Free Late Fee & Interest Calculator for Invoices
Work out what's owed on an overdue invoice — as a percentage interest rate that grows with the days overdue, or as a simple flat late fee.
Example result
- You enter
- Invoice amount
- $3,000
- Monthly interest rate
- 1.5%
- Days overdue
- 30
- You get
- Interest owed
- $45.00
- Total amount due
- $3,045.00
Fictional numbers, to show how the result is laid out.
How it works
Late payment charges on an invoice generally take one of two forms: a percentage-based interest rate applied to the outstanding balance for each day it stays unpaid, or a simple flat fee charged once the invoice crosses its due date regardless of how long it stays unpaid. Interest scales with both the amount owed and how late it is; a flat fee is the same dollar amount whether the invoice is 5 days or 50 days late.
A common interest structure is 1-1.5% per month, which works out to roughly 12-18% annualized — high enough to matter to a client, but not so high it looks punitive. A flat fee is usually a smaller, round number ($25-$50 is typical for small invoices) chosen more to discourage lateness than to compensate for it precisely.
The catch with either method is that you generally can't add it after the fact — most jurisdictions only enforce a late charge if the rate or fee was disclosed upfront, in the original invoice or contract terms the client agreed to. Check your own invoice template and local law before adding a charge that wasn't already stated, and see the Overdue Invoice Email Generator for a way to actually send the request.
A worked example: a $3,000 invoice that is 30 days overdue, at the default monthly rate of 1.5%, accrues $3,000 x 1.5% / 30 x 30 = $45.00 in interest, so the total due is $3,045.00. The tool takes a monthly rate and turns it into a daily rate by dividing by 30, so if your contract states an annual rate, divide it by 12 first. A second mode adds a flat late fee instead.
Interest is only worth adding if your contract or the law allows it, so check that first. Rules differ by country, and the UK, for example, publishes a statutory rate for business-to-business payments while many other places rely on the contract.
Who uses this
| Business type | How it's used |
|---|---|
| Contractors & tradespeople | Calculate interest owed on a client invoice that's gone unpaid past the agreed terms on a job. |
| Freelancers & consultants | Work out the exact interest to add to an overdue project invoice before following up with a client. |
| Wholesalers & suppliers | Calculate late fees on an overdue B2B order under net-30 or net-60 payment terms. |
| Photographers & creative freelancers | Add interest to an unpaid invoice for a shoot or project delivered weeks ago. |
| B2B service providers | Work out interest owed on a past-due retainer or service invoice. |
Frequently asked questions
Q.Should I charge interest or a flat late fee?
A flat fee is simpler to communicate and enforce, and works well for smaller invoices. Interest scales better for larger invoices or payments that stay outstanding a long time, since a flat $25 fee barely matters on a $50,000 unpaid balance. Many small businesses use a flat fee for simplicity.
Q.Can I actually charge interest or a late fee on a late invoice?
In most places, yes — as long as your original contract or invoice terms disclosed the rate or fee before the sale. Charging it retroactively without having stated it upfront is generally not enforceable, so check your invoice terms and local law.
Q.What's a reasonable late fee or interest rate?
1-1.5% per month (12-18% annually) is common for interest, and $25-$50 is a typical flat fee on small business invoices — though some jurisdictions cap the maximum you can charge. Check your local regulations before setting either.
Q.Is this the same as a late fee calculator?
Yes — late payment interest calculator, late fee calculator, and "calculate interest late payment" all describe the same tool: one that works out either a percentage-based interest charge or a flat fee on an overdue invoice.
Sources
- GOV.UK: Charging interest on late commercial payments: UK statutory interest of 8% plus the Bank of England base rate for business-to-business transactions
- GOV.UK: Claim debt recovery costs on late payments: fixed recovery costs by size of debt