How often should you shock a pool?
Shock when a test calls for it rather than on a fixed day, which in peak summer often works out to about once a week; a routine 5 ppm shock in a 20,000 gallon pool is 102 fl oz of 12.5% liquid chlorine.
The weekly shock printed on most labels is a habit, not a rule. A pool that holds enough chlorine for its stabilizer every day rarely needs one, and a pool that keeps dipping below it needs one sooner.
The readings that say shock now
Shock when combined chlorine, your total reading minus your free reading, is above 0.5 ppm; when free chlorine falls more than 1 ppm overnight with no sun on the water; or when the water turns dull or the walls feel slippery. Each means something is using chlorine faster than you add it. Visible algae is a different job: it needs the full shock level for your CYA, 16 ppm at CYA 40, held until the water clears.
Events worth a routine shock
A party or a week of heavy swimming, a hot spell, a heavy rainstorm, and opening or closing the pool each justify a routine shock that evening. Add it at dusk with the pump running, and test again the next morning.
Hold chlorine up and you shock less
Free chlorine should not fall below about 7.5% of your CYA: 3 ppm at CYA 40, 4.5 at CYA 60 and 6 at CYA 80. Pools kept above that line every day seldom fail the tests above, which is why owners who top up daily with liquid chlorine or run a salt cell can go weeks without shocking.
Pick the product for regular shocks
The same routine shock is 1 lb 5 oz of 65% cal-hypo or 1 lb 8 oz of dichlor, but what they leave behind adds up. 20 weekly shocks with dichlor add about 90 ppm of CYA, enough to take a pool from CYA 40 to 130 and its algae shock level from 16 to 52 ppm. With cal-hypo the same season adds about 70 ppm of calcium hardness. Liquid chlorine leaves only a little salt, so it is the one to use for shocks you repeat.
Work out your own numbers
Use the chlorine dosing calculator, pool shock calculator and the CYA 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 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? See all pool care guides.