Simple Interest Calculator
Calculate simple interest, principal, rate or time for any loan.
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Fill in the known values to calculate the missing one
Calculate simple interest or find any missing variable — principal, rate, or time — using the SI formula. Supports four calculation modes, three time units (years, months, days), and displays the substituted formula for full transparency.
Key Features
Four calculation modes
Switch between Calculate SI, Find Principal, Find Rate, and Find Time. Each mode hides the unknown field and solves for it from the other three known values.
Time unit conversion
Enter time in years, months, or days. The calculator automatically converts to the annual basis required by the formula.
Formula display
The exact calculation with substituted values is shown below the inputs, making it useful for checking homework, loan documents, or explaining how a result was reached.
Four result cards
Principal, rate, interest, and total amount are always visible together so you can see the full financial picture at once.
How to Use
- 1Select the mode that matches what you are trying to find: Calculate SI, Find Principal, Find Rate, or Find Time.
- 2Fill in the three known values. The unknown field is hidden for the selected mode.
- 3Read the answer from the highlighted result card.
- 4The substituted formula line confirms exactly how the answer was derived.