How is critical rating calculated in Wizard101?

I have around 30 critical rating in Wizard101. Is that the percent of chance of critical? Or is it how much extra damage?

The way critical rating is calculated is that 500 = 100% of critical. So, 30 would be a 6% chance.
Its basically like this:
500 = 100
30 = xwhere x is the % of critical.
If you get a critical, you deal double the damage you would have normally done.