The Actual/365 day count convention calculates interest by counting the exact number of calendar days in a period, dividing by a fixed 365, and multiplying that fraction by the annual rate and the outstanding principal. Because the denominator stays at 365 while the numerator tracks the real calendar, a 31-day month costs slightly more interest than a 28-day month on the same balance at the same rate. That is the whole idea, and everything else follows from it.
The Formula and Its Three Inputs
Three numbers drive the calculation: days in the period, the annual interest rate, and the principal balance.
Interest = Principal × Annual Rate × (Actual Days ÷ 365)
The numerator counts every calendar day between two dates, weekends and holidays included. A billing cycle from March 1 through March 31 is 31 days. The denominator is always 365, even in a leap year. That fixed denominator is what makes this the “365 Fixed” variant in industry shorthand.
The fraction (Actual Days ÷ 365) is often called the time factor. For a 30-day cycle it is 30 ÷ 365 = 0.08219. For a 31-day cycle it is 31 ÷ 365 = 0.08493. Small on paper, but it explains why monthly interest charges are never identical even when your balance and rate hold steady.
A Worked Example
Take a $50,000 loan at 6% annual interest, and figure the interest for a 31-day month. Time factor: 31 ÷ 365 = 0.08493. Times the annual rate: 0.06 × 0.08493 = 0.005096. Times the principal: $50,000 × 0.005096 = $254.79. That is the month’s interest.
Now run the same loan through a 28-day month. The interest drops to $230.14. The $24.65 difference comes entirely from three fewer days in the cycle. Nothing else moved. This is the practical result of counting actual days instead of assuming every month has the same length.
Per Diem Interest and Payoff Quotes
The per diem is the daily cost of carrying the balance, and it is the building block of every Actual/365 figure. Divide the annual rate by 365, then multiply by the principal. On that $50,000 loan at 6%, the per diem is $50,000 × (0.06 ÷ 365) = $8.22 a day.
Per diem matters most when you are paying a loan off early or closing on a property mid-month. Payoff statements typically quote a balance good through a specific date and add a per diem for each additional day it takes the funds to arrive. If your payoff is valid through Friday but the wire clears the following Tuesday, you owe four more days of per diem interest. Overlooking that is one of the most common reasons borrowers come up short and leave a small residual balance that keeps accruing.
Check the per diem on any payoff statement you receive. Annual rate divided by 365, times the principal, should land within a few cents of the quoted figure. If it does not, the lender may be using a different day count convention than you assumed.
How Leap Years Change the Math
A leap year has 366 calendar days, but Actual/365 keeps the denominator at 365. So a full leap year of interest works out to the annual rate times 366/365, roughly 1.00274 times the normal annual amount. On a $200,000 balance at 5%, that is about $27.40 extra for the year.
The daily charge on February 29 is not special. It is the same per diem as any other day. The total simply reflects one more day of accrual. Software has to recognize the leap day; skipping it shortchanges the lender by exactly one day of interest and misaligns every later figure in the amortization schedule.
Some contracts sidestep the issue by using Actual/Actual, which puts 366 in the denominator during a leap year so the daily rate dips slightly and the full-year total stays aligned with the stated annual rate.
How Actual/365 Compares to the Other Conventions
Three conventions dominate lending and fixed-income markets, and mixing them up can cost real money. The differences come down to how each one defines a day and a year.
Actual/360
Actual/360 counts real calendar days on top but divides by 360. Smaller denominator, bigger daily rate. Because you pay that daily rate for all 365 or 366 actual days in the year, an 8% stated rate produces an effective annual rate of about 8.11%. Compared with Actual/365 at the same stated rate, that is roughly 1.4% more interest over a year.
This is standard in U.S. dollar money markets and common in commercial real estate lending. Borrowers sometimes miss it until they notice their annual interest cost runs slightly above the headline rate. If your documents say “Actual/360” or “365/360,” multiply the stated rate by 365/360 to see the true annual cost.
30/360
30/360 ignores the calendar. Every month is 30 days, every year is 360. That produces perfectly uniform monthly payments, which is why residential mortgages in the United States overwhelmingly use it. Your mortgage payment in February matches your payment in July even though February is shorter. The trade-off is a small loss of precision that borrowers rarely notice because the amortization schedule is built around the convention from the start.
Actual/Actual
Actual/Actual counts real calendar days in the numerator and uses 365 or 366 in the denominator depending on whether the period sits in a leap year. U.S. Treasury securities use this method, basing yields on actual day counts relative to the true length of the year.1U.S. Department of the Treasury. Interest Rates – Frequently Asked Questions The effective annual rate stays equal to the stated rate regardless of leap years, at the cost of more complex math for periods that straddle a year boundary.
Where You Actually Encounter Actual/365
Actual/365 is not a blanket standard in U.S. lending. It shows up in specific places rather than everywhere.
Credit cards are the most familiar example. Most issuers divide the APR by 365 to get a daily periodic rate, then apply that rate to the average daily balance across the days in the billing cycle. That is Actual/365 in operation, even when the card agreement never uses the phrase. Auto loans and personal loans structured as simple-interest products work the same way: interest accrues daily on a 1/365th slice of the annual rate applied to the outstanding balance.
Commercial real estate lenders use all three major conventions depending on the deal. Some CRE loans use Actual/365, but Actual/360 and 30/360 are at least as common. The loan documents will specify which method applies.
What to Check in Your Loan Documents
Federal lending rules require lenders to disclose the finance charge, the annual percentage rate, and the payment schedule on closed-end loans.2Consumer Financial Protection Bureau. 12 CFR 1026.18 Content of Disclosures For open-end credit like credit cards and home equity lines, periodic statements must show the balance the finance charge was computed on and explain how that balance was determined.3eCFR. 12 CFR 1026.7 – Periodic Statement No federal regulation specifically requires lenders to disclose which day count convention they use. The convention typically lives in the note or loan agreement itself, not in the standardized disclosure forms.
The burden falls on you to find it. Look in the promissory note for language like “interest shall be computed on the basis of a 365-day year” or “actual days elapsed divided by 360.” A 360-day denominator on a commercial loan means your effective rate will run slightly higher than the headline number. If you cannot find any day count language, ask the lender directly before you sign.
One boundary worth knowing: if a third-party debt collector later tries to collect interest that was not authorized by the original agreement, or misrepresents the amount owed by using a different calculation method, the Fair Debt Collection Practices Act bars collecting any amount not expressly authorized by the agreement and prohibits false representations of a debt’s character or amount.4Federal Trade Commission. Fair Debt Collection Practices Act That protection applies to collectors, not to your original lender, but it gives you a concrete basis to dispute inflated interest during collection.