Agree it before you need it
The term belongs in the engagement letter or the quote, and then on every invoice. A fee that first appears on a reminder is a new condition invented after the fact, and a client who declines to pay it is usually right.
Word it so it can be applied
- The rate, and whether it is monthly or annual
- What it is charged on — the overdue amount, not the original total
- When it starts — the day after the due date, or after a stated grace period
- Whether it compounds, which for most small businesses it should not
Choose a rate you can defend
The point of the fee is to move you up the payment queue, not to earn interest. A rate that looks punitive invites a negotiation about the whole invoice, and some states cap what may be charged to consumers — so check your own state's rules before you set one.
What a standard late fee looks like
A flat fee is the usual alternative on small invoices, where a percentage would come to a few cents and cost more to chase than it is worth. Whichever you choose, it is charged on what is still unpaid, and it never compounds unless the contract says so in as many words.
How to calculate it
Multiply the overdue balance by the monthly rate, once for each month the invoice is late. Here is a $2,000 invoice at 1.5% a month, simple interest:
One month late: $2,000 × 1.5% = $30
Two months late: $30 + $30 = $60 — not $60.45, which is what compounding would give
If the client paid $800 during the first month, the second month's fee is charged on the $1,200 still owed: $18
Put the fee on the invoice as its own line, dated, rather than folding it into the original total. A client can then see exactly what the original work cost and what the delay added, which is the version of the number that holds up if anyone disputes it.
Applying it is a decision, not an automatic process
Having the term is what gives you the choice. Waiving the fee for a client who calls to explain a delay costs you nothing and buys goodwill; waiving it silently, every time, teaches everyone that the term is decorative.
Common questions
- How do I calculate a 1.5% late fee on an invoice?
- Multiply the overdue balance by 0.015 for each month it is late. On $2,000 that is $30 a month, charged on whatever is still unpaid, and without compounding unless your terms say otherwise.
- What is a standard late fee on an invoice?
- Between 1% and 1.5% a month on the overdue balance is common, or a flat fee on small invoices. There is no legal standard, only caps, and the caps on consumer invoices vary by state.
- Is it legal to add a late fee to an invoice?
- Generally yes, if the client agreed to the term before the invoice was late and the rate stays within your state's limits. A fee added after the fact is a new condition the client can refuse.
- Can I charge a late fee if it was not on the invoice?
- In practice, no. Without prior agreement you are proposing a new term after the fact, and a client can simply decline. Put it on the engagement letter and every invoice from the start.
- Should the fee be a percentage or a flat amount?
- A percentage of the overdue balance scales with the risk and reads as standard practice. A flat fee on a small invoice can look disproportionate, which is exactly when a client disputes it.
- Does a late fee hurt the relationship?
- Announced in advance and applied consistently, it rarely does — it is a term of business. Applied by surprise, it does, because it reads as a penalty rather than a policy.
Figures on this page last reviewed 2026-10.