Use the result for a focused comparison.
- Check classroom examples and contracts that explicitly state simple interest.
- Separate original principal from interest earned or charged.
- Compare a non-compounding result with a compound-interest projection.
Formula and calculation method.
I = P × r × t; ending amount = P + IP is the original principal, r is the annual rate written as a decimal, and t is time in years. Unlike compound interest, previously earned interest is never added to the base used for later periods.
Follow the numbers in context.
$5,000 principal at 6% per year for 3 years
Working: $5,000 × 0.06 × 3
Result: $900 interest and a $5,900 ending amount.
Interpretation: Each year adds the same $300 because the interest base remains $5,000.
Read the estimate with its assumptions.
Simple interest grows in a straight line when principal, rate, and time remain fixed. Doubling the time doubles the interest.
Many real savings products and amortizing loans do not use this exact model, so verify the contract before relying on it.
The formula is most informative when the agreement explicitly applies one annual rate to an unchanged principal. If principal is repaid during the period, interest should normally be calculated against each outstanding balance instead. If earned interest is credited back to an account, later interest may compound and the simple-interest answer will understate growth.
Keep the time unit aligned with the quoted rate. Six months is 0.5 years for an annual rate, while 45 days requires the day-count convention stated by the agreement. Dividing by 365 is common in examples, but some financial contracts use 360 days or count actual days differently.
What the model includes—and leaves out.
- The principal does not change.
- The annual rate remains fixed.
- Time is entered in years and proportional years are allowed.
- No interest is reinvested or added to principal.
The calculator does not model compounding, periodic repayments, changing balances, fees, taxes, day-count conventions, or leap-year adjustments. It is an educational arithmetic tool, not an account quote or lending disclosure. Rates advertised for deposits or credit may be effective annual rates, nominal rates, or regulated APR figures; those labels are not interchangeable with the simple annual percentage entered here.
Unusual inputs need extra care.
Interest is zero and the ending amount equals principal.
Decimals such as 0.5 model a proportional half-year under the simple formula.
Use the Compound Interest Calculator when interest is periodically added to the balance.
Questions about this calculation.
What is simple interest?
Simple interest is calculated only on the original principal: principal multiplied by annual rate and time.
How is simple interest different from compound interest?
Compound interest adds accumulated interest to the balance used for later growth. Simple interest keeps the original principal as the calculation base.
Can I enter months?
Convert months to years by dividing by 12. For example, 6 months is 0.5 years.