Exact date calculation
Calculate simple interest between two dates
This calculator is designed for situations where “30 days” is not precise enough. Enter the principal, annual rate, start date, and end date. The calculator counts the dates using the rule you select and then applies simple daily interest using the selected day-count basis.
Exact date calculations are useful for private notes, payoff estimates, judgment-interest checks, closing adjustments, and any loan that accrues interest according to actual calendar days.
Why date-count rules can change the answer
“August 1 through August 25” can be counted differently depending on whether the start date, end date, both dates, or neither date is included. The governing note, statute, judgment, payoff statement, or other agreement determines the appropriate rule.
The most important point is consistency: do not calculate the number of days one way and then compare it with a statement that used another convention.
Actual/360 versus Actual/365
The date count and the denominator are separate choices. Actual/365 divides the annual rate by 365. Actual/360 divides the annual rate by 360, which produces a slightly larger daily rate for the same annual percentage. The results table compares the common denominators without changing your selected dates.
When the balance changes during the period
If a payment, advance, credit, or other transaction changes principal between the two dates, one single calculation may not be sufficient. Break the period into balance segments and calculate the daily interest separately for each segment.
Need a complete schedule with changing dates or payments?
Amortization Pro lets you edit individual dates, payment amounts, interest rates, and notes while preserving the rest of the loan schedule.