How are credit card interests calculated?

This is pretty nasty. 28% p.a for most credit cards. That's really ridiculous rate and the interest compounds monthly because the fees are charged every month when you do not pay off your credit card debt.
That's excluding all the late charges and additional interest fees.

The total amount of interest you are paying:

• 28% p.a interest on your credit card debt (X) you owe
• Additional late charges monthly (Y)
• (X +Y) x 28% p.a will compound agressively and very quickly.

Rule of thumb:

• NEVER get a credit card if you cannot control your spending
• Always pay off your credit card debt monthly in full
• Don't be too lazy to waive off your credit card annual fees (this compounds also if you dont pay it off)