Calculate simple interest and the final amount from a principal, rate, and time in your browser. Estimates only, not financial advice.
Simple Interest Calculator Online
Calculate simple interest and the final amount from a principal, rate, and time in your browser. Estimates only, not financial advice.
The Simple Interest Calculator runs entirely in your browser and the figures you enter never leave your device. Results are estimates only and are not financial advice.
Simple Interest Calculator finds the interest earned and the final amount owed or earned from a principal, an annual interest rate, and a time period. It uses the classic I = P × r × t formula, applying the rate to the original principal only for every period of the term rather than compounding on top of previously earned interest. Enter a principal, an annual rate as a percentage, and a time period in years, months, or days, and it instantly shows the interest and the resulting total. It is useful for short-term loans, promissory notes, certain bonds, and any calculation where interest is described as a flat rate rather than compounded. These are estimates only and not financial advice — confirm figures with a lender or advisor before acting on them. Everything runs locally in your browser.
Features
Computes interest with the standard I = P × r × t simple-interest formula
Shows both the interest earned and the total amount together
Accepts time in years, months, or days with a one-click unit switch
Handles fractional time periods, such as 18 months or 45 days
Copy button for a plain-text summary of principal, rate, time, interest, and total
Clear error message when principal, rate, or time is missing or invalid
Contrasts simple interest with compound interest so you know which formula applies
Runs entirely in your browser with no financial figures sent anywhere
How to use the Simple Interest Calculator
Enter the principal amount you are lending, borrowing, or investing
Enter the annual interest rate as a percentage
Enter the time period and choose years, months, or days
Read the interest earned and total amount, and copy the summary if needed
Example
Input
$1,000 at 5% for 3 years
Output
Interest: $150
Total: $1,150
Common errors & troubleshooting
The rate was entered as 0.05 instead of 5, giving an interest figure that looks far too small. — Enter the annual rate as a percentage number, such as 5 for 5%, not as a decimal fraction.
The result looks lower than expected compared to a bank statement. — Banks and savings accounts usually compound interest. Simple interest only applies to the original principal, so it will always be lower than compound interest over the same term.
Time was entered in months but the total looks 12 times too large. — Switch the time unit to Months before typing the value — the calculator converts months or days to years internally based on the selected unit.
Interest shows as $0 even though the fields look filled in. — Check for stray spaces or letters in the principal, rate, or time fields — the Simple Interest Calculator needs plain numbers in each field.
Frequently asked questions
What formula does the Simple Interest Calculator use?
It uses I = P × r × t, where P is the principal, r is the annual interest rate as a decimal, and t is the time in years. The total amount is the principal plus that interest.
How is simple interest different from compound interest?
Simple interest is calculated only on the original principal for the entire term. Compound interest adds earned interest back to the principal periodically, so future interest is calculated on a larger balance. Try the Compound Interest Calculator to compare the two on the same numbers.
Can I enter the time period in months or days instead of years?
Yes. The Simple Interest Calculator has a Years / Months / Days toggle and converts your entry to years internally before applying the formula.
Does the Simple Interest Calculator give financial advice?
No. It provides estimates only, based on the numbers you enter, and is not financial advice. Confirm important figures with a lender or financial professional before making decisions.
What kinds of loans use simple interest?
Short-term personal loans, some auto loans, promissory notes, and certain bonds often quote a flat simple-interest rate rather than a compounded one, so this calculator matches how those terms are usually described.
Is my financial data uploaded anywhere?
No. The Simple Interest Calculator runs entirely in your browser. The amounts, rates, and time periods you enter never leave your device.
Related tools
Interest Rate Calculator — Solve for the interest rate of a loan from its amount, payment and term.
Loan Calculator — Calculate the monthly payment, total interest and payoff of any amortized loan.
Savings Calculator — See how a starting balance plus regular deposits grow with interest over time.
Investment Calculator — Solve for the ending amount, return, contribution or time of an investment.