How the fee is calculated
The calculator uses simple interest, counted by day:
- Percent per month: amount × rate × days late ÷ 30. A $1,200 invoice at 1.5% per month, 45 days late: $27.
- Percent per year: amount × rate × days late ÷ 365.
- Flat fee: one fixed amount, whatever the number of days.
Only charge what you agreed
A late fee is easiest to enforce when the client agreed to it before the work started: in your contract, your estimate or the payment terms on the invoice itself. Write it plainly, for example “Payments more than 30 days late are charged 1.5% per month.”
Limits on late fees and interest depend on where you and your client are, and whether the client is a business or a consumer. Some places set a statutory rate for business-to-business debts. Check the rules that apply to you before adding a fee.
Often a reminder works better
Most late invoices are forgotten, not disputed. A friendly reminder on the day after the due date, a firmer one a week later and a phone call after that get most invoices paid without any fee. Our guide to overdue invoice emails has ready-to-copy wording.
Shorter terms help too: Net 30 vs Net 14 explains the options.
Last checked October 23, 2026.