How do you calculate interest?

Use the simple interest calculator to find A, the final investment value, using the simple interest formula. A=P (1+rt) where P is the principal amount of money to be invested at an interest rate r% per period for t number of time periods. Where R is in decimal form; R=R/100; r and t are in the same units of time.

Well let’s say you put $100 in the bank. And they charge 10% interest. And 10% of $100 is 10. So it would be $10 interest.