Calculate simple interest, end balance, principal, rate, or term. Pick a tab, enter the known values, and get instant results with step-by-step math, schedule, and charts.
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Simple interest is interest that is calculated only on the original amount of money you deposited or borrowed — called the principal. Unlike compound interest, simple interest does not earn interest on previously earned interest. This makes it easier to calculate and understand, and it's commonly used for short-term loans, car loans, some personal loans, and certain types of bonds.
Simple interest is the opposite of compound interest in terms of benefit. As a borrower, simple interest works in your favor — you pay less total interest because you're not charged interest on interest. As a lender or investor, compound interest works in your favor — you earn more because interest builds on interest over time.
The basic simple interest formula is:
Where:
The end balance is then calculated as:
Depending on which value you're solving for, the simple interest formula can be rearranged:
The calculator above uses these rearrangements when you switch between the Balance, Principal, Term, and Rate tabs. The math is identical — only the unknown variable changes.
Simple interest can also be calculated over shorter or longer periods than one year. If the interest rate is expressed per month, per quarter, or per day, the formula becomes:
Where:
For example, if you have a 5% monthly interest rate and want to know the interest over one year, the calculation is: P × 0.05 × 12. The calculator supports both yearly and monthly rate inputs — just pick the right option in the dropdown.
Suppose you deposit $20,000 in a savings account that pays 3% simple interest per year for 10 years. Here's how the numbers work:
Notice that the interest earned each year is always $600 — it never grows. Compare that to compound interest, where the interest would grow every year as the balance increases.
Here's the critical difference between simple and compound interest, using the same numbers above but with compound interest (compounded annually):
The difference over 10 years is $878. Over 30 years, the difference becomes dramatic — simple interest would yield $38,000 while compound interest would yield ~$48,545. The longer the time horizon, the bigger the gap.
Simple interest is used in a wide variety of real-world financial products:
Simple interest is best used for:
Yes, completely free. No sign-up, no limits, no ads. All calculations run in your browser.
Yes. Just ignore the "$" sign and enter values in your own currency. The math is identical.
Each tab solves for a different variable in the same formula. "Balance" solves for the end balance, "Principal" for the starting amount, "Term" for the duration, and "Rate" for the interest rate. Use whichever matches what you're trying to find.
Yes. In the Interest Rate field, switch the dropdown from "per year" to "per month." The calculator will handle the conversion automatically.
As a borrower, yes — you'll pay less total interest. As a saver or investor, no — you'll earn less. It depends on which side of the transaction you're on.
No — this calculator shows pre-tax interest. If interest is taxable in your country, the actual amount you keep will be lower. For a calculator that handles tax, see our Savings Calculator.
No. This calculator runs entirely in your browser with JavaScript. Nothing is uploaded or stored anywhere.