Late Fee Calculator
Work out the late fee and updated balance on an overdue invoice — by monthly percentage or a flat charge.
Written by Dorothy Ibrahim, 10+ years in banking & finance
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How we calculate this
This calculator works out the late fee and updated balance owed on an overdue invoice, whether you charge a monthly percentage or a flat amount. It converts the number of days an invoice is overdue into months, applies the fee, and shows the new total the client owes. A flat fee, when set above zero, replaces the monthly-percentage calculation entirely rather than being added to it.
The formulas
- Months late
- days late ÷ 30
- Late fee
- flat fee, if set above $0 — otherwise invoice amount × monthly fee % × months lateA flat fee above $0 overrides the monthly-percentage method; the two are never combined.
- New balance due
- invoice amount + late fee
Worked example
- With the defaults — a $5,000 invoice that is 45 days late — months late = 45 ÷ 30 = 1.5.
- No flat fee is set, so the fee uses the monthly percentage: $5,000 × 1.5% × 1.5 months = $112.50.
- The new balance due is $5,000 + $112.50 = $5,112.50.
Rates, benchmarks & sources
- A 1% to 1.5% per month late fee (roughly 12–18% annualized) is a common freelance norm, not a legal requirement. — Standard freelance contract practice (rule of thumb)
- Many states cap the maximum interest or late-payment rate that can be charged on overdue commercial invoices. — State usury and late-payment statutes (vary by state)
Figures current as of 2026-07-02. See our methodology & editorial standards for how constants are versioned and verified.
What this tool doesn’t model
- Does not determine whether a late fee is legally enforceable — that depends on your contract terms and state law, which this tool does not check.
- Does not check your state’s usury or late-payment rate caps; the monthly percentage you enter is a manual input with no built-in limit beyond the field range.
- Assumes a single invoice and a single fee method; it does not track partial payments, multiple overdue invoices, or compounding fees over several billing cycles.
Frequently asked questions
How much can I charge as a late fee?
A common freelance late fee is 1% to 1.5% of the invoice per month, roughly 12% to 18% annualized. Some states cap the maximum interest you can charge on overdue commercial invoices, so check your state’s limit and keep the fee reasonable before applying one.
Can I charge a late fee if it wasn’t in the contract?
Generally no. For a late fee to be enforceable, it should be stated in your signed contract or on the original invoice before the work is done. Adding a fee after the fact, with no prior agreement, is difficult to collect and can damage the client relationship.
Flat fee or percentage — which should I use?
A percentage per month scales with the invoice size and keeps growing the longer payment is late, which creates urgency on larger jobs. A flat fee is simpler and predictable, which suits small, fixed-price work. In this calculator, setting a flat fee above $0 overrides the percentage method.
How do I actually collect a late fee?
Send a polite but firm reminder that references your agreed terms and shows the new balance including the fee. Consistent, documented follow-up on a schedule collects most late invoices; for persistent non-payment, escalate to a formal demand letter or small-claims court.
Does this tool check my state’s legal limits?
No. This is a general contracting calculator, not legal advice, and it does not look up state usury or late-payment caps. Late fees are enforceable only when agreed in advance, and some states limit the maximum rate — confirm your contract terms and local law before charging one.