How to number invoices
Invoice numbering looks like a formatting choice and is actually a control. The sequence is what demonstrates that no invoice has been quietly removed — which is why in many countries it is expected to run without gaps, and why an auditor's first question about a missing number is rarely a friendly one. The good news is that a scheme that works is easy to set up and then requires no thought.
A scheme that survives contact with reality
Pick a prefix, a year and a counter, and stop there: INV-2026-0001. The prefix separates your invoices from your proformas and quotes, which need their own sequences. The year makes the number legible at a glance and gives you a natural reset point. The counter should be zero-padded to a fixed width, four digits being enough for most businesses, so that numbers sort correctly in every system that will ever touch them — INV-2026-0009 and INV-2026-0010 sort in the right order, INV-2026-9 and INV-2026-10 do not. The app increments the trailing digits and keeps your padding, so you set the format once on the first document.
Why gaps matter
A gap in a sequence has two possible explanations: an invoice was cancelled and documented as such, or an invoice existed and no longer does. Only you know which, and only the first one is comfortable. So do not delete an invoice that has been issued: if it is wrong, issue a credit note or a corrected document that references the original, and keep the original in the sequence marked as cancelled. Duplicates are the same problem from the other direction — two documents with one number cannot both be the one being referred to.
When to reset
Yearly is the common choice and it is a good one: the year in the number tells you the period without opening the file, and the counter never grows unwieldy. Reset on your financial year rather than the calendar year if the two differ, so a sequence maps to one set of accounts. What you should not do is reset monthly without putting the month in the number, which produces several different invoices numbered 0001 in one year and no way to tell them apart.
Prefixes for more than one thing
If you invoice under more than one brand, from more than one location, or in more than one currency, a prefix per stream keeps each sequence intact and readable: ACME-2026-0001 and NORTH-2026-0001. Resist encoding much more than that. Numbers that carry the client, the project and the month become impossible to keep unique, and the moment a number needs a human decision to allocate it, the sequence starts developing gaps.
Frequently asked questions
Do invoice numbers have to be sequential?
Many countries expect a sequence without gaps, and it is good practice everywhere because it is what shows nothing was removed. Check the rule that applies to you.
Can I start at 1000 so it looks like I have more clients?
You can start wherever you like; the requirement is about the sequence being unbroken from then on, not about where it begins. Whether it is a good idea is a separate question.
What if I need to cancel an invoice?
Do not delete it. Issue a credit note or a corrected document referencing the original, and keep the original number in the sequence marked as cancelled.
Should proformas use the invoice sequence?
No. Give them their own prefix and counter, otherwise your invoice sequence appears to have gaps.
How many digits should the counter have?
Four is plenty for most businesses and sorts correctly in spreadsheets and file listings. Keep the width fixed once you have chosen it.
Can two invoices share a number?
No. A number has to identify one document, otherwise a payment reference is ambiguous.
Invoices where the totals actually add up. Invoice Maker builds invoices, proformas, quotes and receipts in your browser, does the money in whole cents so the VAT line never drifts, and exports a PDF. Nothing is uploaded.
Make an invoice in the browser