Invoice due date and late fee calculator
Enter the invoice date and the payment terms to get the due date. If you also enter the date you were paid, or today's date, the page counts the late days and applies the annual rate, flat fee, or daily fee you choose.
Formula
Due date = invoice date + payment terms in calendar days Days late = max(0, as-of date − due date − grace days) Interest = amount × annual rate × days late / day count Flat percent fee = amount × flat rate, only when days late is above zero Flat fee = the flat amount, only when days late is above zero Per-day fee = daily amount × days late Total fee = interest + flat percent fee + flat fee + per-day fee Amount due = amount + total fee
Money is rounded to the nearest cent. Each fee is rounded on its own, then added.
Worked example
An invoice dated 1 January 2026 with net 30 days is due on 31 January 2026. The amount is $1,000.00. Checked on 15 February 2026, it is 15 days late. There is no grace period.
At 12% a year on a 365-day count, interest is 1,000 × 0.12 × 15 / 365, which rounds to $4.93. The flat fees in this example are zero, so the amount due is $1,004.93. The same 15 days on a 360-day count would be exactly $5.00.
FAQ
How is the due date counted?
Net 30 means the due date is 30 calendar days after the invoice date. An invoice dated 1 January is due on 31 January. Weekends are not skipped. Net 0 means the due date is the invoice date itself.
When does the late fee start?
The fee starts the day after the due date, unless you enter grace days. Days inside the grace period are not charged. A payment on the due date has no late fee, and neither does a payment that is still inside the grace period.
Which day-count should I use?
365 is the default simple year. Some contracts divide the annual rate by 360 instead. The annual rate is divided by the day-count you pick, then multiplied by the days late and the invoice amount. A flat percent, a flat amount, or a per-day amount is added only when the invoice is actually late.
Is this the interest the law requires?
No. Statutory interest and contractual late fees differ by country and by the words on the invoice. This page applies only the rate and the fees you type. Read the payment terms before you add a charge to a customer's bill.
The result is a planning estimate from the numbers on this page. It is not an invoice, a tax filing, or a marketplace statement.