How do you find forgotten invoices?

You find forgotten invoices by looking for activity without an invoice. Six techniques catch most of them: gaps in a customer's regular invoicing rhythm, work order or sales order numbers with no invoice line, work in progress that has been open too long, costs on a customer project with no matching revenue, customers with activity but no invoice, and annual invoices that have not gone out this year. A forgotten invoice leaves no trace of its own, so you have to follow the trail of the work, not of the invoice.

Why forgotten invoices are so hard to find

You can find a wrong invoice, because it exists. A forgotten invoice does not. There is no document to check, no amount that looks off, no customer on the phone. The work is done, the customer is happy and pays the invoices they do receive. Nobody misses anything.

That is why searching your billing system for forgotten invoices does not work. You have to start with something else that does leave a trace: the work, the costs, contact with the customer or the rhythm of earlier invoices. This is the principle of a completeness check, set out more broadly in how to check that all revenue is invoiced. Below are six concrete techniques.

1. Gaps in the invoicing rhythm

Customers on a fixed contract get a regular invoice: every month, every quarter or every year. That rhythm is an expectation you can check.

How to do it. For each customer with recurring invoicing, list the invoice dates over the past two years. Look for months or quarters with no invoice, where the periods before and after do have one.

What you find. A month that was skipped because the invoice run was done by hand that month. A customer that was not carried over after a system migration. A subscription that was paused and never switched back on. For recurring revenue more broadly, including wrong amounts and quantities, see how to find errors in recurring revenue.

2. Numbers without an invoice

Work orders, sales orders, projects and variation orders usually have a number. Every number should end up on an invoice, or have a status explaining why not.

How to do it. Export all numbers for a period, for example all completed work orders from the past six months. Match them to the invoice lines. Every number with no invoice line and no status such as cancelled, warranty or included is a candidate.

What you find. Work orders left in a van or an inbox. Orders that were delivered but never marked as invoiced. Projects completed without a final invoice. In service businesses this is often the largest source, and it is closely linked to revenue leakage from manual administration.

3. Work in progress that is too old

Work in progress is work that has been carried out and not yet invoiced. Some of it is normal: work that, by agreement, is invoiced at the end of a phase. But work that sits in progress for months without a planned invoicing moment has often been forgotten.

How to do it. Sort work in progress per project or customer by age. Investigate everything older than sixty days that has no future invoicing moment.

What you find. Completed projects for which nobody raised the final invoice. Hours booked to a project after the last stage invoice went out. Additional work that was carried out but never flagged as additional work.

4. Costs without revenue

When you incur costs for a customer, you expect revenue in return. Materials booked to a project, a subcontractor who worked for a customer, travel to a customer site. If the costs are there and the revenue is not, that is a signal.

How to do it. For each customer or project, put the costs booked next to the revenue invoiced over the same period. Flag customers or projects with costs but no revenue, or with a margin much lower than normal.

What you find. Rechargeable costs that were never recharged. Projects whose invoice was forgotten while the purchasing was processed as usual. A subcontractor who did a job that you never invoiced.

5. Activity without an invoice

Customers you serve leave traces outside billing. Support tickets, engineer visits, logins to your platform, emails with questions, appointments in the calendar.

How to do it. List the customers with activity in the past three months, from your support system, scheduling or platform. Compare that list with the customers who received an invoice in the same period.

What you find. Customers whose contract has ended but who are still being helped. Customers receiving free support that is chargeable under the terms. Customers who never got a contract but are using your services.

6. Annual invoices missing this year

Some invoices come once a year: an annual licence, an annual maintenance contract, an inspection, a certification. Because they are so infrequent there is no routine, and one is easily forgotten.

How to do it. Find all of last year's invoices that are annual in nature. For each one, check whether this year's invoice has already gone out, or when it is planned.

What you find. Annual licences that roll over silently with no invoice. Maintenance contracts whose renewal invoice was never raised. An inspection that was carried out and not invoiced because the employee who always handled it has left.

Worked example

Worked example: suppose an installation company with EUR 5 million in revenue applies the six techniques to the past year.

Not all of it can still be invoiced. Work from ten months ago is harder than work from last month. But even what you can no longer invoice tells you where your process leaks.

Can you still send a forgotten invoice?

Legally, you can usually send a forgotten invoice later than most business owners assume, but limitation periods and contract terms differ, so check the rules in your jurisdiction. Commercially it is a different story. A customer who receives an invoice for work from a year ago is surprised and sometimes irritated. The older the invoice, the greater the chance of a dispute.

A few rules of thumb from practice:

• Work from the past three months: simply invoice it, with a short explanation if needed.
• Work from three to twelve months ago: contact the customer first, explain what happened, then invoice.
• Older than a year: judge case by case. Sometimes a conversation is wiser than an invoice. If in doubt about the legal side, take advice.

The lesson is the same: the sooner you find a forgotten invoice, the more likely you can still send it without damaging the relationship. That is the main reason to run these checks continuously rather than once a year. How to set that up is covered in how to detect revenue leakage automatically.

Plan for this week

1. Choose the technique that fits your largest revenue stream: work orders, sales orders, subscriptions or projects.
2. Export the data for the past six months.
3. Run the comparison and list the candidates.
4. Investigate the ten largest and decide for each: invoice, contact the customer or record why not.
5. Note the cause for each invoice found. After ten you will see a pattern.
6. Adjust the process to that pattern, and repeat the check next month.

For the broader framework of where revenue leaks away in your revenue chain and how to search for it systematically, see how to find revenue leakage in a business.

Frequently asked questions

What is the most common cause of a forgotten invoice?

A manual handover that does not happen. The work order never reaches the office, the email about additional work is missed, the colleague who always did it is on holiday. It is rarely a system error.

How far back should I look?

Start with six months. That is recent enough to still send the invoices you find and long enough to see patterns. If you find a lot, look further back.

Can I prevent forgotten invoices instead of searching for them?

Largely. Sequential numbering, a fixed deadline between completing work and invoicing it, and an automatic alert for work with no invoice prevent most cases. A periodic check is still needed to see whether it works.

What should I say to a customer who receives a late invoice?

Be honest: the invoice got stuck on your side. Give the details of the work so the customer can recognise it. For larger amounts or older invoices, a phone call beforehand is better than an invoice with no explanation.