How Much Shock Does My Pool Need?
By The Aqualitx team at STEMania · Updated
"Shocking" just means raising free chlorine well above its everyday level for a short time, to burn off whatever the normal dose isn't keeping up with. How much it takes depends on why you're shocking, how big the pool is, and how much stabilizer is in the water.
Why you're shocking sets the target
- Combined chlorine above 0.5 ppm (chloramines, the "chlorine smell"): raise free chlorine to about 10 times the combined chlorine reading. At 0.8 ppm CC, that's 8 ppm FC. This is breakpoint chlorination — see free chlorine vs combined chlorine.
- Routine shock after a pool party, a storm or a heat wave: raise free chlorine by about 10 ppm.
- Algae: much higher, and held there until the water clears — scale it to your CYA (below).
How much chlorine that takes
To raise free chlorine 1 ppm in 10,000 gallons:
- Liquid chlorine (12.5%): about 10 fl oz
- Cal-hypo (65%): about 2 oz by weight
So a routine 10 ppm shock in a 15,000-gallon pool is 10 × 10 × 1.5 = 150 fl oz of liquid chlorine (just over a gallon), or 10 × 2 × 1.5 = 30 oz of cal-hypo. The shock calculator does the math for your pool and reading.
Stabilizer changes the target
Chlorine's strength depends on how much CYA is holding it in reserve, so a shock level that works at 30 ppm CYA does far less at 80. For algae, a common rule is to raise free chlorine to about 40% of your CYA and hold it there until the water is clear and an overnight test loses less than 1 ppm. At 40 ppm CYA that's 16 ppm FC; at 80 it's 32 — usually a sign it's time to lower CYA instead. See ideal CYA levels.
Which shock to use
- Liquid chlorine adds nothing but chlorine and a little salt. It's the default for most pools.
- Cal-hypo adds calcium along with chlorine — fine for plaster pools that run low on calcium, but it builds up in hard-water areas.
- Dichlor adds stabilizer with every dose — about 0.9 ppm of CYA for each 1 ppm of chlorine. Fine now and then; as a habit it drives CYA up fast.
How to shock
- Test first: free chlorine, combined chlorine, pH and CYA. Bring pH to about 7.4 before shocking — chlorine works much better there.
- Shock at dusk, with the pump running. The sun burns chlorine off faster than it can work.
- Pour liquid chlorine slowly in front of a return jet. Pre-dissolve cal-hypo in a bucket of pool water, and never mix it with any other chemical.
- Leave the pump running overnight and re-test in the morning.
Signs you didn't use enough
If combined chlorine is still there in the morning, or free chlorine is close to zero, the shock was used up before it finished the job. Re-dose to the same target. Falling short of breakpoint can make chloramines worse for a while, so a second round beats calling it done.