Simple Interest Calculator
Calculate interest on the original principal from an annual rate and a time in years, including fractional years.
Calculator arithmetic runs locally in your browser and does not require an API.
The interactive calculator loads here when JavaScript is enabled.
How to use Simple Interest Calculator
Use 0.5 years for six months. Keep the time unit consistent with the annual rate.
Formula and method
I = P × r × t; final amount = P + I. Divide the annual percentage by 100 to obtain r.
Understanding the results
The interest amount grows linearly with time. Accrued interest does not join the principal, so this is different from compound growth. Fees and taxes are excluded.
Worked example
For 1,000 at 5% for 10 years, I = 1,000 × 0.05 × 10 = 500. The final amount is 1,500.
These are mathematical estimates, not financial advice or an offer of credit.
Frequently asked questions
Does the earned interest earn more interest?
No. Only the original principal earns interest in this model.
Can the duration be less than a year?
Yes. Enter a decimal year, such as 0.25 for three months.
Can this model a monthly amortized loan?
No. An amortized loan reduces principal with each payment; use the Loan Calculator for that repayment model.