US standard: 1.5% per month (18% APR).
This invoice is 30 days overdue.
Original amount$1,000.00
Interest (1.5% / month × 30 days)$15.00
Amount due today$1,015.00
Client hasn't paid on time? Enter the invoice amount, its due date, and your monthly interest rate. The calculator returns the days overdue and the exact interest + total amount owed today.
US standard: 1.5% per month (18% APR).
This invoice is 30 days overdue.
According to industry surveys, roughly half of B2B invoices in the US are paid late. For a solo contractor or small business, even a single 30-day-overdue invoice can push payroll and rent into the red. Charging a late fee is not about punishment — it’s about signaling to the client that on-time payment is a term of doing business.
Interest = Invoice amount × (monthly rate / 100) × (days overdue / 30) + flat fee
This is simple interest, which is legally cleaner than compound interest and easier for the client to accept. Most US small businesses use 1.5% per month (18% APR) as the industry standard.
A $2,500 invoice was due 45 days ago at 1.5% monthly interest. Interest = $2,500 × 1.5% × (45/30) = $56.25. Add a $25 flat late fee and the client owes $2,581.25 today. Send a fresh invoice showing the breakdown and reference the original invoice number in the notes — that’s exactly what our invoice generator is built for.