FC Shock Level?


LifeTime Supporter
Mar 22, 2010
Ridgeway, SC
Overnight FC loss from 11.5 to 7..... so, time to shock. If I look at the Chlorine/CYA chart, with my CYA at 80 (about the only thing in my pool where I want it!), it recommends FC to 30ppm (SWCG). However, Pool Calculator, based on my current readings for TA, Borates, CH, CYA calculates 21ppm. I shocked to 21.... do I need to go to the 30 number?

I have a pretty Irish looking pool now, by the way. Is that to be expected?

It's exciting to "be in control" of what's in the pool - as opposed to at the mercy of the pool store - but, I'll sure be glad when I get to some level of stability!



LifeTime Supporter
Jan 11, 2009
Cupertino, CA
The Pool Calculator and the FC/CYA chart use different methods to arrive at their shock levels. The exact shock level is not critical; anywhere between them will be fine. Pick a target and stick to it, that's the important thing.


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
The reccomendations in the pool calc as well as pool school are guidelines. You will do little harm if you stray somewhat from those guidelines so I'd split the difference and set my goal to 25ppm. (21 would work (a little slower), also and 30 would work as well. (perhaps a little faster)

AS you become more familiar with testing and the parameters, you see that none of the numbers are "set in cement"....i.e. your pool won't go green if FC is 20 or be perfect if FC is 21.

By far, the most important thing is that you choose a number (say, 25ppm) and then stick with it. Always replenish your chlorine back to that number so the subsequent loss is measurable.