Simple Interest Calculator by Term or Exact Dates
Get the ordinary answer first, or audit exact calendar days and fixed denominator differences.
Calculate ordinary simple interest on a fixed principal. No compounding.
Compare the same actual calendar-day count across three fixed denominators, then reconcile one convention by calendar month.
Calendar-month accrual schedule
Quick term calculation stays first
Enter principal, an annual percentage rate, and a term in years or months. The result shows simple interest and the final principal-plus-interest amount without adding earned interest back into the principal.
Compare one real date interval three ways
The exact-date mode counts YYYY-MM-DD calendar days once, then applies fixed 360-, 365-, and 366-day denominators side by side. Dollar and percentage deltas use fixed 365 as a neutral comparison baseline; they do not declare a contract winner.
Reconcile accrual by calendar month
Select one fixed denominator to split the interval into partial first and last months plus complete middle months. Every row exposes its start, end-exclusive boundary, day count, incremental interest, and cumulative unrounded interest before TSV export.
Scope: arithmetic, not a financial or legal determination
This is a fixed-principal simple-interest model. It does not handle payments, amortization, fees, compounding, variable rates, exchange rates, tax, or jurisdiction-specific contract interpretation. Confirm the required convention with the governing document or institution.
Frequently asked questions
What is the simple interest formula?
Simple interest is principal × annual rate × time. The interest does not join the principal and earn more interest, so this page does not model compounding.
How does the exact-date calculator count days?
It uses pure YYYY-MM-DD calendar dates. The default interval is [start, end): the start is included and the entered end is excluded. Choosing include end adds exactly one calendar day. Browser time zones and daylight-saving transitions do not change the count.
What do Actual/360, Actual/365, and Actual/366 mean here?
Each comparison uses the actual counted calendar days divided by a fixed 360-, 365-, or 366-day denominator. Actual/366 on this page is a fixed 366-day comparison; it is not Actual/Actual and does not change denominator across years.
Which day-count convention should I use?
Use the convention written in the relevant loan, deposit, invoice, judgment, or policy. This calculator compares arithmetic outcomes but does not choose the contractual or legal method for you.
Does the monthly schedule compound interest?
No. Every calendar-month row uses the same fixed principal and annual simple rate. Row interest is accumulated for reconciliation only; it is never added to principal for the next row.
Why might displayed monthly cents not add to the displayed total?
The calculation sums unrounded row values once, then formats values for display. The TSV export retains unrounded browser-number values so a spreadsheet can audit the same total without adding pre-rounded cents.