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Invoice Late Fee Calculator

Monthly rate × past-due balance — the standard, defensible way to size a late fee.

How the math works

A late fee is the monthly rate times the past-due balance, charged once for each month the invoice stays unpaid. On a $1,150 invoice at 1.5% per month, that's $17.25 a month — charged against the original balance, not fee-on-fee compounding, because simple interest is the version you can explain in one sentence and clients accept without argument. The full rules — what makes a fee enforceable, state rate caps, when to waive it — are in How to Charge Late Fees on an Invoice (Legally).

Calculator

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1.5%/month (18%/year) is the standard most service businesses use.

5–10 days is the common practice — it reads as fair.

Full months past the due date + grace period.

Late fee per month
$17.25
Total accrued (simple, no compounding)
$17.25
New balance
$1,167.25

Print this on your invoice

"1.5% per month on balances more than 10 days past due."

Simple, not compounded

The calculator charges the fee against the original past-due amount every month. On that $1,150 invoice, month two adds another $17.25 — not 1.5% of $1,167.25. At these scales the dollars barely differ, and simple is the version courts, clients, and you-at-tax-time can all verify in one line.

A fee only holds up if the client agreed to it before the work started — in your contract, estimate, or written terms — and it's repeated on the invoice next to the due date. If the policy first appears on a past-due reminder, it isn't enforceable. The four-step setup is in the enforceability section of the late-fee guide.

Check your state before you set the rate

This calculator does the arithmetic; it isn't legal advice. Usury and late-fee caps vary by state and are often stricter when your client is a consumer rather than a business, so verify your state's current rules before settling on the number — the late-fee guide covers what to check and links a maintained state-by-state roundup.

Next step

Put the policy line in your standard terms today and on every invoice from here forward — then apply it calmly, and waive it deliberately when a good client deserves the break. If you build invoices by voice, the policy is one spoken line: "Note: one point five percent monthly late fee on past-due balances."

Voice Invoice turns the spoken version of an invoice — policy line included — into a client-ready PDF in about a minute. Free for 3 invoices a month.

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