I spent some time looking for a simple compound interest calculation function in .NET today. I couldn't find anything usable, so I grabbed the source formula and reduced it to .NET.

Here's the formula:

A = P * (1 + r / n) ^ n * t
A = final total amount
P = principal amount
r = interest rate (decimal)
n = number of periods per year
t = number of years

The C# code:

decimal CalculateTotalWithCompoundInterest(decimal principal, decimal interestRate, int compoundingPeriodsPerYear, double yearCount)
    return principal * (decimal)Math.Pow((double)(1 + interestRate / compoundingPeriodsPerYear), compoundingPeriodsPerYear * yearCount);

For example, you can calculate the total loan amount for $10,000.00 (principal and interest), compounded daily for 2 years like so:

decimal total = CalculateTotalWithCompoundInterest(10000m, 8.5m, 365, 2);