Invoice Late Fee Calculator
Compute a simple-daily-interest late fee on an overdue invoice. US-default 18% APR with a configurable grace period and optional flat fee.
How this is calculated
The fee uses the simple-daily-interest formula every US incumbent cites: split the annual percentage rate over 365 days and multiply by the number of days past the grace period.
days_overdue = max(0, payment_date − due_date − grace_period)
simple_interest = invoice × (annual_rate / 365) × days_overdue
late_fee = (days_overdue > 0) ? flat_fee + simple_interest : 0
total_due = invoice + late_feeThe default 18% APR (= 1.5% / month) matches US small-business norms (FreshBooks, Housecall Pro). Eight US states cap late-fee rates; when the computed APR exceeds 18%, we surface a soft "check your state" warning rather than hard-coding 50 statutes.
The calculator uses simple interest, not compounding. Compound mode is documented as a future enhancement.
The fee line only makes sense next to the terms it follows from. What net 30 payment terms covers is the due-date language itself, and the invoice generator prints those terms at the bottom of every invoice it builds.
When the account needs a full letter rather than a single line, the past due invoice letter template writes the reminder, the second notice and the final notice around the figure this page returns. Getting to the point where you rarely send one is the subject of how to get paid faster, and the rest of the billing toolkit sits on the billing tools hub.