How to Calculate Interest & Principal on a Personal Loan

1). Find the periodic interest rate by dividing the annual rate by the payments per year. The periodic rate is the rate charged between payments. If you make monthly payments, the periodic rate would be the monthly interest rate. For example, if your annual rate equals 7.02 percent and you make monthly payments, divide 0.0702 by 12 to get 0.00585.
2). Calculate the interest paid on your personal loan. Multiply the periodic interest rate on your personal loan by the loan balance. In the example, if you still owe $6,000 on your loan, the interest paid equals $35.10 (0.00585 x $6,000).
3). Get the principal paid. Subtract the interest paid from the amount of your loan payment to figure out how much of the principal you paid down. In the example, if your monthly payment is $91.41, the principal paid equals $56.31 (91.41 - 35.10).