Banks use 360 days instead of 365 for interest because the convention started as a hand-calculation shortcut, and once it was embedded in contracts and accounting systems, no one had reason to change it. The side effect, which is why it survives, is that dividing an annual rate by 360 produces a slightly larger daily rate than dividing by 365. The bank then charges that larger daily rate across all 365 actual days of the year, so a stated 8% loan quietly becomes an effective 8.11%.
Where the 360-Day Year Came From
Before calculators, loan officers computed interest with pen and paper. Months of 28, 29, 30, and 31 days made billing thousands of accounts a slow, error-prone job. Treating every month as 30 days and every year as 360 turned awkward division into clean fractions: the monthly rate was the annual rate divided by 12, and the daily rate was the annual rate divided by 360.
The idea wasn’t new to modern banking. Merchants in ancient Mesopotamia organized their calendars around 12 months of 30 days for the same reason, and European banks adopted the framework centuries later. By the time American commercial lending took shape, the 360-day year was already standard. Once it was written into contracts, loan systems, and accounting standards, switching to 365 would have required renegotiating enormous volumes of existing debt. Inertia carried it the rest of the way.
How the Smaller Denominator Produces More Interest
The mechanics are simple. A 6% annual rate divided by 365 gives a daily factor of 0.016438%. The same rate divided by 360 gives 0.016667%. That gap looks tiny, but the bank still charges you for every real calendar day, including the five “extra” days the 360-day year pretends don’t exist.
On a $500,000 loan at 6%, a true 365-day calculation produces $30,000 in interest over a year. The 360-day method produces about $30,417, because that slightly larger daily factor runs across all 365 real days. The borrower pays an extra $417 a year with no change to the rate on the contract. The effective annual rate works out to roughly 6.083% instead of the stated 6%.
On a $10 million commercial loan at 8%, the gap widens past $11,000 a year. The quick formula is to multiply the stated rate by 365/360. At 8%, that gives 8.111%. Over five years on that $10 million balance, the extra cost crosses $55,000. For large balances, this is real money that never shows up in the headline rate.
Actual/360 vs. 30/360
Not every 360-day calculation produces the same result. The two common methods are actual/360 and 30/360, and only one of them creates the extra cost described above.
Under actual/360, the bank counts the real number of days in each month (31 for January, 28 or 29 for February) but divides the annual rate by 360 to get the daily factor. That combination is what pushes the effective rate above the stated rate. You’re paying a 1/360th daily charge for all 365 days.
Under 30/360, the bank assumes every month has exactly 30 days. The daily rate is still based on a 360-day year, but the bank also counts only 360 days of charges per year (12 months of 30 days each). The stated rate and the effective rate end up much closer together. A 30/360 borrower doesn’t see the same rate bump that actual/360 creates.
The practical difference: actual/360 benefits the lender more, because a higher daily rate is applied across more days. If your loan uses actual/360, expect to pay roughly 1.39% more interest annually than the stated rate suggests.
Which Loans Use Which Method
The 360-day year is overwhelmingly a commercial lending convention. Actual/360 is standard on commercial real estate loans, construction financing, bridge loans, commercial mortgage-backed securities, and most revolving credit facilities. Lines of credit and short-term working capital loans use it almost without exception.
Residential mortgages are different. Most home loans use either 30/360 or actual/365, depending on the lender and the loan type. The 30/360 convention is common in fixed-rate residential mortgages because it produces identical monthly payments. Adjustable-rate mortgages sometimes use actual/360, but it varies by lender and should be spelled out in the loan documents.
Conventions also vary by currency. Sterling-denominated loans in the United Kingdom typically use a 365-day year, while U.S. dollar short-term instruments default to 360. On cross-border financing, the day-count convention can follow the currency rather than the lender.
Checking Your Own Loan
The day-count convention is in the fine print, usually in a section labeled “interest accrual,” “computation of interest,” or “payment terms.” Some notes state it plainly: “interest is computed on the basis of a 360-day year for the actual number of days elapsed.” Others reference it indirectly.
Three variables control your real interest cost:
- The principal balance the lender is charging interest on, which may differ from the original loan amount if you’ve made payments or drawn on a line of credit.
- The nominal annual rate stated in the contract, before the day-count convention adjusts it.
- The day-count method itself: actual/360, 30/360, or actual/365.
To verify a statement, take the principal balance, multiply by the annual rate, divide by 360, and multiply by the actual number of days in the billing period. If that matches the charge, the bank is using actual/360. If the math only works when you use 30 days per month, it’s 30/360. Running this check on the first statement catches errors before they compound.
Federal law backs you up on seeing the real number. The Truth in Lending Act requires lenders to disclose the annual percentage rate before closing, and the APR is meant to capture the true yearly cost of borrowing, including the effect of a 360-day base.1Office of the Law Revision Counsel. 15 USC 1601 – Congressional Findings and Declaration of Purpose When a lender uses actual/360, the disclosed APR should be higher than the stated nominal rate.2Consumer Financial Protection Bureau. CFPB Laws and Regulations TILA
When 360-Day Math Crosses Into Usury
The convention creates one legal issue worth knowing about. If a loan is written at or near a state’s maximum legal interest rate, the actual/360 method can push the effective rate above the ceiling. In states with strict usury statutes, that overage can void the loan.
The leading case is Cagle v. Boyle Mortgage Co., a 1977 Arkansas Supreme Court decision. The note was written at 10%, which was the state’s usury cap. The bank calculated daily interest on a 360-day basis while charging for all 365 days, producing an effective rate of roughly 10.14%. With monthly compounding, the true rate reached 10.62%. The court found the loan usurious and canceled both the note and the mortgage. The lender’s argument that the overcharge was an unintentional mistake of law was rejected.
Other states have gone different ways. Some permit 360-day calculations on short-term loans of less than a year while barring them on longer debt. Others have held that the 360-day method alone doesn’t trigger a usury violation regardless of the effective rate. For a borrower whose stated rate sits near the legal maximum, the day-count convention is more than a billing detail; it can be a defense.
For most borrowers the stakes are smaller, but the principle is the same. The 360-day year isn’t neutral math. It’s a convention that quietly raises the price of credit, and knowing it’s there is the difference between paying the rate on the contract and paying the rate the bank actually charges.