Invoice Late Fee Calculator
Calculate a late fee on an overdue invoice using simple interest, based on the annual rate stated in your contract or invoice terms and the number of days payment is late.
Simple interest, priced by the day
The annual rate from your terms is spread over a 365-day year and applied to the unpaid invoice - there is no compounding and no monthly step:
late fee = invoice × (annual rate ÷ 100) × (days late ÷ 365)
total owed = invoice + late feeA $1,000 invoice at 18% a year, 30 days past due, earns $14.79, so $1,014.79 is owed. Leave it 90 days and the fee is $44.38. The fee always sits on the original amount, so unpaid interest never starts earning interest of its own.
Rates and what they come to
| Annual rate | Per month equivalent | Fee after 30 days on $1,000 |
|---|---|---|
| 5% | 0.42% | $4.11 |
| 12% | 1.0% | $9.86 |
| 18% | 1.5% | $14.79 |
| 24% | 2.0% | $19.73 |
Terms are often written as "1.5% per month" rather than as an annual figure. Multiply by 12 to get the rate this box wants, but expect small differences: a flat monthly charge of 1.5% on $1,000 is $15.00 whether the month runs 28 days or 31, while daily interest gives $14.79 for 30.
Frequently asked questions
How much is the late fee on a $1,000 invoice 30 days overdue?
At an 18% annual rate, $14.79, bringing the total to $1,014.79. The same invoice 90 days late carries $44.38.
What is 1.5% per month as an annual late fee rate?
18%. Enter that in the rate box and the days do the rest, though a per-day calculation will differ by pennies from charging a flat 1.5% each time a month rolls over.
Should I actually charge my client the late fee?
Its real job is to make paying on time cheaper than not, which needs the term visible up front. Plenty of contractors invoice the fee and then waive it once the balance clears; what does not work is inventing one after the invoice is already late.