How to Follow Up on an Unpaid Invoice

An invoice that has passed its due date needs two things from you: a reminder the customer can act on, and a clear view of what the law already provides while you wait. This guide offers a reminder schedule you can use, with each message written out, and sets it beside the rules on when a business invoice counts as late and whether a reminder changes anything.

The schedule is our suggestion. It is not a legal requirement, and it is not a statistic about what other businesses do. The legal references are to United Kingdom law and to the EU's late-payment directive, with a short note on how Germany and Spain treat reminders. For invoices between private businesses in the United States, the terms come from the contract and from state law, which this guide does not cover.

First, confirm the invoice is overdue

A reminder sent before the due date is an avoidable mistake. Work out the due date from the invoice date and the terms you agreed; the Invoice Due Date Calculator does that for you, and the guide on how to write payment terms on an invoice covers stating the terms so the date is not open to argument.

Under UK law the date matters for more than courtesy. The Late Payment of Commercial Debts (Interest) Act 1998 applies to contracts for goods or services where the purchaser and the supplier are each acting in the course of a business (section 2). Where the two agreed a payment day, statutory interest starts to run on the day after it. Where they agreed none, it starts on the day after the last day of a 30-day period, which begins with the later of the day the supplier performed and the day the purchaser had notice of the amount (section 4).

Does a reminder change anything in law?

That depends on whose law governs the invoice, and the difference is worth knowing before you write the first message.

• United Kingdom: section 4 fixes the start of statutory interest by the dates above. Nothing in it makes interest wait for a reminder.
• EU directive: Article 3 of Directive 2011/7/EU entitles the creditor to interest for late payment "without the necessity of a reminder", where the creditor has fulfilled its own obligations and has not received the amount on time, unless the debtor is not responsible for the delay.
• Germany: § 286 BGB works differently. A debtor who does not pay after a reminder (Mahnung) sent once the debt is due is in default through that reminder. No reminder is needed where the time for payment is fixed by the calendar, and the debtor of a payment claim is in default at the latest 30 days after the due date and receipt of an invoice.
• Spain: under article 5 of Ley 3/2004 the debtor is in default automatically on missing the agreed or statutory deadline, with no notice of the due date and no demand from the creditor.

Under the UK Act, the directive and the Spanish law, then, interest does not depend on the reminder. Under the German code a reminder can be the event that puts the debtor in default.

A reminder schedule you can use

The schedule below counts days from the due date: a first message on day 1, a second on day 7, a third on day 14 that states the interest, and a formal letter on day 30. If those intervals do not suit your terms, choose your own and keep to them. Date every message, name the invoice in each one, and keep a copy.

Day 1. The first message gives the customer everything needed to pay without looking anything up.

Subject: Invoice 1043, due September 14, 2026

Hi Dana,

Invoice 1043 for £2,400 was due yesterday, September 14, 2026, and I have not yet received payment. You can pay by bank transfer to the details on the invoice. I have attached the PDF again in case that is easier.

Thank you, Sam

It names the invoice number, the amount and the due date, gives one way to pay, and attaches the document again.

Day 7. The second message asks two questions.

Subject: Invoice 1043, 7 days past due

Hi Dana,

Invoice 1043 for £2,400 is now seven days past its due date of September 14, 2026. Could you tell me the date you expect to pay, or let me know if anything on the invoice needs correcting?

Thanks, Sam

The second question is there for a reason. A missing purchase order number or a queried line is something you can fix only once you have been told about it.

Day 14. The third message states the interest, with the rate and the law it rests on. "Interest may apply" tells the customer nothing; a figure does.

Subject: Invoice 1043, 14 days past due, statutory interest

Hi Dana,

Invoice 1043 for £2,400 was due on September 14, 2026 and is unpaid. Statutory interest under the Late Payment of Commercial Debts (Interest) Act 1998 has run since September 15, 2026 at 11.75% a year. That is £10.82 up to September 28, 2026 and £0.77 for each further day. Please let me know the payment date.

Sam

Day 30. If the invoice is still unpaid, the next step is a formal letter that sets out the whole claim. The guide on what to do when a client won't pay covers what that letter contains and which sums it can include.

How the figure in the day-14 message is built

Every input of the example is stated here so that you can replace it with your own.

• Principal: £2,400, with an agreed payment day of September 14, 2026.
• Start: interest runs from September 15, 2026, the day after the agreed payment day (section 4).
• Rate: the Late Payment of Commercial Debts (Rate of Interest) (No. 3) Order 2002 sets 8% a year over the official dealing rate in force on 30 June, for interest that starts to run between 1 July and 31 December. The Bank of England's Bank Rate on that date in 2026 was 3.75%, so the rate is 11.75%.
• Result: £2,400 x 11.75% x 14 / 365 is £10.82 up to September 28, 2026. By October 14, 2026, 30 days after the due date, it is £23.18.

The Late Payment Interest Calculator performs the same calculation for your dates. For a half-year whose reference rate its table does not hold, it asks you to enter the rate from the publisher. Once statutory interest runs, the UK Act also adds a fixed sum; the guide on late payment interest explains how each rate and sum is set.

When writing gets no answer

If two written reminders have had no reply, a phone call is a reasonable next step, because it lets you ask directly whether there is a problem with the invoice. Afterwards, put what was said in writing the same day: "Thank you for the call. To confirm, you will pay invoice 1043 in full by 5 October." A date agreed on the phone is then on the record with the rest of the correspondence.

One invoice handled this way is a habit. Applied to every invoice, it is a process, and the guide to the dunning process for small business covers setting one up, including the court procedures at the end of it.

FAQ

How soon after the due date should I send a reminder?

No rule sets a day. The schedule in this guide starts the day after the due date, with further messages on day 7, day 14 and day 30, and you can change the intervals to suit your terms. Check the due date against the invoice date and the agreed terms before you send anything.

What should the first reminder say?

State the invoice number, the amount, the due date and one way to pay, and attach the invoice again. That gives the customer everything needed to pay without looking anything up.

Does interest wait until I have sent a reminder?

Not under UK law, the EU directive or Spanish law. Section 4 of the Late Payment of Commercial Debts (Interest) Act 1998 starts statutory interest on the day after the agreed payment day, Article 3 of Directive 2011/7/EU grants interest without the necessity of a reminder, and article 5 of Ley 3/2004 makes default automatic. Germany differs: under § 286 BGB a reminder sent after the due date puts the debtor in default, unless the payment time is fixed by the calendar or 30 days have passed since the due date and receipt of the invoice.

From which day does UK statutory interest run?

From the day after the agreed payment day. Where no day was agreed, from the day after the last day of a 30-day period that begins with the later of the supplier's performance and the purchaser's notice of the amount (section 4 of the 1998 Act).

What rate applies to a late UK business invoice?

The rate order of 2002 sets 8% a year over the official dealing rate in force on the 30 June or 31 December before interest starts to run. The Late Payment Interest Calculator applies that rule to your dates.

When should I call instead of writing?

That is your judgment. This guide suggests a call once two written reminders have had no reply, followed the same day by a written note of what was agreed.