Free calculator
Late fee calculator
Enter the overdue amount, your late fee rate and how many days late the payment is. You get the interest, any flat fee and the new amount due.
Late fee
$44.38
- Invoice amount
- $2,000.00
- Interest: 45 days at 18% a year
- $44.38
- New amount due
- $2,044.38
How the late fee is worked out
The calculator charges simple interest by the day. A monthly rate counts twelve times over a year, so 1.5% a month is 18% a year. On a $2,000 invoice 45 days late, that is $2,000 × 18% × 45 ÷ 365 = $44.38.
You can add a flat fee as well, such as $25 for any late payment. Only charge what your terms allow.
Make late fees enforceable
- Agree the late fee before the work starts, in your contract or quote.
- Print it on every invoice: for example, “1.5% per month on balances unpaid after the due date.”
- Stay under your state’s limit. Many states cap interest on overdue accounts; 1% to 1.5% a month is common.
- Send a new invoice or statement showing the original amount, the fee and the new total.
Last updated 2026-09-30. This calculator is a guide, not tax advice.
Frequently asked questions
What is a normal late fee on an invoice?
1% to 1.5% a month on the unpaid balance is common, sometimes with a small flat fee. Whatever you choose must be agreed up front and within your state’s legal limit.
Can I charge a late fee if it wasn’t on the invoice?
Usually not. Late fees are enforceable when the customer agreed to them beforehand, in a contract or in terms they accepted.
Is it simple or compound interest?
This calculator uses simple interest, the usual way to charge late fees on invoices: interest is not charged on earlier interest.