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 By Brad Hostetter Updated March 28, 2017 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. 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. Learn how to calculate monthly interest for loans, bank accounts, credit cards, and more to see how much you pay (or earn) per month. To calculate interest, start by determining the principal, which is the amount of money you'll be calculating interest on. Next, determine the interest rate, which was agreed upon at the outset and should be presented in a decimal number for calculation. Then, determine the length of time, or term, the interest will be accruing, which is measured in years. 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. How to calculate interest expense December 28, 2018 / Steven Bragg. Interest expense is the cost of the funds that have been loaned to a borrower. To calculate interest expense, follow these steps: Determine the amount of principal outstanding on the loan during the measurement period. How to Calculate Interest Earned on Savings Now that you've gathered the information you'll need to calculate for interest, it's time to go over the formulas you'll be using. Even though the interest calculated on personal savings accounts is almost always calculated as compound interest, we'll go ahead and cover both simple and compound interest. But learning how to calculate interest yourself serves two purposes. First, it makes it easy for you to quickly estimate interest on your own, even if you can't do exact calculations in your head. And second, it gives you an appreciation for just how quickly interest rates can add up. 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? Well, essentially, interest is a fee you pay for using someone else’s (usually the bank’s) money.