Calculated from the sample figures using the displayed formula.
What is the simple interest formula?
I = P × r × tSimple interest applies the rate to the original principal rather than to an increasing balance.
What do the inputs mean?
- Principal
- The value entered for principal. Keep its period and units consistent with the other inputs.
- Annual rate (%)
- The value entered for annual rate (%). Keep its period and units consistent with the other inputs.
- Time in years
- The value entered for time in years. Keep its period and units consistent with the other inputs.
How to calculate it step by step
- Define the decision.
Write down what the result will help you compare or decide. - Collect matching inputs.
Use figures from the same period and apply the definitions shown on this page. - Apply the formula.
I = P × r × t. - Check the units.
Confirm whether the result is a currency amount, percentage, number of periods or ratio. - Test a second case.
Change one assumption at a time to see what drives the result.
Worked example
Using the sample figures in the calculator, the result is $2,000.00. Change one figure at a time and recalculate to see which assumption has the greatest effect. A spreadsheet should return the same result when it uses the same formula, units and timing.
When should you use simple interest formula?
Use it for contracts and examples that expressly specify simple interest.
Limitations and common mistakes
Convert rate and time to consistent units before multiplying.
Do not mix annual figures with monthly figures, ending balances with averages, or percentages with whole numbers. Record the definition used so another person can reproduce the result.
Frequently asked questions
What is simple interest formula?
Simple interest applies the rate to the original principal rather than to an increasing balance.
How should I use simple interest formula?
Use the formula only after matching every input to the same period, unit and definition. Then compare the result with a worked example and the decision you are trying to make.
What is the most common mistake with simple interest formula?
Convert rate and time to consistent units before multiplying.
Can I calculate simple interest formula in a spreadsheet?
Yes. Reproduce the displayed equation with separate cells for each input, keep percentages as decimals, and test the spreadsheet against the worked example before replacing the sample figures.
Sources and methodology
This page uses the Investor.gov Compound Interest Calculator for the underlying financial or reporting context. Spreadsheet-related behavior is checked against Microsoft’s financial-function documentation. Examples are illustrative and use the formula displayed on the page.