Invoice Late Fee Calculator
Work out the late fee and interest owed on an overdue invoice from the balance, your late-fee terms and the number of days past due.
How it works
Monthly: fee = balance × rate% × days ÷ 30 · Annual: fee = balance × rate% × days ÷ 365 · Flat: fee applied once past due · New balance = balance + fee
A percentage late fee is really interest on the unpaid balance, so it grows with each day overdue; a flat fee is a single charge that applies once the invoice passes its due date. The classic "1.5% per month" term equals an 18% annual rate, which is why many jurisdictions cap it. Always state your late-fee terms on the original invoice — a fee you never disclosed is hard to enforce.
Worked example
A $2,400 invoice is 20 days late under a 1.5%-per-month term. The fee is 2,400 × 0.015 × 20 ÷ 30 = $24, so the new balance due is $2,424 — accruing about $1.20 a day until paid.
Frequently asked questions
Is a 1.5% monthly late fee legal?
Many jurisdictions allow it, but some cap the maximum interest rate on commercial debt. 1.5% per month is 18% per year — check your local rules and always disclose the rate on the invoice before charging it.
Can I charge both a flat fee and interest?
Some businesses do — a fixed penalty plus running interest — but only if both are stated in the contract or invoice terms. This calculator models one method at a time; add the two figures if your terms combine them.
When does the late fee start?
Typically the day after the due date, sometimes after a short grace period. Count days from the first day the payment is actually late.
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Educational estimate, not financial advice.