Simple Interest Calculator

What is simple interest?

Simple interest is interest calculated only on the original principal — never on interest that's already accrued. That makes it linear: the same dollar amount of interest accrues every period, unlike compound interest where each period's interest is added to the base for the next. It's the formula behind short-term loans, certain bonds, and many auto/personal loans: I = P × r × t, where P is principal, r is the annual rate (as a decimal), and t is time in years.

How this tool works

1. Enter the principal amount — the amount borrowed, invested, or deposited.
2. Enter the annual interest rate as a percentage.
3. Set the time period, in years, months, or days — pick whichever matches how the term was quoted, without converting it by hand first.
4. Choose a currency for the formatted output.

The Summary shows the principal, rate, time period (converted to years), total interest, and total amount (principal + interest). The Year-by-Year Accrual table breaks the term into full years (with the final year prorated if the term doesn't land on a whole year) showing interest earned that year, cumulative interest, and running balance — both copyable and downloadable as CSV.

The formula

Interest = Principal × Rate × Time (time in years). Total Amount = Principal + Interest. Because interest never compounds, the interest earned in year 2 is identical to year 1 — the Year-by-Year table makes that flat, linear accrual visible at a glance, in contrast to a compound-interest schedule where each year's interest grows.

Simple vs. compound interest

Simple interest only ever applies to the original principal. If you need interest calculated on a growing balance instead — savings accounts, most investment growth — use the Compound Interest Calculator instead.

Common uses

• Estimating interest owed on a short-term or add-on personal loan
• Calculating simple-interest earned on a promissory note or certain bonds
• Checking a lender's quoted interest figure against the P × r × t formula
• Comparing how the same rate accrues over different time periods

Privacy

This tool runs entirely in your browser. Your numbers are never uploaded to a server.