This simple mortgage calculator will calculate the monthly principal and interest payments on a mortgage. Just enter the loan amount, the number of years (example - 30 years), and the interest rate. If you're unsure of an interest rate to start with please see the chart below left (automatically updated weekly). Please note that the Freddie Mac rates below are for primary residence "conforming" loans with 20% down. A conforming loan is limited to a maximum loan amount of $417,000 for 2012 except for these areas. Investment loans will generally carry a higher interest rate