DueDateHow it works
Enter the invoice amount and due date. Once any grace period has passed, the calculator adds a flat fee (if you charge one) and simple interest from the due date: amount × annual rate ÷ 365 × days late.
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
When does a late fee start?
After the due date and any grace period in your contract. With a 10-day grace period, an invoice due on the 1st becomes late on the 12th.
Does interest count the grace period?
By default interest runs from the due date once the grace period has passed. The API can start interest at the end of the grace period instead (accrueFrom=grace).
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.