How (and when) to charge late fees on invoices legally

Learn the legal way to add late fees to your contracts and invoices to get paid faster.

Charging interest isn't about getting rich—it's about getting paid. Here is the legal wording you need.

The Legal Requirement: It must be in the contract

You generally cannot surprise a client with a late fee if they never agreed to it. The "Late Fee Clause" must be present in your initial contract or Terms of Service (ToS) that the client signed or agreed to before work started.

Contract Wording (Copy-Paste)

"Payments not received by the due date will result in a late fee of 1.5% per month (18% APR) on the outstanding balance, or the maximum rate permitted by law, whichever is less. The Client will be responsible for all costs of collection, including reasonable attorney fees."

Note: 1.5% per month is standard B2B practice. It is high enough to be annoying, but low enough to be defensible.

How to calculate it

Use the simple interest formula. Do not compound it unless your contract specifically allows for compound interest (which is often illegal in consumer contracts).

The Formula:

(Invoice Amount) x (Interest Rate / 365) x (Days Late) = Late Fee

Example:

Invoice: $10,000
Rate: 18% per year (0.18)
Days Late: 30

Calculation: 10,000 x (0.18 / 365) x 30 = $147.95

When to waive the fee (The Strategy)

The late fee is a negotiation tool. You don't actually want the $147. You want the $10,000.

Step 1: Send a revised invoice WITH the late fee added.
Step 2: When the client complains, offer to waive it IF they pay the original amount within 24 hours.
Step 3: They pay immediately to "save" money. You get paid. Everyone wins.