How to Calculate a Late Fee on an Invoice (Flat, Monthly and Annual Methods)
General business information, not legal advice. Whether and how much you can charge as a late fee depends on your contract and on local law, which varies by state and country.
A late fee is only useful if you can state it clearly and calculate it the same way every time. These are the three common methods, with one invoice run through each.
The example: a $2,500 invoice, 45 days past due. You can also run your own numbers in the late fee calculator.
Method 1: Flat fee
You charge a fixed amount once the invoice is late, for example $50.
| Item | Amount |
|---|---|
| Invoice | $2,500.00 |
| Flat late fee | $50.00 |
| Total owed | $2,550.00 |
Easy to explain, but it does not grow with time or invoice size. On a $200 invoice, $50 is 25 percent; on a $20,000 invoice it is almost nothing.
Method 2: Percentage per month
You charge a percentage of the unpaid balance for each month it is late. Common contract wording is "1.5% per month." Check the maximum your state allows.
Formula: fee = invoice × monthly rate × (days late ÷ 30) if you prorate, or × number of started months if you charge per month.
| Version | Calculation | Fee |
|---|---|---|
| Prorated by day | 2,500 × 1.5% × (45 ÷ 30) | $56.25 |
| Per started month | 2,500 × 1.5% × 2 | $75.00 |
The two versions differ because day 31 is "one more month" in the second one. Write which one you use in your terms.
Method 3: Annual rate, simple interest
You state an annual rate such as 18 percent and charge daily simple interest.
Formula: fee = invoice × annual rate × (days late ÷ 365)
| Calculation | Fee |
|---|---|
| 2,500 × 18% × (45 ÷ 365) | $55.48 |
Note that 1.5 percent a month is 18 percent a year, so methods 2 and 3 give nearly the same answer. Some states cap interest rates, so confirm that your rate is lawful where you work.
What has to be true to charge it
- It is in your contract or agreed terms before the work. A fee you invent after the invoice is late is hard to enforce.
- It is on the invoice along with the due date.
- It is allowed by law where you and the client operate. Some places cap rates; check your state's rules.
- You apply it consistently. Waiving it the first time as a courtesy is fine, but say so in writing so it does not look like you forgot the term.
Where state or local law sets payment deadlines, those can matter too. For example, the "Freelance Isn't Free" laws in New York State, New York City, Seattle, Los Angeles, Minneapolis and Illinois require a written contract for work over $800 and, unless the contract says otherwise, payment within 30 days of completion, per Freelancers Union's summary (details can differ by place). Check whether a similar law covers you.
Wording you can adapt
"Invoices are due 15 days after the invoice date. Unpaid balances accrue a late fee of 1.5% per month, prorated daily, beginning the day after the due date."
Have a professional review any contract language you rely on.
Use the fee as leverage, not income
A late fee mainly signals that dates matter. The bigger lever is a reminder routine; see how to handle late payments and what to do when a client doesn't pay. The invoice itself needs the right details; see the invoice checklist.
If you want ready-made wording for quotes, scope and payment terms, the Proposal & Quote Template Pack in our shop includes editable templates and email scripts. Our own products are listed, along with everything else, on the shop page.