Simple Interest Calculator
Calculate simple interest and total repayment from principal, annual rate, and time in years. Free and private, right in your browser.
Calculate interest that accrues only on the original principal, at a flat annual rate over a set time period, without any compounding folded in. Reach for it when checking a short-term loan, an add-on car loan, a promissory note, or a bond that quotes a simple annual rate.
How to use it
- Enter the principal amount
- Set the annual interest rate and time period
- Read off the interest earned and total payoff
Frequently asked questions
- How is this different from compound interest?
- Simple interest only ever applies to the original principal, so it grows in a straight line. Compound interest applies to principal plus previously earned interest, so it grows faster over time. Use this tool when a loan, bond, or promissory note specifies a flat annual rate with no compounding.
- What's the actual formula being used?
- I = P × r × t, where P is principal, r is the annual interest rate (as a decimal), and t is time in years. Total payoff or maturity value is just P + I.
- Can I calculate for months or days instead of years?
- Yes. Enter your time period in the units you have and the tool converts it to years internally (e.g., 90 days or 6 months), so you don't have to do the fraction math yourself.
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Nothing is uploaded
The file never leaves your device. There is no server to send it to, so there is nothing to leak, log, or subpoena.
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No signup, no limits
No account, no email, no daily cap, no watermark, and no paid tier waiting behind the third file.
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Works offline
Load the page once and it keeps working with the network off, because all the work was always happening locally.