Simple Interest Calculator – Compute Interest on Principal
Calculate simple interest on principal amount.
How to use this simple interest calculator
- Enter the Principal Amount — the initial sum being borrowed, lent, or deposited.
- Enter the Annual Interest Rate as a percentage.
- Select the Time Period unit: Years, Months, or Days.
- Enter the time value in the unit you selected.
- Click the "Calculate" button.
- Review the Simple Interest earned and the Total Amount (principal plus interest).
Frequently Asked Questions
What is simple interest?
Simple interest is calculated only on the principal amount. Unlike compound interest, it does not include interest on previously earned interest.
How is simple interest calculated?
Simple Interest = (Principal × Rate × Time) / 100. The formula assumes the rate is annual, and time can be in years, months, or days.
What is the difference between simple and compound interest?
Simple interest is calculated only on the principal, while compound interest is calculated on the principal plus accumulated interest. Compound interest grows faster over time.
Related Tools
A simple interest calculator helps students, borrowers, lenders, and savers quickly estimate the interest earned or owed on a fixed principal amount over a set period. This tool is useful for classroom finance exercises, working out interest on a short-term loan between friends or family, checking the return on a fixed deposit, or comparing straightforward loan and savings offers. By using a simple interest calculator, you can see exactly how much interest accrues and what the total repayable or maturity amount will be, without needing to work through the formula by hand. The tool supports entering the time period in years, months, or days, and computes the result instantly and entirely in your browser for privacy and speed.
What is simple interest calculator?
A simple interest calculator applies the standard simple interest formula: Interest = (Principal × Rate × Time) / 100, where the rate is an annual percentage and time is expressed in years. If you enter the time period in months, the tool converts it to years by dividing by 12; if entered in days, it divides by 365. The calculated interest is then added to the principal to produce the total amount, representing what would be owed on a loan or received from a deposit. Unlike compound interest, this formula calculates interest only on the original principal for the entire period, so the interest amount grows linearly rather than accelerating over time. All conversion and calculation steps happen instantly in your browser.
Common use cases
- Estimating interest on a short-term personal loan between friends or family
- Checking the maturity value of a fixed deposit or savings certificate
- Comparing straightforward loan offers before choosing a lender
- Completing classroom or textbook finance exercises on interest calculations
- Working out interest owed on an informal or private lending arrangement
- Getting a quick estimate before using a more detailed compound interest or EMI calculator
Limitations and common mistakes
- Uses the simple interest formula only, so it does not account for compounding — actual returns or costs on compound-interest products will differ.
- Converts days to years using a fixed 365-day year, without adjusting for leap years.
- Assumes a single fixed annual rate applied uniformly across the whole time period, with no support for variable or tiered rates.
- Displays results to two decimal places without any currency symbol, since the tool is currency-agnostic.
- Requires a positive principal, a non-negative rate, and a positive time value; invalid or zero inputs trigger a validation alert instead of a result.