UtilityBaseCalculators
Simple Interest Calculator
Calculate simple interest with I = P × r × t, and see the interest and final balance.
Updated July 10, 2026
How to use the simple interest calculator
- 1Enter the principal amount.
- 2Enter the annual interest rate.
- 3Enter the time in years or months.
- 4Read the interest and final balance.
Common uses
- Finding interest on a short-term loan
- Calculating car loan interest
- Homework using the I = PRT formula
- Estimating simple bond interest
Frequently asked questions
What is the simple interest formula?
Interest equals principal times rate times time: I = P × r × t, where r is the annual rate as a decimal and t is the time in years. For $1,000 at 5% for 3 years, that's 1000 × 0.05 × 3 = $150.
How is simple interest different from compound interest?
Simple interest is calculated only on the original principal, so it grows in a straight line. Compound interest is calculated on the principal plus accumulated interest, so it grows faster over time.
When is simple interest used?
It's common for many short-term loans, most car loans, and some bonds, where interest is charged on the principal only. Savings accounts and mortgages usually use compound interest instead.
About this tool
The simple interest calculator works out interest earned or owed on a principal using the formula I = P × r × t, where r is the annual rate and t is the time. Unlike compound interest, simple interest is charged only on the original principal, so it grows in a straight line — the model used for many short-term and car loans. Enter the principal, rate, and time in years or months to see the interest and the final balance, with the formula shown. Everything is calculated in your browser.
Like most tools on UtilityBase, the simple interest calculator runs entirely in your browser — nothing you enter is uploaded or stored on a server. It's free to use with no account required. Browse more calculators here.
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