An invoice is a demand for payment that can be checked later by somebody who was not there. Almost everything on it follows from those two jobs, which is why the lists in different countries look so similar even though the rules behind them are different.
The blocks nearly everyone expects
- The word "Invoice". Sounds trivial. It is what distinguishes a demand for payment from a quote, an estimate or a receipt, and some systems route documents on it.
- A unique invoice number. More on this below — it is the field that most often causes trouble.
- The date it was issued, and separately the date payment is due.
- Who you are. Trading name, address, and your tax or company registration number where you have one.
- Who it is for. The client's legal name, not the name of the person you email. Paying an invoice made out to a person when the contract is with a company causes real friction.
- What you supplied, line by line, with quantities and unit prices — enough for somebody who was not in the room to see what was bought.
- Subtotal, tax and total, kept as separate lines rather than one number.
- How to pay you. Full bank details, and the reference you want them to quote.
The number is not decoration
It has to be unique, and it is usually expected to run in sequence. In several places that is a requirement rather than a convention: a VAT invoice in the European Union must carry a sequential number that uniquely identifies it.
The reason is auditing. A sequence with no gaps is evidence that nothing was quietly removed, which is precisely what an inspector is checking. Which produces two practical rules that surprise people:
- Do not reuse a number. Ever, including for a corrected version of the same job.
- Do not delete an invoice. If it was wrong, issue a credit note against it. A gap in the sequence is the thing you are trying not to have.
A prefix per client or per year is fine — 2026-014, ACME-007 — as long as each series is its own unbroken run. What is not fine is a number that restarts, or one built from a date that two invoices could share.
The payment block decides whether you get paid
This is the part freelancers underinvest in, and it is the part with the most direct effect on cash.
Give a date, not a term. "Net 30" asks the reader to work out when the clock started and to do arithmetic they have no incentive to do promptly. "Due 8 October 2026" is a fact that either has passed or has not, and it can be put straight into a diary.
Put the bank details on the invoice itself, not in the covering email. The invoice is what gets forwarded to whoever actually pays, and the email usually is not.
Give them a reference to quote — the invoice number is the obvious one — because an unreferenced payment lands in a bank account and matches nothing.
One thing worth knowing: in many countries, including across the EU and the UK, a business has a statutory right to interest on a late commercial payment whether or not the invoice mentions it. Saying so on the invoice is not what creates the right, but it does tend to concentrate minds.
Tax is where the rules stop being universal
Everything above travels. Tax does not.
Whether you must show a tax registration number, whether you must break out the rate, what wording a cross-border supply needs, whether an invoice must be issued electronically in a prescribed format, and how long you must keep copies — all of that is decided where you are and where your client is, and it changes.
Two situations that catch people specifically: selling across a border, where the tax treatment often shifts to the buyer and the invoice has to say so in particular words; and selling to consumers rather than businesses, which in many places has different requirements from selling business to business. Both are worth ten minutes with somebody local once, and then it is settled forever.
An invoice is not a receipt
They get confused constantly and they are opposite documents. An invoice is issued before payment and asks for money. A receipt is issued after and confirms money arrived. A client asking for a receipt after paying does not want the invoice again — and for their own bookkeeping, they often need both.
Related: a proforma invoice is a quote formatted like an invoice. It is not a demand for payment, it does not go in your sales ledger, and it should not take a number from your real sequence.
Send a PDF, not a document
A PDF looks the same everywhere and cannot be edited by accident on the way. A word processor file can be, and an image of an invoice cannot be searched or copied from by the person entering it into their system — which slows down the thing you want to happen quickly.
The invoice generator here builds the PDF in your browser, with no account between you and the file, which for a document carrying your bank details and your client list is the arrangement that makes sense. If the client wants it signed, you can add a signature without uploading it either — though for an invoice a signature is almost never required, and our guide on electronic signatures covers when it is.
This is a plain-language summary of what invoices generally carry, not tax or legal advice. The requirements that matter to you depend on where you and your client are registered, and an accountant in your own country is worth more than any web page.