Simple interest is money you can earn by initially investing some money (the principal). A percentage (the interest) of the principal is added to the principal, making your initial investment grow! How to Calculate Interest Rate. If you know the amount of a loan and the amount of interest you would like to pay, you can calculate the largest interest rate you are willing to accept. You can also look at your interest payments in a year. Interest (An Introduction) Interest: how much is paid for the use of money (as a percent, or an amount) Money is Not Free to Borrow. People can always find a use for money, so it costs to borrow money. Learn how to calculate monthly interest for loans, bank accounts, credit cards, and more and see how much you pay (or earn) per month. Learn how to calculate monthly interest for loans, bank accounts, credit cards, and more and see how much you pay (or earn) per month. The Balance How to Calculate Monthly Interest. Simple interest (SI) is a quick method of calculating the interest charge on a loan. Use this simple interest calculator for you to do a loan, deposit rate calculation based on the SI formula. How to Calculate Interest Payments. Not all loans are created equal. Understanding how to calculate a monthly payment, as well as the amount of interest you'll pay over the life of the loan, are very helpful in choosing the perfect loan. Calculate interest on loan. What is interest? When you take out a loan, whether it’s a car loan, home loan or credit card, you’ll have to pay back both the amount you borrowed and interest on top of it. But what do we mean by that? Use the compound interest calculator to gain a picture of how the interest on your savings or investments might grow over a period of months and years. Using the compound interest formula, you can determine how your money might grow with regular deposits or withdrawals. See how to calculate loan interest every month or over the life of a loan. Get tips for standard home or auto loans as well as credit card debt. The ability to calculate interest is a vital part of understanding how well you are managing your finances. Not only is interest likely a major player in whatever debt you may owe, but it can also be a key factor in making your money grow.