DueDateAnnual vs monthly rates
An annual rate (APR) is applied daily: amount × rate ÷ 365 × days. A monthly rate such as 1.5% per month is applied per 30-day period; the API can prorate, round up or ignore a partial month, and can compound monthly. 1.5% per month is 18% a year simple, slightly more when compounded.
API
On Starter, Team and Business plans, POST /v1/late-fee returns days late, a flat fee, interest and total due with a step-by-step explanation. It also supports monthly rates (simple or compounded), caps, business-day grace periods using DueDate holiday calendars, and due dates computed from an invoice date and term like Net 30. Each request counts as one API call. See pricing.
FAQ
What is 1.5% per month as an annual rate?
18% a year simple. Compounded monthly it is about 19.56% a year.
Does a leap year change the result?
The annual-rate method uses a 365-day year with the actual number of days late, so Feb 29 counts as one more day late.
Is this legal advice?
No. DueDate only does the arithmetic. Check your contract and your state's law before charging late fees or interest; many states cap them.
How is rounding done?
Interest and percentage-based fees are each rounded to the nearest cent, half away from zero.