Irish Calculators
Saving & investing

Simple Interest Calculator with Dates or Duration

Enter a principal, annual simple rate and duration to calculate interest without compounding. For dated periods, choose the day-count convention specified in your agreement.

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Your numbers

%
Display currency only. Convert all inputs to this currency before calculation.
Your figures stay on this device.

How the calculation works

Simple interest accrues on the original principal without adding earlier interest to the interest-bearing balance. The date mode makes the year-fraction convention explicit, while duration modes accept a stated number of months or years.

Interest = principal × annual simple rate × year fraction. No interest is added to principal during the period.

  1. Establish the year fraction

    Actual bases divide elapsed calendar days by their stated denominator. European 30E/360 adjusts day numbers to its thirty-day-month convention; month mode divides months by twelve.

  2. Apply the annual rate

    Multiply principal, annual rate as a decimal and year fraction. Because the principal stays unchanged, interest grows proportionally with the chosen time fraction.

  3. Read interest and total separately

    The interest result is the change attributable to the rate. Principal plus interest includes the original amount and is not additional earnings.

Worked example: Check Simple interest

Sample inputs demonstrate the method. They are not a forecast or product offer.

The first example runs from 1 January to 30 June 2026, an elapsed 180 days. On an Actual/360 basis, 1,000 at 10% earns 1,000 × 0.10 × 180/360 = 50.

Principal
1000
Annual simple rate (%)
10
Duration input
Start and end dates
Start date
2026-01-01
End date
2026-06-30
Day-count basis
Actual/360
Currency for all amounts
Euro (EUR)

Expected result

Simple interest
€50.00
Worked example: Check Simple interest

Sample inputs demonstrate the method. They are not a forecast or product offer.

Principal
1000
Annual simple rate (%)
10
Duration input
Start and end dates
Start date
2026-01-01
End date
2026-06-30
Day-count basis
Actual/365 fixed
Currency for all amounts
Euro (EUR)

Expected result

Simple interest
€49.32

Understanding your result

Different contractual bases can produce different interest on identical dates and rates. The difference is a convention effect, not evidence that one calculation is automatically incorrect.

This model does not handle changing balances, staged repayments, compounding or a full bond coupon-accrual schedule. Those require a more specific cash-flow model.

Questions you might have

What does the result include?

Calculate interest without compounding using an explicit day-count convention or a duration in years or months.

Which limits should I check?

A contractual day-count basis can change the answer. Select the convention from your agreement.

Choosing the right inputs

Day-count basis
For dates, select the convention in the agreement: Actual/365, Actual/360 or European 30E/360. These are not interchangeable labels.
Duration input
Choose dates when the actual interval matters, or months or years for a stated duration. The duration field follows the selected unit.
Annual simple rate
Use an annual simple rate. Entering a compound annual return does not make this a compound-interest model.

Common mistakes to avoid

  • European 30E/360 is not US/NASD 30/360.
  • Do not count both date endpoints manually and then add an extra day to the engine's elapsed-date result.

Assumptions to check

  • A contractual day-count basis can change the answer. Select the convention from your agreement.
  • All calculations run on your device. Figures are scenarios based on your inputs, not investment recommendations.

Sources and method

Read our calculation and testing approach.