Late Payment Fee Calculator
When an invoice is overdue, you need an accurate updated balance before contacting the client. This tool calculates the number of days overdue, the fee owed under your chosen method (flat, percentage, or daily interest), and the new total — so your follow-up is specific and defensible.
Days overdue
20
Fee amount
$25.00
Total owed
$2,525.00
Reminder email: Your invoice is now 20 days overdue. Updated balance: $2,525.00 including late fee.
When and how to apply late payment fees
Late fees only apply if they were agreed in writing before the project began — they should appear in your contract and be referenced on the invoice. You cannot introduce a late fee for the first time in a chasing email and expect it to be enforceable. The clause in the contract is the prerequisite; this calculator gives you the accurate number once the clause is triggered.
In the UK, even without a contractual late fee clause, the Late Payment of Commercial Debts (Interest) Act 1998 gives freelancers a statutory right to claim interest at 8% above the Bank of England base rate on overdue B2B invoices. Fixed debt recovery costs (£40–£100 depending on invoice value) can also be claimed. You do not need a contractual clause to exercise this statutory right, but referencing it in your final notice makes the legal basis explicit.
Fee calculation methods compared
| Method | How it works | Example (£2,000 invoice, 30 days late) |
|---|---|---|
| Flat fee | Fixed amount applied once after due date | £50 flat fee → total £2,050 |
| Monthly percentage | Percentage of outstanding balance per month | 1.5%/month × 1 month = £30 → total £2,030 |
| Daily interest | Percentage of balance per day overdue | 0.05%/day × 30 days = £30 → total £2,030 |
| Statutory (UK) | 8% + BoE base rate annually, pro-rated | ~13% annual / 365 × 30 days × £2,000 ≈ £21.37 + £40 fixed cost |
Rates shown are illustrative. Use the calculator above to get the precise figure for your invoice.
How to use the fee amount in a reminder
Once you have the updated balance, reference it precisely in your follow-up email: invoice number, original amount, fee applied, and the new total due. Specificity signals that you are tracking the debt carefully and reduces the chance of a client claiming ignorance of the terms.
For example: “Invoice INV-042 for £2,000 remains unpaid and is now 30 days overdue. Per our agreement, a late fee of £30 has been applied. The outstanding balance is £2,030, due by [date].”
Keep the message short and factual. The Late Payment Email Template below has copy-ready text for each stage of escalation.
After calculating the updated balance, copy the reminder language from the Late Payment Email Template and read the full escalation approach in the late payment guide. To regenerate a clean version of the original invoice, use the Invoice Generator.
How to read the results
Use the calculated total in your payment reminder rather than the original invoice amount. A precise figure that includes the late fee signals you are tracking the debt carefully and have contractual grounds to apply it.
Days overdue, fee amount, and updated total are your three outputs. Use all three in the reminder email — not just the original invoice amount. If the client disputes the fee, point to the contract clause and the invoice where it was disclosed.
Best practice
Only apply late fees if they are in your contract. Introducing fees for the first time in a chasing email is not enforceable and damages the relationship unnecessarily. The clause needs to be agreed before work begins.
Worked example
An invoice for £2,000 overdue by 30 days at 1.5% monthly generates a fee of £30 and a total due of £2,030. Reference both figures in the reminder.
Swap your own assumptions to create a quote-ready number or policy clause.
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FAQ
In most jurisdictions, no — you cannot apply a contractual late fee without prior agreement. However, in the UK, the Late Payment of Commercial Debts Act gives B2B freelancers a statutory right to interest and fixed recovery costs without a contract clause. Check equivalent legislation in your country.
Both approaches are valid depending on the relationship and payment history. For a first-time late payment from a reliable client, referencing that fees may apply without immediately charging them can be enough to prompt payment. For a client with a pattern of late payment, applying fees consistently teaches them that terms are real.
Point to the clause in the signed contract and the reference to late payment terms on the original invoice. If both are present, the dispute is likely about willingness, not legality. For significant amounts, consult a solicitor or use formal debt recovery.
Yes, and sometimes that is the right call. If you waive, do it explicitly in writing — 'I am waiving the late fee on this occasion, but future invoices will have the terms applied as agreed' — so it does not set a precedent of terms being optional.