the bottom before anyone gets in.
Work out your own numbers
Use the pool shock calculator, CYA calculator and the chlorine dosing calculator with your own readings. If you do not know your volume yet, measure it with the pool volume calculator.
More guides
Can you over-shock a pool? How many bags of shock does my pool need? Why is my pool still green after shocking? How do you lower CYA in a pool? How much chlorine do you need to open a pool? How do you raise pool alkalinity without raising pH? How much does it cost to run a pool pump? How often should you shock a pool? See all pool care guides.