Simple Interest Calculator
Enter the principal, interest rate, and time period to instantly calculate simple interest and the total amount payable.
How to use the Simple Interest Calculator
Enter the principal amount, the annual interest rate, and the time period in years, then click "Calculate." The tool instantly shows the interest amount and the total amount (principal plus interest).
What is simple interest?
Simple interest is calculated only on the original principal amount, unlike compound interest which also earns interest on previously accumulated interest. It grows at a steady, linear rate over time, using the formula: Interest = Principal × Rate × Time.
Simple interest vs compound interest
Simple interest is common for short-term loans, certain bonds, and some car loans, where the interest doesn't compound. For savings accounts, investments, and most loans where interest builds on itself over time, use the Compound Interest Calculator instead — the growth will typically be faster.
Frequently asked questions
What's the formula used?
This calculator uses I = P × R × T, where I is the interest, P is the principal, R is the annual interest rate (as a decimal), and T is the time in years. The total amount is P + I.
Is my data stored anywhere?
No. This tool is part of the Toolyard collection of free, browser-based utilities. All calculations run locally in your browser — nothing you enter is uploaded to a server or stored anywhere.
Simple interest versus compound interest
Simple interest is calculated only on the original principal amount for the entire loan or investment term — it never changes, regardless of how much time passes. Compound interest, by contrast, is recalculated on principal plus previously earned interest, so it grows faster over time. Simple interest is common in short-term loans, certain auto loans, and some bonds; compound interest is more typical for savings accounts, credit cards, and most long-term investments.
The formula behind the number
Simple interest is calculated as principal × rate × time, meaning a $1,000 loan at 5% annual interest for 3 years accrues exactly $150 in interest regardless of how the payment schedule is structured — there's no interest-on-interest effect to account for, which makes it more predictable and easier to calculate by hand than compound interest.
Common uses
Understanding exactly how much interest a short-term loan will cost before signing, checking a bond's expected return, and comparing a lender's simple-interest offer against a compound-interest alternative to see which actually costs less over the loan term are typical reasons to use this calculator.