Calculate simple interest on loans or investments. Enter principal, rate, and time to find interest earned or owed plus the total amount.
Simple interest is the most straightforward way to calculate the cost of borrowing or the return on an investment. Unlike compound interest, simple interest is calculated only on the original principal — it does not compound on previously earned interest. The formula is elegantly simple: Interest = Principal × Rate × Time.
Simple interest is commonly used for short-term loans, auto loans, personal loans, Treasury bills, and some bonds. It is also the foundation for understanding more complex financial calculations. If you are evaluating a flat-rate loan, estimating short-term investment returns, or simply want to understand how interest works, this calculator gives you instant results.
Enter your principal amount, annual interest rate, and time period to calculate the total interest earned or owed, plus the combined total (principal plus interest). The calculator supports daily, monthly, and annual time periods for flexibility. Because simple interest does not compound, it is typically used in auto loans, short-term personal loans, and some government bonds, making this calculator a useful reference point for those common borrowing scenarios.
Simple interest is the building block of all financial math. Understanding it helps you evaluate loans, estimate investment returns, and compare financial products. This calculator provides instant, accurate results so you can quickly determine how much interest you will earn on savings or owe on a loan without needing to do the math manually.
Simple Interest: I = P × r × t Total Amount: A = P + I = P × (1 + r × t) Where P = principal, r = annual rate (decimal), t = time in years. For months: t = months / 12. For days: t = days / 365.
Result: $1,500 interest, $11,500 total
With a $10,000 principal at 5% annual simple interest for 3 years: I = $10,000 × 0.05 × 3 = $1,500. The total amount after 3 years is $10,000 + $1,500 = $11,500. With compound interest, the same investment would yield $1,576 — simple interest produces $76 less because it does not compound.
The formula I = P × r × t is one of the most fundamental equations in finance. P represents the principal or initial amount, r is the annual interest rate expressed as a decimal, and t is the time period in years. To use months, divide by 12; to use days, divide by 365 (or 360 for some financial conventions).
While compound interest dominates modern finance, simple interest is still used in specific contexts: Treasury bills, some corporate bonds, certain auto loans, and short-term commercial lending. Some countries use simple interest conventions for specific financial products as mandated by regulation.
Understanding simple interest is essential before moving to compound interest, amortization, and more complex financial math. Every loan payment, investment return, and financial projection builds on these fundamental concepts. Mastering simple interest gives you the foundation to evaluate any financial product.
Simple interest is interest calculated only on the original principal amount, without compounding. The formula is I = P × r × t, where P is the principal, r is the annual rate, and t is the time in years. It is the simplest form of interest calculation and is used for some loans and short-term financial instruments.
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus any previously accumulated interest. Over time, compound interest grows faster because you earn "interest on interest." The difference becomes more significant over longer time periods and at higher rates.
Some auto loans, personal loans, and short-term loans use simple interest. Most mortgages and credit cards use compound interest. Federal student loans accrue simple interest during certain periods. Always check your loan agreement to understand which type applies to your specific loan.
For borrowers, simple interest is better (cheaper) than compound interest because you pay less total interest over the life of the loan. For investors, compound interest is better because you earn more over time. Simple interest is neither inherently good nor bad — it depends on which side of the transaction you are on.
You cannot directly convert between the two because they grow at different rates over time. However, you can calculate the equivalent compound rate that produces the same total interest over a specific period. For short periods, simple and compound interest are nearly identical.
No. By definition, simple interest does not compound at all. It is calculated once based on the original principal, rate, and time. If interest compounds monthly, it is compound interest, not simple interest.