How to use this calculator
- Enter constant principal, annual simple rate (%), start date (yyyy-mm-dd), end date (yyyy-mm-dd), annual day-count denominator.
- Choose a display currency, then select Calculate. Results also update after a brief pause while you edit.
- Read the result with the assumptions below. Export details or save a second scenario to compare your choices.
Formula and method
Assumptions and limitations
Start excluded, end included as elapsed days; constant principal and rate, no compounding. Not Actual/Actual or 30/360.
Worked example
From 1 January to 31 January 2026 there are 30 elapsed days. A constant 10,000 principal at 5% using Actual/365 fixed earns 10,000 × 0.05 × 30 / 365 = 41.095890, displayed as 41.10 in a two-decimal currency.
What the result means
Calculate actual elapsed-day interest using explicitly selected 360- or 365-day denominators.
Common questions
How is the calculation performed?
Days=UTC end date−start date; interest=principal×annual rate×days/basis.
What does the model leave out?
Start excluded, end included as elapsed days; constant principal and rate, no compounding. Not Actual/Actual or 30/360.
Methodology and transparency
Calculation specification reviewed on 2026-10-14. Mathematical implementation is tested against independent reference fixtures. This does not imply review by a licensed financial professional. Read the testing and precision policy.