What Is a Daily Interest Accrual Calculator?

A daily interest accrual calculator works out how much interest builds up on a balance over a specific number of days, using the daily periodic rate rather than a monthly one. Credit cards, student loans, margin accounts and many lines of credit accrue interest this way: instead of charging a flat monthly amount, they divide the annual rate by the day-count basis and apply it to the balance for every day it is outstanding. The result is that interest is far more sensitive to the exact timing of payments than a monthly schedule would suggest.

Because interest is charged on the actual balance each day, a payment made early in a cycle saves more than the same payment made late, and a balance that sits untouched for a long period grows by an amount that depends on the precise number of days, not the number of months. This calculator makes those daily mechanics explicit and reports the effective annual rate so you can compare a daily-accrual product against one quoted on a monthly basis.

How It Works

The calculator divides the annual percentage rate by the day-count basis you select, either 365 or 360, to obtain the daily periodic rate. It then multiplies that rate by the balance and by the number of days. In simple mode the same daily amount is charged throughout, so the total is the daily interest multiplied by the days; in compound mode each day's interest is added to the balance, so the next day's charge is calculated on a slightly larger amount, using the familiar growth formula balance multiplied by (1 + daily rate) to the power of the days minus one.

Because the two day-count bases produce different daily rates, the calculator also reports the effective annual rate implied by each basis: the daily rate compounded across a full year. That figure is what makes a daily-accrual product directly comparable to a nominal rate quoted elsewhere, since it captures the effect of compounding within the year.

Understanding Your Results

The primary card shows the interest accrued over the period you entered, with its subtext stating the days, the annual rate and the day-count basis so the assumption is always visible. The three statistics beside it give the daily interest amount, the monthly equivalent using the annual rate divided by twelve, and the effective annual rate, which together let you translate between daily and monthly views of the same cost.

The breakdown table lists the daily periodic rate, the balance, the days charged and the accrual method, so you can verify each step of the calculation. Summary Statistics adds the annual interest and repeats the effective annual rate, which is the number to use when comparing a daily-accrual loan against a product quoted on a different basis. Treat the results as an estimate for a single, stable balance; if the balance changes during the period, interest is charged on each day's actual balance, so divide the period into segments and add the results for the most accurate figure.