How to use the simple interest calculator
Enter the original principal, annual simple interest rate, and time in years. The calculator applies the rate only to the original principal for the full period.
Simple interest formula
Simple interest does not add earned or charged interest to the principal for future interest calculations. That is the key difference from compound interest.
Example: Leah calculates three years of interest
Leah places $10,000 at an illustrative 5% simple annual rate for three years. Interest is $10,000 × 0.05 × 3 = $1,500, giving an ending total of $11,500.
The second and third years each add the same $500 because this model does not earn interest on earlier interest. A compounding account follows a different formula.
Simple interest vs. compound interest
Simple interest is calculated from the original principal throughout the modeled period. Previously earned interest does not become part of the base used for later interest calculations.
Compound interest is different because interest can be added to the balance and then earn additional interest. Over longer periods, that difference can materially change the ending value even when the stated annual rate is the same.
Matching the rate and time units
The standard simple-interest formula uses an annual rate and time expressed in years. If the time period is given in months, convert it to a fraction of a year before applying the formula.
For example, six months is 0.5 years. A 6% annual rate on $1,000 for six months produces $30 of simple interest: $1,000 x 0.06 x 0.5.
Where a simple-interest estimate is useful
The formula is useful for classroom problems and financial scenarios that explicitly use simple interest. It should not be substituted for a compound-interest account, amortizing loan, or product with fees and irregular cash flows.
Assumptions and limitations
What the estimate assumes
Payment schedules, fees, and compounding are not included.
Simple Interest Calculator FAQs
What is simple interest?
Simple interest is calculated from the original principal for each period rather than from a balance that includes prior interest.
How is simple interest different from compound interest?
Compound interest can earn or charge interest on previously accumulated interest. Simple interest does not.
Can I enter part of a year?
Yes. For example, 1.5 years represents one year and six months.
Does the calculator include payments?
No. It calculates a simple principal-rate-time relationship.
What is the simple interest formula?
Simple interest is I = P x r x t, where P is principal, r is the annual rate as a decimal, and t is time in years.
Does simple interest earn interest on interest?
No. Simple interest continues to use the original principal as the interest base. Compound interest can add previously earned interest to the balance.
For educational use. Actual accounts and loans can use different day-count conventions, payment timing, and compounding rules.