How to use this simple interest calculator
Enter the principal and annual simple interest rate, using 5 to represent 5%. Enter a nonnegative duration and choose its unit. Months are converted to years by dividing by twelve. For days, choose 365 or 360 days per model year based on the intended calculation. The tool accepts a supplied number of days; it does not calculate a date interval or derive a contractual day-count convention. Keep the time unit aligned with the annual rate and check whether the principal truly stays constant.
How the calculation works
The basic relationship is I = P × r × t, where P is principal, r is an annual decimal rate, and t is time in years. Interest does not join the principal for subsequent interest calculations. Consequently, each additional equal time interval contributes the same interest when the rate and principal remain fixed. The total amount is P + I; it is not interest alone. Identifying these separate quantities prevents a repayment balance from being mistaken for an earned return.
For an eighteen-month duration, the year fraction is 18/12 = 1.5. For a day-based duration, the selected denominator converts the supplied day count into a model year fraction. Ninety days on a 365-day basis gives 90/365 year, while a 360-day basis gives 90/360 year. The second fraction is larger. The tool divides the entered days by the chosen denominator; it does not implement calendar-based 30/360 date adjustments or an Actual/Actual convention.
Real products described as simple-interest loans may accrue interest on a declining outstanding balance as payments arrive. In that case, I = Prt applies separately to each interval with its own remaining principal and elapsed time. Applying the original principal across the entire term would overstate or misrepresent interest. This calculator intentionally handles one unchanged-principal interval. Use the Loan Calculator for the separately documented equal-monthly-payment model, and verify daily-accrual contracts against their actual payment dates.
No reinvestment, intermediate payment, fee, tax, inflation adjustment, or rate change is included. If principal changes, break the problem into valid segments rather than extending one constant-principal equation over the whole history. A quoted rate alone cannot establish the correct day basis or product behavior. Read the account or loan agreement for its rate definition, day-count method, posting rules, and any transaction that changes the amount on which interest is calculated.
Formulas
Simple interest and total
Time conversion
Worked examples
A two-year illustration
A principal of 2,000 at 4% for two years gives I = 2,000 × 0.04 × 2 = 160. The ending amount is 2,160. Each year contributes 80 because the principal stays at 2,000. Reinvesting the first year's interest would be a different compound-interest calculation, not this fixed-principal model.
An eighteen-month interval
For 5,000 at 6% over eighteen months, t = 18/12 = 1.5 years. Interest is 5,000 × 0.06 × 1.5 = 450 and principal plus interest is 5,450. Entering 18 as years instead of months would make the interest twelve times larger, so the selected unit is a critical part of the input.
Compare day-count bases
For 10,000 at 5% over ninety supplied days, the 365-day model gives 10,000 × 0.05 × 90/365 ≈ 123.29 interest. The 360-day model gives 125.00. Neither denominator is chosen automatically from a country or currency; use the convention required by the example or the relevant contract.
Common uses
Solve a principal-rate-time example
Enter known principal, annual percentage, and duration to verify the textbook I = Prt relationship. Confirm whether the question asks for interest alone or the total amount. Showing both values makes it easier to check a handwritten solution without confusing money already present with the additional interest.
Compare explicit day denominators
Keep the supplied number of days constant and switch between 365 and 360 to understand the effect of the chosen year denominator. This comparison does not determine the contract's required convention. A calendar-based rule may change both the denominator and the count of days, which this interface does not derive.
Check a fixed-principal interval
Use the model for a period in which no payment or deposit changes principal. If a transaction occurs, calculate separate intervals using the new balance and the relevant duration for each one. A single full-term calculation is not an adequate replacement for a dated transaction ledger.
Accuracy and limitations
Months are divided by twelve, supplied days by the selected 365 or 360 basis, and years are used directly. The converted term must remain within zero to one hundred years. Arithmetic retains precision until monetary formatting, without adding intermediate compounding or a schedule of payments.
The result depends on constant principal and rate. It does not count dates, infer leap-year conventions, implement 30/360 date adjustments, or include fee capitalization. A lender's use of the phrase simple interest can describe daily accrual on a changing balance and may require a different calculation.
Sources and methodology
These references support the formulas and financial concepts explained on this page. Product-specific rates, fees, and contract conventions still need to be checked with the provider.
Frequently asked questions
Should the rate be annual?
Yes. This interface expects an annual simple interest percentage. Duration is converted into a year fraction before applying it.
Does 360-day mode implement 30/360?
No. It divides the supplied day count by 360. A date-based 30/360 convention adjusts date counts and requires a separate procedure.
Does interest earn more interest?
No. Interest remains separate from principal in this model. Use the Compound Interest Calculator for reinvested interest.
Can I calculate interest after partial repayment?
Calculate each constant-principal interval separately with the appropriate remaining balance and time. This tool does not process a transaction history.
Why do I see both interest and total?
Interest is the additional amount calculated by I = Prt. Total is the initial principal plus that interest, so it includes money already present.
What happens with zero time or rate?
Interest is zero and principal plus interest equals the principal. Zero principal also produces zero interest.