In order to calculate the account's ending balance (also known as Future Value, or FV) by using either simple or compound (w/re-investing) interest formulas, follow the steps:

1. Enter the Principal amount as number (dollars.cents)
2. Enter the annual interest rate as a number (no % sign)
3. Enter the number of payments per year (frequency)
4. Enter the period of accruing the interest (years)
5. Select either "Simple" or "Compound" radio-button
6. Click the on-screen button to calculate the balance

The formula for the ending balance calculation using simple interest is shown below:

FV = PV(1+rt/100)

The formula for the ending balance calculation using compound interest is shown below:

FV = PV(1 + r/n/100)^(nt)

More reading on this topic is available online at:

1. Compound Interest Calculator
2. Compound Interest on wiki
3. Compound Interest on wiki