Simple Interest Calculator
Calculate interest on an unchanged principal using years, months, or an explicit 360- or 365-day basis.
Last updated: November 4, 2026
On this page
Quick summary: Calculate interest on an unchanged principal using years, months, or an explicit 360- or 365-day basis.
Read the complete guide
Updated reference sections in this guide are in English.
Interest on the original principal
Use this form when the interest calculation is based on a principal that stays unchanged during the selected period. Enter the principal, the annual simple rate and a length of time. If you enter months, the calculator divides by twelve. If you enter days, choose a 360-day or 365-day convention. The selected day convention does not change calculations entered directly in months or years.
The simple-interest relationship
Total = principal + interest
For months: t = months ÷ 12
For days: t = days ÷ selected days per year
The rate is entered as a percentage and divided by 100 before calculation. Interest is not added back to principal to earn additional interest. That distinction makes this a different model from compounding, where the balance used to calculate the next period’s growth can increase.
Worked examples
An 18-month period
For a principal of 5,000 at 6% simple annual interest over 18 months, time is 18 ÷ 12 = 1.5 years. Interest is 5,000 × 0.06 × 1.5 = 450, and principal plus interest is 5,450.
The day convention changes the estimate
For a principal of 10,000 at 5% over 90 days, a 365-day basis gives interest of approximately 123.29. A 360-day basis gives 125.00. Those calculations differ because the same 90 days represent different fractions of a convention year.
Check this scenario in the calculator:
- Principal
- 5000
- Annual simple rate (%)
- 6
- Time
- 18
- Time unit
- Months
- Day-count basis, when using days
- 365 days per year
- Currency (display only)
- USD — US dollar
When this is the wrong loan model
A loan with regular principal repayments has a changing outstanding balance. Multiplying the original loan amount by rate and the full term will generally not reproduce its amortizing payment schedule. Use the fixed-rate loan payment calculator for the specific monthly-payment model it describes. Similarly, an account that compounds interest requires a compounding model rather than simple interest.
Limitations and contract conventions
The form does not implement actual/actual, 30/360 date adjustments, daily principal changes, fees, penalties or a repayment schedule. It converts a supplied number of days using one constant denominator; it does not infer a contract’s day-count rule from calendar dates. Rates and periods must be nonnegative, and the supported equivalent period is no longer than 1,000 years. Use the convention specified in your agreement when checking a contractual amount.
Simple-interest questions
Does the interest itself earn interest?
No. Every part of this model’s interest is calculated from the original principal.
Does a 365-day basis automatically account for leap years?
No. It is a constant denominator of 365. Actual/actual conventions require more specific dates and rules.
Can I enter a fraction of a year?
Yes. For example, 0.5 years represents half of a convention year in this model.