Joyce, here's a simple rule of thumb: Whatever the strength of the chlorine is...5.25%, 6%, 8.25%, 12%...
One gallon will add EXACTLY that much free chlorine to 10,000 gallons of water.

So...one gal of 5.25% adds 5.25 ppm of free chlorine to 10,000 gallons. One gal of 6% adds 6ppm, and so on.

If your pool is 20,000 gallons, you need 2 gallons to get the same effect.

If your pool is 5,000, you need 1/2 gallon to get the same effect.

Adjust accordingly to your pool's size and the amount of bleach/LC you add.