What is Simple Interest The rate at which you borrow or lend money is called the simple interest. If a borrower takes money from a lender, an extra amount of money is paid back to the lender. The borrowed money which is given for a specific period is called the principal. The extra amount which is paid back to the lender for using the money is called the interest. You calculate the simple interest by multiplying the principal amount by the number of periods and the interest rate. Simple interest