Learning how to issue an invoice properly is not about finding a good-looking template. It is about a document that carries every required detail, has a number nobody else has, and reaches the client while the work is still fresh. This guide walks through the whole thing: what belongs on the document, how numbering works, when payment falls due, and the mistakes that cost you time.
What an invoice must legally contain is set by the country you invoice from, and the rules differ more than you would expect. This guide covers what holds true almost everywhere; check your own rules before you settle on a template.
What to have ready
Before you open a blank form, gather four things:
- Your own details. Legal name, registered address, company registration number, and your VAT number if you have one.
- Whether you are registered for VAT. This single answer decides whether tax appears on the document at all, and it changes what else is required.
- The client's details. Name, address and registration number. In Lucanto you can type a company number and the details of a Slovak or Czech client fill in from the public register — including whether they are VAT-registered.
- A numbering series. Decide the format before you issue the first document, not after the twentieth.
With those in hand, filling the invoice in takes minutes.
What belongs on the invoice
Whatever the jurisdiction, an invoice answers the same questions. Somebody reading it — the client, their accountant, an auditor two years later — needs to see:
- Who is invoicing whom. Both sides by legal name and address, with registration numbers.
- A unique number. One per document, never reused.
- When it was issued, and when the goods or service were actually supplied. These are two different dates and they are often not the same day.
- What was delivered. A description specific enough to recognise, plus quantity and unit price.
- What is owed. The total, the currency, and how to pay it — account number and a payment reference.
If you are registered for VAT, add your VAT number, the rate applied and the tax amount. If a special scheme applies — reverse charge, margin scheme, an exemption — the document usually has to say so in words, not just leave the tax off.
Invoicing without VAT
If you are not VAT-registered, leave tax out completely. No rate, no tax amount, no VAT number. Everything else stays exactly as above.
The most common mistake here comes from copying someone else's template: the VAT rate stays in and the invoice claims tax you have no right to charge. A short line saying you are not registered for VAT prevents the follow-up email.
Numbering
Every invoice needs a unique number, and the sequence should be unbroken and chronological. Gaps and duplicates are the first thing anyone checking your books will ask about.
The format is your choice. Year plus a counter — 2026001, 2026002 — is common because it sorts correctly and tells you at a glance which year a document belongs to. Keep separate series for separate document types: invoices, advance invoices, credit notes.
Done by hand, this means tracking the last number you used and hoping two documents never collide. Software assigns the number and refuses to issue the same one twice.
Payment terms
Put a due date on every invoice. Terms are usually a matter of agreement rather than a legal requirement of the document, but without a date "overdue" means nothing — and you have no ground to chase.
Many countries set a default period that applies when you agreed nothing, and a maximum for business-to-business terms. Fourteen days is a common practical choice: long enough for the client's payment run, short enough that the money is not sitting on someone else's account for a month.
Sending it
Email as a PDF is the norm, and an electronic invoice is worth exactly as much as a paper one. In most places the recipient accepts the electronic form simply by taking the document and processing it.
Where a payment QR standard exists, use it. The client scans the code in their banking app and the payment is pre-filled — no retyping an account number, no transposed digits, no invoice sitting unpaid because paying it was a chore. What happens to the document after you send it is covered in the invoice statuses guide.
The mistakes that cost you
- A missing or duplicated number. Two invoices sharing a number is a problem in your books and an obvious one to anyone checking.
- A VAT rate on an invoice from someone not registered for VAT. Almost always a copied template.
- No date of supply. It is not the same as the issue date, and where it is required, leaving it out invalidates the document.
- Editing an invoice after it went out. Once issued, an invoice is corrected with a credit note, not by overwriting the original.
- Sending it late. Some countries put a hard deadline on issuing the document. Even where they do not, every day you wait is a day added to getting paid.