Free, online auto loan calculators allow you to skip the pencil-and-paper math and calculate your interest in seconds. You can experiment with different amounts, loan terms and rates to see how they affect your monthly payment and total interest charges.There are several ways to calculate your monthly auto loan interest payment. Online payment calculators have the formulas built in. If you work directly with a lender, they’ll plug the figures into their loan systems to calculate your interest and monthly pa ...