Your interest, a surcharge on a loan amount, should be stated in annual terms. That means, to calculate a monthly payment with 10 percent interest, you are really calculating a portion of that interest that must be added to the base amount each month. By figuring out the term of the loan and then adding in a corresponding monthly sum for the interest, you should be able to ascertain the monthly payment. Check the terms of the loan to ensure there are no special riders or conditions regarding your monthly payments.

Find the base amount of the loan, and figure the monthly base payment. For example, if you borrow $4,800 on a 12-month term, divide $4,800 by 12 to get the base monthly payment. Here, it comes to $400.

Divide the interest rate by 12. In this example, 10 percent (0.10) divided by 12 comes to 0.00833.

Multiply the base amount by the decimal from Step 2. In this example, $4,800 times 0.0083 yields $40.00. That is the month's interest.

Add the interest from Step 3 to the base monthly payment. Here, $400 plus $40 interest comes to $440 as a monthly payment.