Work out the late fee on an overdue invoice.

It depends on your state, and sometimes on the type of customer and what your contract says. Some states cap the rate or the amount, and a fee above the limit may not be enforceable. This calculator cannot tell you the limit where you work, so check your state's rules or ask a lawyer before you set one.

A late fee is easiest to stand behind when the customer agreed to it before the work began, in your quote, contract or invoice terms. Adding one afterwards is harder to justify and may not be allowed where you are. The simple habit is to state your late fee terms the same way on every quote and invoice.

It keeps the arithmetic predictable: the monthly rate is spread evenly over 30 days, so an invoice 15 days late carries half a month's fee. Your terms may count differently, for example charging a full month as soon as an invoice is one day late. If so, enter 30 days for each month you charge.

A flat fee is easy to explain and the same on every invoice. A percentage grows with the size of the invoice and with how late it is. Whichever you choose, it has to sit within your state's limits, and it should be written into your terms so it is never a surprise to the customer.