Short answer: yes — if the fee was in your agreement or on your invoice terms before the work started, or the client has since agreed to it. If it wasn't in writing anywhere, chase the base amount first and add the clause for your next client. Then use this to work out the number.
Worth getting right before you write it down, because the label changes the rules. A late fee is a fixed charge for breaking the payment terms — a flat $25, or a set percentage of the overdue balance for each month it sits. It is priced as compensation for the hassle, and it stops growing when you say it stops.
Interest is a running charge on money you are owed, accruing over time on the unpaid balance. Calling it interest is what pulls in a different rulebook: many states cap the interest rate a business may charge, the cap is usually stricter when your client is a consumer rather than another business, and some places require the rate to be disclosed in a particular way before it can be charged at all. A modest late fee that was agreed in writing up front is the lower-risk route for a solo freelancer, and it is what this calculator writes for you. If you want to charge true interest, or the sum is large, check your state's rules — or ask someone local who knows them — before you invoice it.
Either way, the arithmetic here is the same: rate times balance times periods. It is the wording and the ceiling that differ.
The pack is the rest of the job: the six-email follow-up sequence, the invoice template, the payment-terms clause that makes this fee enforceable next time, and the phone and text scripts for when email stops working. This calculator does the math — the pack does the chasing.
Want the schedule first? Read the free six-touch follow-up calendar and two full emails.