A typical template figures monthly interest as the balance times the annual rate, divided by 12. Many loans count interest by the day instead. Reinhart Boerner Van Deuren, a law firm, shows that an 8% rate charged by the day on a 360-day year costs 8.11% a year2. On a $10,000,000 interest-only loan, that's $11,111 more than the $800,000 a borrower might expect.
Interest on $1,000,000 at 8%, by day count
| Day count | How it counts | 31-day month | Full year |
|---|
| 30/360 | Every month is 30 days of a 360-day year | $6,666.67 | $80,000.00 |
|---|
| Actual/365 | Actual days over a 365-day year | $6,794.52 | $80,000.00 |
|---|
| Actual/360 | Actual days over a 360-day year | $6,888.89 | $81,111.11 |
|---|
Illustrative figures for a 365-day year. On Actual/360, a year of interest works out to 8.11%.
One agreement can even use two day counts. Zevia, a beverage company, included a 2024 amendment to its bank loan in an SEC filing. It counts base rate loans on a 365- or 366-day year and Term SOFR loans on a 360-day year3, and notes that the 360-day basis means more interest. The bank signed it alone, as the lenders' agent, when the loan's old benchmark rate was being replaced.
For a floating rate, record the index, the margin, when the rate resets and any floor. In Zevia's agreement, Term SOFR is CME's published rate for the coming one, three or six months, taken two business days before each interest period. A fixed adjustment is added (0.11448% for one month), and the result counts as 0% if it would fall below zero. Update the rate on each reset date, not when the next statement arrives.