Here is a little function that calulates the compound interest of an amount, over the specified period.

Function CompoundInterest(cStartAmount As Currency, fInterestPerPeriod As Single, lPeriods As Long) As Currency
    Dim i As Long
    Dim cTotal As Currency
    cTotal = cStartAmount
    For i = 1 To lPeriods
        cTotal = cTotal + (cTotal * (fInterestPerPeriod))
    Next i
    CompoundInterest = cTotal - cStartAmount
End Function

Private Sub cmdCalculate_Click()
    '// 100 = starting amount
    '// 0.1 = 10% Interest per period
    '// 2   = 2 periods
    MsgBox CompoundInterest(100, 0.1, 2)
End Sub