Formulas for Compound Interest
There are two formulas that are used for compound interest:

Discrete Compound Interest Formula
This is used for interest that is not compounded continuously.

The varibles are defined below:
A = the amount after time t
P = the initial amount or principal
r = the interest rate in decimal form
n = the number of compounding periods in 1 year
t = time in years.

If the interest is compounded yearly, n is 1.  If the interest is compounded semi-annually, n is 2.  If the interest is compounded quarterly, n is 4.  If the interest is compounded monthly, n is 12.
Continuous Compound Interest Formula
This is used for interest which is compounded continuously.

The varibles are defined below:
A = the amount after time t
P = the initial amount or principal
r = the interest rate in decimal form
t = time in years.