I am trying to use an excel work sheet to figure out what the monthly interest and principal is from the over all mortgage payment and show the reduction in the overall loan but for some reason the amount of the interest and the principal does not equal what the monthly over all payment is and it's driving me crazy. What am I doing wrong? a perfect example would be: monthly payment is $800.00, the interest for that payment is showing as $445.50, but the principal amount is showing as $367.62. the principal and interest amounts don't even come close to equalling $800.00.