Calculation desk / interest & growth
Simple Interest Calculator
Estimate interest when the original principal earns a fixed annual rate without compounding.
Simple Interest Calculator: Calculate interest earned on an unchanged original principal at a fixed annual rate for a stated time in years. Earned interest is kept separate and never added to the amount earning further interest. The result shows that interest and the combined principal-plus-interest total, without modelling periodic repayments or deposits. Runs 100% locally in your browser with zero server file uploads.
- Category
- Calculators
- Runs
- In your browser
- Cost
- Free · no sign-up
- Availability
- Ready to use
Total amount
$1,150.00
- Interest earned
- $150.00
Interest = principal × (annual rate ÷ 100) × years; total = principal + interest- This is an educational/planning estimate only, not personalized financial advice.
- Actual products, taxes, fees, contribution dates, and returns differ; confirm terms with the relevant provider.
- Inputs follow common calculator conventions such as initial amount, recurring contribution, time, estimated annual rate, and compounding frequency described by Investor.gov: https://www.investor.gov/financial-tools-calculators/calculators/compound-interest-calculator
- Simple interest does not add earned interest back to the principal.
Formula and worked example
Interest = principal × (annual rate ÷ 100) × years; total = principal + interest
A worked example with sample values:
| Inputs | |
|---|---|
| Principal | $1,000.00 |
| Annual rate | 5% |
| Time | 3 years |
| Results | |
|---|---|
| Total amount | $1,150.00 |
| Interest earned | $150.00 |
An unchanged principal for three years
Let P be original principal, r annual interest rate as a decimal and t duration in years. Interest I = P × r × t. Total A = P + I. A rate entered as 5 means 5%, so the formula uses r = 0.05 rather than multiplying the principal by 5.
For P = $1,000, an illustrative 5% annual rate and t = 3 years, I = 1,000 × 0.05 × 3 = $150. The total is 1,000 + 150 = $1,150. Each full year adds $50 under these assumptions; the second year's interest is not calculated on $1,050. At a zero rate, interest is zero and the total remains $1,000.
Simple rate, yield and purchasing power
OpenStax's simple-interest treatment (https://openstax.org/books/intermediate-algebra-2e/pages/2-2-use-a-problem-solving-strategy) sets out I = Prt. The rate in this equation is an annual simple rate. APY reflects compounding under CFPB's definition (https://www.consumerfinance.gov/rules-policy/regulations/1030/2/), and a loan APR can include charges beyond interest, so neither label alone establishes the right input here.
The total is nominal currency and leaves inflation and tax out. The rule of 72 estimates doubling with compound growth, so it is not the simple-interest doubling formula. Under positive simple interest, doubling requires r × t = 1: at the example's 5% rate, that is 20 years with principal unchanged, before any other adjustments.
How to use it
- Enter the amounts, rate and duration shown in the worked example or your own scenario.
- Calculate to update the result and review the assumptions.
- Compare the formula and example before using the result for planning.
Privacy & limitations
Inputs and calculations stay in your browser. No values are uploaded.
Related tools
Frequently asked questions
How do I enter six months of simple interest?
Use 0.5 years for half a year, or divide a month count by 12. If your agreement calculates days, identify its day-count basis first; 365-day, 360-day and actual-year conventions need not give the same time fraction. This page accepts years and does not select a contractual day count.
Does a compounding frequency change simple interest?
There is no compounding-frequency input because interest is always based on the original principal. At positive rates over several years, reinvested interest can produce a larger compound balance. Use compound interest when earned interest is credited back and itself earns a return.
Can I use this total for a loan with monthly repayments?
Not directly. The formula holds principal unchanged for the whole duration. Regular principal repayments change the outstanding balance and therefore the interest calculation. Use a loan payment or amortisation calculator when repayments reduce what is owed each month.
Embed this tool
Free to use on any website. Paste the code where the tool should appear; the link under it lets readers open the full page.
<iframe src="https://toea.com/embed/simple-interest-calculator" title="Simple Interest Calculator" width="100%" height="990" style="border:0;max-width:680px" loading="lazy"></iframe>
<p style="margin:4px 0 16px;font-size:13px"><a href="https://toea.com/simple-interest-calculator">Simple Interest Calculator</a> · TOEA</p>
<script>addEventListener("message",function(e){if(e.origin!=="https://toea.com"||!e.data||e.data.type!=="toea-embed-height")return;document.querySelectorAll("iframe").forEach(function(f){if(f.contentWindow===e.source)f.style.height=e.data.height+"px"})})</script>Free tool · runs in your browser · no account required