How much chlorine do you need to open a pool?
Opening a cloudy or green 20,000 gallon pool with a stabilizer reading of 40 ppm takes 2 gal 72 fl oz of 12.5% liquid chlorine, enough to lift free chlorine from zero to the 16 ppm shock level; clear water needs far less.
Most covered pools reach spring with no chlorine left at all. How much you need turns on two things, your stabilizer reading and how the water looks, so test before you buy.
Clear water: bring it up, then test overnight
If the water is clear and the walls are not slippery, a 3 ppm rise is a normal start: 61 fl oz of 12.5% in 20,000 gallons. Add it at dusk and test at dawn. If free chlorine fell more than 1 ppm overnight, something is growing and the pool needs the full shock below.
Cloudy or green: shock to your CYA
The kill level is about 40% of CYA, never under 10 ppm: 12 ppm at CYA 30, 16 at CYA 40 and 24 at CYA 60. From zero in 20,000 gallons that is 1 gal 118 fl oz, 2 gal 72 fl oz and 3 gal 108 fl oz of 12.5% liquid. With 1 lb bags of 65% cal-hypo, the CYA 40 shock is 5 bags, each adding calcium hardness. Hold the level, retesting morning and evening, until the water is clear and passes the overnight test. That takes days, not one dose.
Test CYA before anything else
Winter rain overflow and a spring drain-down both dilute stabilizer, so last autumn's reading is out of date. Near zero, sunlight strips new chlorine within hours: add stabilizer up to 30 to 50 ppm, and expect it to take a week to read correctly.
The order to work in
Top up the water, clean the filter and run the pump for a few hours before testing. Bring pH under 7.8 first, since chlorine works poorly above it, then add chlorine at dusk in front of a return. If you fill from a well, add a metal sequestrant before shocking, or iron can turn the water brown.
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
How long after shocking can you swim? 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 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.