How many bags of shock does my pool need?
Killing algae in a 20,000 gallon pool takes 4 one-pound bags of 65% cal-hypo, not the 2 the label suggests, because at a stabilizer level of 40 ppm the water has to reach 16 ppm of free chlorine and each bag adds only 3.9 ppm.
The label's 1 lb per 10,000 gallons is a maintenance dose. The double, triple and quadruple shock advice for green water is really tracking your CYA reading, which is the number to test first.
What double, triple and quadruple shock mean
One 1 lb bag of 65% cal-hypo in 10,000 gallons lifts free chlorine 7.8 ppm. Doubling it gives 15.6 ppm, close to the 16 ppm shock level at CYA 40. Tripling gives 23.4 ppm, near the 24 ppm needed at CYA 60, and quadrupling gives 31.2 ppm, near the 32 ppm needed at CYA 80. The rule of thumb works because it follows stabilizer, so choose the multiple from your CYA test, not from how green the water looks. In a 20,000 gallon pool that is 4, 6 and 8 bags.
Check the bag before you count
Not every bag weighs a pound or holds 65%. Stronger 73% cal-hypo adds 8.8 ppm per pound per 10,000 gallons, so it goes further, while some bags hold less than a pound. Dichlor shock adds stabilizer with every bag, which raises the shock level you need next time.
When liquid is the better buy
A green pool rarely clears on one dose, and every bag of cal-hypo also adds calcium hardness. Liquid chlorine leaves nothing behind but a little salt: 2 gal 51 fl oz of 12.5% takes the same 20,000 gallons to 16 ppm, and you can repeat it daily without changing your water balance.
Adding it safely
Pre-dissolve each bag in a bucket of pool water, adding the shock to the water rather than the other way round, and pour it in front of a return at dusk with the pump running. Never put cal-hypo in a skimmer or feeder that has held trichlor tablets: the two react violently. Retest after an hour and top up to hold the level.
Work out your own numbers
Use the pool shock calculator, CYA calculator and the shock table for 20,000 gallons 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? 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.