What this simple interest calculator does
Simple interest is the most honest kind of interest there is: a fixed percentage of the original amount, for every year the money is borrowed or invested, with no compounding and no surprises. This calculator takes the three ingredients — principal, annual rate, and time — and returns the interest, the total, and the interest broken down per month, using the classic formula I = P × r × t.
You'll run into simple interest more often than you'd expect. Many auto loans accrue interest on a simple daily basis. Promissory notes between family members or business partners are almost always written with simple interest, because it's easy for both sides to verify. Some CDs, treasury instruments, and most late-fee and penalty calculations use it too. And if you're a student, it's the version of interest your math or personal-finance class starts with — this tool will confirm your homework answers and show the steps implied by the numbers.
The per-month figure in the results is worth a look when you're lending or borrowing informally: it translates an abstract "4% for three years" into "this loan costs about $16.67 a month," which is usually the framing that makes an agreement between two people feel fair and concrete.
How to use it
- Enter the principal — the original amount borrowed or invested.
- Enter the annual rate. If your agreement quotes a monthly rate, multiply it by 12 first.
- Enter the time in years or months — the months option handles the divide-by-12 conversion for you.
- Read the interest, the total, and the monthly equivalent. Use Copy summary to paste the figures into a note or an agreement.
Frequently asked questions
What is the simple interest formula?
I = P × r × t — interest equals principal times the annual rate (as a decimal) times time in years. For $5,000 at 4% over 3 years: 5,000 × 0.04 × 3 = $600 of interest, for a total of $5,600. That's the exact calculation this tool performs.
What's the difference between simple and compound interest?
Simple interest is always calculated on the original principal, so it's the same dollar amount every year. Compound interest is recalculated on the growing balance, so it snowballs. At 5% for 3 years the gap is small ($750 versus about $788 on $5,000); at 5% for 30 years it's the difference between $7,500 and over $16,600 in interest. For long horizons, use our compound interest calculator instead.
Where is simple interest actually used?
Many auto loans (accruing daily on the outstanding balance), short-term personal loans, promissory notes between individuals, some CDs and bonds, and most late-payment penalty math. Its appeal is transparency — anyone with a calculator can verify the charge, which is exactly why informal lending agreements favor it.
How do I handle months or partial years?
Time in the formula is in years, so convert: divide months by 12. Nine months is 0.75 years; 18 months is 1.5. The months option in this calculator does the conversion automatically, and decimals work fine in the years field too — 2.5 years is a valid input.
Is this calculator free? Do you keep my numbers?
Free and unlimited, no account. Everything runs in your browser; nothing is stored or transmitted.
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This calculator is for informational purposes only and is not financial advice. Loan and deposit agreements vary in how they accrue interest; confirm the method your agreement uses.