Work out interest on an overdue invoice
A polite reminder lands harder when it comes with a real number. This calculator works out the interest accrued on an overdue invoice based on the monthly rate you've agreed and the number of days it's late, then shows the total now due.
The math behind it
Late fee = invoice amount × (monthly rate ÷ 30) × days overdue. This prorates the monthly interest down to a daily amount, so a payment that's 10 days late isn't charged a full month's interest.
A worked example
Say a $2,000 invoice is 30 days overdue and your contract sets a 1.5% monthly late fee. Here's what's now due:
| Input | Value |
|---|---|
| Invoice amount | $2,000 |
| Monthly interest | 1.5% |
| Days overdue | 30 |
| Late fee accrued | $30 |
| Total now due | $2,030 |
Thirty dollars won't get rich, but that's not the point — the fee turns "please pay" into "the meter is running," which is what actually changes behaviour. The longer it goes unpaid, the more it accrues.
Setting a fair — and legal — late fee
A monthly rate of around 1–1.5% is common, but two rules matter more than the number: it must be written into your contract before the work starts, and it must stay within the legal limits where you operate. Some places cap interest rates; some (for example, B2B invoices in parts of the EU and UK) even give you a statutory right to interest and recovery costs. Check before you charge.
Late payments are mostly preventable. See our full guide on getting clients to pay on time for deposits, terms, and a calm escalation ladder — and make sure your invoices state the terms clearly.
Frequently asked questions
How do I calculate a late payment fee? Invoice amount × (monthly rate ÷ 30) × days overdue. That prorates the monthly interest to a daily figure, so 10 days late isn't a full month's charge.
What's a reasonable late fee? Around 1–1.5% per month is common, but it must be in your contract beforehand and within local law.
Can I legally charge late fees? Only if it's in your contract and within local law — some places cap rates, some grant statutory interest on B2B invoices. Verify both.
How do I bring it up? Reference the clause the client already agreed to: "As per our agreement, a 1.5% monthly late fee now applies."
How do I avoid late payments? Deposits, clear written terms, prompt invoicing, and a calm reminder schedule. See getting clients to pay on time.
Flat fee or interest? Either, if it's in your contract. Interest scales with the delay; a flat fee is simpler. Some combine both after a grace period.