Simple Interest Calculator
Calculate simple interest earned on your principal, compare it to compound interest, and visualize growth over time.
Understanding Simple Interest
Simple interest is one of the most straightforward ways to calculate the cost of borrowing money or the return on an investment. Unlike compound interest, which reinvests earned interest back into the principal, simple interest is always calculated on the original amount only. This makes it predictable and easy to understand — you earn (or owe) the exact same dollar amount of interest every year.
The Simple Interest Formula
The formula is I = P × r × t, where I is the total interest, P is the principal (starting amount), r is the annual interest rate expressed as a decimal, and t is the time in years. For example, $10,000 at 5% for 3 years earns $10,000 × 0.05 × 3 = $1,500 in interest. The total amount you'd have is $11,500. Notice that you earn exactly $500 per year, every year — the interest doesn't change.
Simple vs Compound Interest
The key difference is how earned interest is treated. With simple interest, $10,000 at 5% earns $500 every year regardless of how long you hold it. With compound interest, the first year also earns $500, but the second year earns interest on $10,500 instead of $10,000 — so you earn $525. This snowball effect means compound interest always produces more over time. The gap widens significantly with higher rates and longer time periods.
When Simple Interest Works in Your Favor
If you're a borrower, simple interest loans save you money. Auto loans, for example, commonly use simple interest — your interest is calculated on the remaining principal balance, and making extra payments directly reduces future interest charges. For investors, understanding simple interest helps you evaluate whether a quoted return accounts for compounding. A "5% simple return" over 10 years is very different from a "5% compounded annually" return — the compound version yields about 28% more total interest.
Practical Applications
Treasury bills, short-term promissory notes, and some bonds use simple interest. When you see a bond paying a "5% coupon," that's simple interest on the face value paid out periodically rather than reinvested. Understanding this distinction helps you compare investment options accurately and set realistic expectations for returns.
Frequently Asked Questions
What is simple interest?
Simple interest is calculated only on the original principal amount. Unlike compound interest, earned interest is not added back to the principal, so you earn the same dollar amount of interest each period. The formula is I = P × r × t, where P is principal, r is the annual rate, and t is time in years.
How is simple interest different from compound interest?
With simple interest, you earn interest only on your original deposit. With compound interest, you earn interest on both the original deposit and on previously earned interest. Over time, compound interest grows exponentially while simple interest grows linearly. The longer the time period and higher the rate, the bigger the gap between the two.
Where is simple interest commonly used?
Simple interest is used for most auto loans, some personal loans, U.S. Treasury bills, and short-term lending between individuals. It's also used to calculate interest on bonds (coupon payments) and some certificates of deposit that pay interest out rather than reinvesting it. Credit cards and savings accounts typically use compound interest instead.
Is simple interest better for borrowers or savers?
Simple interest is generally better for borrowers because you pay less total interest over the life of the loan compared to compound interest. For savers and investors, compound interest is better because your money grows faster. If you're comparing two offers, always check whether the quoted rate uses simple or compound interest.