What Is Simple Interest?
Simple interest is a method of calculating the cost of borrowing money or the earnings on a deposit. The interest amount is based solely on the original principal balance and the interest rate, multiplied by the length of time the money is borrowed or invested. Because the base never changes, each period produces the same amount of interest.
How It Is Calculated
The standard formula is:
Interest = Principal × Rate × Time
Here, the rate is usually an annual percentage and the time is expressed in years. For example, if you borrow 1,000 at an annual simple interest rate of 5 percent for 3 years, the interest is 1,000 × 0.05 × 3, which equals 150. The total amount owed at the end of the term is 1,150.
Simple Interest Compared With Compound Interest
The key difference is what the interest is applied to. With compound interest, accrued interest is added to the principal, and future interest is calculated on that larger total. Over short periods the two methods produce similar results, but over long periods compounding typically generates noticeably more growth on savings or more cost on debt.
Where It Is Used
Simple interest is common in certain consumer and short-term loans, such as some auto loans and personal loans, and in some short-term bonds and certificates. Many loan agreements specify whether interest is simple or compounded, so reading the terms carefully helps clarify how much a borrower will actually pay.
Why It Matters to Investors
Understanding simple interest helps investors compare offers accurately. A savings product that advertises a rate may be using simple or compound calculation, and that choice affects the final return. Knowing the method also makes it easier to estimate the cost of debt and the earnings from fixed-income holdings over a specific period.
Updated October 20, 2026. For general education only; not financial, tax or legal advice.