What Is The Formula For Compound Interest?

The multiplier applied to the principle is
k = (1 + r/n)^nt

Where r is the annual rate, n is the number of compounding periods in a year, t is the number of years.

Example
An amount is deposited into an account with 5% interest compounded quarterly. How much is there after 6 years?
k = (1 + 5%/4)^4*6
k = 1.0125²⁴
k ≈ 1.347351

There will be 1.347351 times as much in the account after 6 years as there was to start. In other words, at the end of the 6-year period, there will be 34.7351% more money in the account. If the original amount was $1000, the final amount will be $1347.35.