Simple Interest Calculator
Calculate simple interest (I = P × r × t) from a principal, annual interest rate, and time period in years, months, or days — get the total interest and final amount plus a year-by-year accrual schedule.
Input
Output
| Metric | Value |
|---|---|
| No data yet | |
| Year | Interest This Year | Cumulative Interest | Balance |
|---|---|---|---|
| No data yet | |||
Guides
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
- Enter the principal amount — the amount borrowed, invested, or deposited.
- Enter the annual interest rate as a percentage.
- Set the time period, in years, months, or days — pick whichever matches how the term was quoted, without converting it by hand first.
- 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.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/simple-interest-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"principal": "10000",
"rate": "5",
"time": "3",
"timeUnit": "years",
"currency": "USD"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `simple-interest-calculator` tool (Simple Interest Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.