What actually has to be on an invoice
There are two questions hiding in this one: what an invoice needs to get you paid, and what it needs to be a valid record for tax. They overlap, but they are not the same list.
To get paid
Someone in an accounts department is deciding whether to pass your invoice for payment. They need, without hunting:
- Who you are — trading name, address, and a way to reach a human.
- Who it is for — the right legal entity, not just the site name. "Flat 3" is not who pays.
- A reference they recognise — a purchase order number, a job number, a site address. Invoices without one sit in a pile.
- What the work was, in words the person paying will recognise from the quote they approved.
- The amount, and how to pay it — bank details or a payment link, on the invoice itself.
- A due date, as a date. "30 days" makes the reader do arithmetic, and readers who do arithmetic pay later.
For the tax authority
This part depends entirely on where you are, and it is worth ten minutes on your own tax authority's website rather than trusting a blog. That said, the same items appear in most jurisdictions:
- A unique, sequential invoice number. Sequential is the part people get wrong — see below.
- The date of issue, and sometimes separately the date the work was done.
- Your tax registration number, if you are registered.
- The tax treatment: the rate applied, the amount of tax, and the total before and after it. Where a reverse charge or an exemption applies, it usually has to be stated in words.
If you are registered for a sales tax and your invoice does not show what it must, your customer may not be able to reclaim it — which turns a paperwork problem into an argument with someone who owes you money.
The numbering rule people trip on
Most jurisdictions require invoice numbers to run in an unbroken sequence. The reason is auditing: a gap suggests an invoice was issued and then hidden.
The practical consequences are worth knowing:
- Do not assign a number when you start writing. Assign it when you issue. Drafts that get abandoned should not burn numbers.
- Do not delete an invoice you have sent. Cancel it — issue a credit note, or mark it void — so the number still exists with an explanation attached.
- Be careful with two devices, or two people, issuing at once. Both will happily produce number 0104.
The parts nobody requires, that get you paid faster
- Payment terms in a sentence, not a paragraph of legal text.
- A payment link or QR code. Removing the step where someone retypes an account number removes the day they meant to and forgot.
- What was already paid. If there was a deposit, show it, and show the balance. An invoice that appears to ask for the full amount again gets queried, and a query is a fortnight.