How the Late Invoice Fee Calculator Works
Interest is calculated as simple interest using a 365-day year. The one-time percentage and fixed fee are added once; interest is not compounded.
Formula and method
Simple interest = principal × annual rate × days overdue ÷ 365. Late percentage = principal × one-time late-fee rate. Estimated amount due = principal + simple interest + late percentage + fixed administration fee.
Practical tips
Check the signed agreement, invoice terms, local limits, notice requirements, and whether a fee can legally be charged before requesting payment.
Important limitations
The tool does not determine whether interest or penalties are legally enforceable. It excludes compounding, partial payments, grace periods, variable rates, and jurisdiction-specific rules.