Simple Interest Calculator
Interest that grows in a straight line — calculated only on the original principal, never on prior interest.
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$1,500
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Last updated: August 6, 2026 · Reviewed by the DoCalc team
What Is Simple Interest?
Simple interest is the most basic way to calculate interest: it's charged (or earned) only on the original principal, for the whole time period, and never on interest that's already accumulated. That makes it grow in a straight line — the same dollar amount of interest every year — unlike compound interest, which accelerates because each period's interest gets added to the balance that future interest is calculated on.
The Formula
Where I is the interest earned or owed, P is the principal (starting amount), r is the annual interest rate (as a decimal), and t is time in years. The total amount at the end is simply P + I — principal plus the interest that accumulated.
Worked Example
$10,000 principal at a 5% annual rate for 3 years:
Notice the interest is exactly $500 per year, every year — $1,500 over 3 years is simply 3 × $500. That flat, predictable growth is the entire distinguishing feature of simple interest versus compound interest, where the same starting numbers would produce a slightly larger total because year 2's interest would itself start earning interest.
Simple vs. Compound: Same Inputs, Different Outcomes
| Year | Simple interest total | Compound interest total (annual) |
|---|---|---|
| 1 | $10,500 | $10,500 |
| 2 | $11,000 | $11,025 |
| 3 | $11,500 | $11,576 |
The gap is small over 3 years but widens substantially over longer periods — compounding is why long-term investment horizons matter so much.
Who Should Use This Calculator
Use it if you're working with a loan, bond, or short-term note that specifically uses simple interest — check the loan or investment terms, since most modern savings products and credit cards compound instead.
Use the Compound Interest Calculator instead if you're modeling a savings account, investment, or any product where interest is added to the balance periodically and then itself earns interest.
Common Mistakes to Avoid
The most common mistake is assuming a loan or account works this way without checking — most everyday financial products (savings accounts, credit cards, most mortgages after the first payment) actually compound, so applying the simple interest formula to them will understate the true cost or return.
Worth knowing: some short-term loans and add-on interest auto loans are structured with simple interest specifically because it's predictable and easy to verify — if you're unsure which type applies to your situation, ask the lender directly rather than assume.
Frequently Asked Questions
What is simple interest?
Simple interest is interest calculated only on the original principal, for the entire time period — unlike compound interest, it never earns interest on previously accumulated interest.
What's the difference between simple and compound interest?
Simple interest grows in a straight line — the same dollar amount of interest each period. Compound interest grows faster over time because each period's interest is calculated on a balance that already includes prior interest.
Where is simple interest actually used?
Some short-term loans, certain bonds, and some auto loans use simple interest. Most savings accounts, credit cards, and long-term investments use compound interest instead, since it's more common in modern banking.
Does the time period have to be in years?
No — the formula works with any consistent time unit as long as the rate matches it. This calculator uses years with an annual rate; convert months to years (divide by 12) if you're working with a monthly time period.
Is simple interest better for borrowers or lenders?
Generally better for borrowers, since interest never compounds on itself — the total interest owed grows linearly rather than accelerating over time, all else being equal.
Conclusion
Simple interest is the baseline case — a straight-line formula worth knowing before layering on compounding, which is where most real-world savings and debt actually live. Knowing which one applies to your specific loan or account is the first step to getting the math right.