Free Tool
Free late fee and interest calculator
Enter the invoice amount, days past due, and your annual rate to get the interest owed and the total now due. Free, no signup.
1.5%/mo = 18%/yr is common
Private by design — everything you type stays in your browser. Nothing is uploaded, stored, or seen by us.
Put a number on a past-due invoice
When an invoice goes past due, a clear late fee gives you a reason to follow up and a number to put on the statement. This calculator figures the interest so you are not guessing.
It uses simple interest: the invoice amount times your annual rate times the days past due, divided by 365. A common contract term is one and a half percent per month, which is eighteen percent a year. Add a flat late fee on top if your contract allows one.
Two rules make this collectible. First, only charge interest or a fee if your contract or invoice says you will, in writing, before the work. Second, stay within your state's cap, since some limit the rate you can charge on a past-due balance.
The best late fee is the one you never have to use. A clear "net 30, interest after" line, sent with every invoice, gets most customers to pay on time.
Late Fee Calculator questions
How do I calculate a late fee on an invoice?
Multiply the invoice by your annual interest rate, then by the days past due, divided by 365. This calculator does it and adds any flat fee.
What is a normal late fee for contractors?
One and a half percent per month, or eighteen percent a year, is a common term. Check your state's cap, since some limit the maximum rate.
Can I charge a late fee without it in the contract?
Generally no. Put the term on the contract and the invoice before the work so it is enforceable. This tool is an estimate, not legal advice.
Is this really free?
Yes. No email, no signup, and nothing you type leaves your browser.
Get paid before it is late
Werx sends the invoice, automatic reminders, and a pay-by-card link, so fewer invoices ever reach past-due. 14-day free trial, no credit card.
No credit card required · Free migration assistance · Cancel anytime
Disclaimer: These free tools and templates are provided for general information and estimating purposes only. All results — including quantities, material and cost estimates, load, sizing, and span figures, scores, and any guidance or warnings — are rough approximations that may not fit your specific project, site conditions, local building codes, or current prices. They are not engineering, architectural, structural, legal, financial, or other professional advice, and are not a substitute for a licensed engineer, architect, contractor, or other qualified professional. Always confirm quantities, sizing, and structural, electrical, mechanical, and code requirements with a licensed professional and your local authority having jurisdiction before purchasing materials or performing any work. Werx makes no warranty as to accuracy and accepts no liability for any decision made using these tools. Use at your own risk. See our Terms of Service.