Simple Interest Calculator
Work out the interest and total cost of a loan using a flat, simple rate.
This calculator runs entirely in your browser. Nothing you type is uploaded or saved.
How to use this tool
This simple interest calculator works for any flat rate loan, so it is handy for sizing up a personal loan, a short-term cash advance, or a basic lending fee from a friend. Enter three things:
- The principal, which is the amount you borrow or invest.
- The annual interest rate as a percentage, for example 5 for five percent.
- The loan term, then pick whether that number is in years, months, or days.
Press Calculate to see the total interest and the full amount you will repay. Use Copy result to grab the numbers for an email or a spreadsheet.
The formula for simple interest
Total = Principal + Interest
Here the rate is written as a decimal (5% becomes 0.05) and the time is measured in years. When you choose months or days, the calculator converts the term into a fraction of a year first: months are divided by 12 and days are divided by 365. Simple interest is "flat" because it is always charged on the original principal. Unlike compound interest, it never charges interest on interest, which is why the math stays so easy.
A real example
Say you take a $10,000 personal loan at a 5% annual rate for 3 years. The interest is 10,000 x 0.05 x 3 = $1,500. Add that to the principal and the total you repay is $11,500. If the same loan ran for 18 months instead, the time becomes 18 / 12 = 1.5 years, the interest is 10,000 x 0.05 x 1.5 = $750, and the total repayment is $10,750. This makes it quick to compare the flat rate credit cost of different loan lengths.
Common questions
What is the difference between simple and compound interest?
Simple interest is charged only on the original principal, so it grows in a straight line. Compound interest adds earned interest back to the balance, so future interest is charged on a larger and larger amount. For the same rate and term, compound interest always costs more.
Is this the same as the APR on my loan?
Not exactly. APR can fold in fees and may compound, while this tool gives a plain flat-rate estimate. Use it to understand the basic lending fee, then check your loan agreement for the official APR and any extra charges.
Can I use this for an investment instead of a loan?
Yes. The same formula tells you the interest you would earn on a deposit or bond that pays simple interest. The "total" then represents your principal plus the interest you collect.
Why convert months and days into years?
The interest rate you enter is an annual rate, so the time in the formula must also be in years. The calculator divides months by 12 and days by 365 so everything is measured on the same yearly scale.
Does it handle very large numbers or decimals?
Yes. You can enter decimals for the rate and term, and large principals are formatted with commas in the result. All values are processed locally in your browser for privacy.
This calculator gives an educational estimate of simple interest only. It is not financial advice. Check your lender's terms for the exact cost of any loan.