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.
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.
Quick questions
Can I legally charge late fees? Only if it's in your contract and within local law. 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."