Simple Interest Calculator
Calculate simple interest in seconds. Enter a principal, an annual interest rate, and a time in years or months to instantly see the total interest earned and the final balance, using the classic I = P × r × t formula.
ToolsSoup's Simple Interest Calculator works out the interest on a loan, deposit, or investment using the classic simple interest formula. Enter the principal, an annual interest rate, and a time in years or months, and you instantly see the total interest earned and the final balance. Unlike compound interest, simple interest is charged only on the original principal, so the amount added is the same every period — which makes it the standard way to price many short-term loans, car loans, and bonds. Everything runs in your browser: no uploads, no sign-up, and your figures never leave your device.
What is simple interest?
Simple interest is interest calculated only on the original principal — the amount you borrowed or deposited — and never on any interest that has already accrued. Because the base never changes, the interest added each period stays constant, so the balance grows in a straight line rather than the accelerating curve you get with compound interest. Simple interest is common for short-term personal loans, many car loans, and some bonds and savings products, where it keeps the math predictable for both sides.
How to calculate simple interest
Simple interest needs just three inputs, and this tool does the math automatically as you type:
- Enter the principal — the amount borrowed, deposited, or invested.
- Enter the annual interest rate as a percentage.
- Enter the time and choose years or months. Read the total interest and final balance below.
What is the simple interest formula?
The formula is I = P × r × t, where I is the interest, P is the principal, r is the annual interest rate written as a decimal (5% becomes 0.05), and t is the time in years. The final balance is simply the principal plus the interest, A = P + I = P × (1 + r × t). When you enter a time in months, the calculator converts it to years by dividing by 12 before applying the formula, so a 6-month term uses t = 0.5.
What's the difference between simple and compound interest?
Simple interest is calculated only on the original principal, so it grows by the same fixed amount every period. Compound interest is calculated on the principal plus all previously earned interest, so the amount added grows each period. Over a single year the two match, but over longer terms compound interest pulls ahead because interest starts earning interest. If you need to model compounding instead, use ToolsSoup's compound interest calculator; this tool keeps things to the straight-line simple interest used by most short-term loans.
Why use this simple interest calculator?
- Instantly shows the total interest and final balance from the principal, rate, and time.
- Accepts the time in years or months and converts months to years for you.
- Uses the exact I = P × r × t formula, rounding money to two decimals.
- Handles 0% rates and non-round amounts cleanly, with clear validation messages.
- Updates live as you type, runs entirely in your browser, and is 100% free with no ads or sign-up.
Frequently asked questions
How do I calculate simple interest?
Use the formula I = P × r × t, where P is the principal, r is the annual rate as a decimal, and t is the time in years. Enter your numbers above and the calculator returns the interest and final balance instantly.
What is the simple interest on $1,000 at 5% for 10 years?
Simple interest of 1000 × 0.05 × 10 = $500, so the final balance is $1,500. Change any field above to model your own principal, rate, and time.
Can I enter the time in months?
Yes. Switch the time unit to months and the calculator divides the value by 12 to convert it to years before applying the formula, so a 6-month term uses t = 0.5 years.
How is simple interest different from compound interest?
Simple interest is charged only on the original principal, so the same amount is added every period. Compound interest is charged on the principal plus accrued interest, so it grows faster over time. For longer terms, compound interest produces a larger balance.
What happens if I enter a 0% interest rate?
With a 0% rate there is no interest, so the total interest is zero and the final balance equals the principal you entered.