Invoice Late Fee Calculator
Calculate flat late fees, accrued interest, total fees, and total due on overdue invoices.
Last updated: 2026-03-27
Invoice late fee calculator
Enter your values
Estimate overdue charges from the invoice balance, lateness, and the clause in your terms.
Total Late Fees
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Enter invoice amount, days late, flat fee, interest rate, and interest mode to estimate the new balance due.
Calculation History(0)
Example calculations
Tap an example to prefill the calculator with sample values.
Freelance invoice at 45 days
$5,000 invoice with a flat fee and daily interest
A common late-payment clause structure used in service agreements and invoice terms.
Result: The late fee can move from symbolic to meaningful once interest keeps accruing beyond the first month.
Small retainer overdue
Monthly interest on a lower-dollar invoice
Useful for recurring retainers or small-business invoices where the clause is written in monthly terms rather than daily terms.
Result: Monthly simple interest is easier to explain to clients, though the economic effect can still add up.
Larger B2B invoice
Higher balance with no flat fee
A cleaner interest-only clause where the size of the invoice matters more than the administrative fee.
Result: On bigger invoices, even a moderate annual rate can create meaningful carry cost quickly.
How the late fee estimate works
The calculator starts with the overdue invoice amount and adds any one-time flat late fee you charge. It then applies either daily simple interest or monthly simple interest to the principal for the number of late days entered.
The result shows interest charges, total late fees, and the new amount due. This is a planning model only, so it does not decide whether a clause is enforceable or whether notice steps were met.
Invoice late fee FAQs
Match the estimate to the clause you actually wrote into the invoice or service agreement.
Why offer both daily and monthly interest modes?
Because invoice terms vary. Some contracts specify a daily rate, while others describe a monthly late charge such as 1% or 1.5% per month. The calculator lets you match the clause you actually use.
Does this tell me what is legally enforceable?
No. It only calculates the amount implied by the terms you enter. Enforceability can depend on contract language, local law, notice requirements, and whether the clause is considered reasonable.
Should interest apply to the flat fee too?
Most simple invoice clauses apply interest to the overdue principal balance, not to the flat administrative late fee. This calculator follows that simpler and more common planning approach.
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Contract and legal note
This calculator does not determine whether a late-fee clause is valid in your jurisdiction or contract. Review your agreement and local law before relying on the estimate for billing or collections.