Simple Interest Calculator
Calculate simple interest, total interest earned or paid, and ending balance.
Educational estimate only. This is not financial, investment, tax, legal or lending advice, a quote or a guarantee. Read the disclaimer.
Your inputs
How this calculator works
P is principal, a is the annual percentage ÷ 100 and y is time in years. No interest earns interest. The model assumes unchanged principal with no installments, fees or taxes. Fractional years are allowed, but calendar dates and 360/365-day accrual conventions are not modeled.
Dollar results are in USD and generally rounded to the nearest whole dollar; salary equivalents and percentage outputs use two decimal places. Calculations retain JavaScript floating-point precision between steps rather than rounding every payment to cents. Displayed components may not sum exactly to displayed totals. Payoff times use whole monthly payment periods. Defaults are examples, not current rates or recommendations.
Full methodology · About the publisher and corrections
Content and calculation checks: October 12, 2026. Educational estimates; no independent professional certification is claimed.
Worked example: $10,000 at 5% for 3 years
At 5% simple interest, $10,000 generates $1,500 of interest over three years, for an ending amount of $11,500. Because this is simple interest, previously earned interest does not itself earn interest.
Practical guide
When simple interest is useful
- Use it when a contract or classroom example explicitly uses simple interest.
- Do not substitute it for a compounding loan or savings product unless the terms actually use simple interest.
- Check whether the real product uses a 360-day or 365-day convention.
- For compounding scenarios, use the Compound Interest Calculator.
How to use the result
Change one assumption at a time to compare scenarios. For example, test a lower rate, larger contribution, different term or extra payment. This makes the calculator most useful as a decision-support tool rather than a single-number answer.
Important financial decisions can depend on taxes, fees, eligibility rules, lender policies, market returns and personal circumstances that this simplified calculator does not capture.
Frequently asked questions
Is this simple interest calculator free?
Yes. SumStride calculators are free to use and do not require an account.
Are the results exact?
No. Results are estimates based on the values you enter and the assumptions described on this page. Real-world fees, taxes, rates, policies and timing can differ.
Does SumStride save my inputs?
No account is required. The calculator runs in your browser; SumStride does not need your entered values to perform the calculation.