Unpaid Principal Balance
You want to know your unpaid principal loan balance that is remaining after you make your first payment, it is easy to compute. First, take your principal loan balance of $100,000 and multiply it times your 6% annual interest rate. The annual interest amount is $6,000. Divide the annual interest figure by 12 months to arrive at the monthly interest due. That number is $500. Since your December 1 amortized payment is $599.55, to figure the principal portion of the payment, you would subtract the monthly interest number ($500) from the principal and interest payment ($599.55). The result is $99.55, which is the principal portion of your payment. Now, subtract the $99.55 principal portion paid from the unpaid principal balance of $100,000. That number is $99,900.45, which is the remaining unpaid principal balance as of December 1. You now know that your principal balance after your December payment will be $99,900.45. To figure your remaining balance after your January 1 payment, you will compute it using the new unpaid balance: $99,990.45 x 6% interest = $5,994.03 / by 12 months = $499.50 interest due for December. Your January payment is the same as your December 1 payment because it is amortized. It is $599.55. You will subtract the interest due for December of $499.50 from your payment. That leaves $100.05 to be paid to principal on your loan.