"Shock" is a verb that means raise chlorine levels high enough to kill anything living in the water, and metabolize all organic compounds. That's ALL it means. Chemicals called "shock" is a mis-nomer designed to fool you, the consumer, into thinking it's more than it is--namely some sort of chlorinating chemical.

To shock you add more chlorine, and it doesn't matter what the source of that chlorine is. Since I use bleach to chlorinate, I use bleach to shock--just a lot more of it.

It's VERY easy to calculate how much bleach to use. You need to know:
1) The volume of your pool in gallons (liters is OK, but then you measure the bleach in liters as well).
2) Your current Free-Chlorine (FC) level
3) Your level of stabilizer/CYA
4) Your shocking level of Free Chlorine which is based solely on your stabilizer level (see the Best Guess table).

You subtract your current FC from your shock-level FC and that tells you how much more FC you need.

Then you either use MWSmith2's Bleach calculator program to tell you how much to use, or you use the formula:

(1,000,000/pool volume) * (bleach concentration) * (gallons of bleach) = FC added

Or you can use the rule of thumb and guessitmate off of it:
1 gallon of regular bleach (5.25%) adds EXACTLY 5.25ppm of FC to 10,000 gallons of water. You need twice as much for 20,000 gallons OR to increase 10,000 gallons by 10.5ppm

1 gallon of Ultra bleach (6%) adds EXACTLY 6ppm of FC to 10,000 gallons,
A 3 quart jug of Ultra adds 4.5ppm of FC to 10,000 gallons.
etc.

For example:
Say your pool has 20,000 gallons
and your CYA is 40ppm
and your current FC is 4.5ppm.

At CYA=40, the correct shock level (from the best-guess table) is 15ppm.

That means you need 10.5ppm MORE of FC to shock your pool.

Well, since 1 gallon of 5.25% adds 5.25ppm to 10,000 gallons, you need 4 gallons of regular bleach to get to 15ppm.

That's all there is to it!

But that's why we ALWAYS want to know the gallonage, the current FC and the current CYA before telling you how much bleach to use.