What actually has to be on an invoice
There are two questions hiding inside this one, and they have different answers. What does an invoice need in order to get you paid? And what does it need in order to be a valid record for tax?
The lists overlap. They are not the same list, and it is the first one that decides whether you are chasing money in five weeks.
What gets it paid
Picture the person deciding. On a domestic job it is the customer at their kitchen table. On anything commercial it is somebody in accounts with forty invoices to process before Thursday, no memory of your job, and no appetite for ringing you about it.
That second person needs a small number of things, none of which they should have to hunt for.
Who you are, meaning a trading name, an address and a way to reach a human. Who it is for, meaning the correct legal entity rather than the site: "Flat 3" is not who pays, and neither is "the Hendersons" if the work was ordered by a letting agent. A reference they recognise, which on commercial work means the purchase order number and on domestic work means the job or the address. And what the work was, described in the words the person paying will recognise from the quote they approved, rather than in your own shorthand.
Then the money: the amount, how to pay it, and a due date written as an actual date. "Payment terms: 30 days" makes the reader do arithmetic, and readers who have to do arithmetic pay later. "Due 14 October 2026" does not have that problem.
Miss the purchase order number and none of the rest matters. The invoice is not rejected, which would at least tell you something. It sits.
What the tax authority needs
This part depends entirely on where you are, so ten minutes on your own tax authority's website beats any blog, including this one. What follows is the UK position, and most of the same items turn up in most countries.
If you are not registered for VAT the requirements are light: your name and address, the customer's, a unique invoice number, the date, a description of the work and the amount. Close to what you would write anyway. HMRC lists them on its own page about what an invoice must include, which is worth five minutes.
Registered is a different document. A full VAT invoice carries your VAT number, the tax point (usually the invoice date, though on staged work it can be the payment date), the rate applied to each line, the total before VAT, the VAT itself, and the total after it. Where a special treatment applies it has to be stated in words rather than implied by a zero. The domestic reverse charge for construction is the one that catches people out: on qualifying work between VAT registered contractors the invoice shows no VAT and says so on its face, and the customer accounts for it instead.
Getting that wrong is not only your problem. If your invoice does not show what it must, your customer may be unable to reclaim the VAT, which turns a formatting mistake into an argument with the person who owes you money.
Showing tax so that nobody has to guess
State whether a figure includes tax. Every time, in words.
£2,400 plus VAT at 20% is £480 of VAT and £2,880 to pay. The same £2,400 described as including VAT is a £2,000 job with £400 of tax on it. That is a £480 difference on a single line, arrived at entirely through a missing preposition. Show all three numbers, net, tax and gross, and never make anyone reverse engineer one of them.
The numbering rule people trip on
Invoice numbers have to be unique and, in most places, sequential. The reason is auditing: a gap in a sequence suggests an invoice was issued and then made to disappear.
Three consequences follow, and they are all about when a number is assigned rather than what it looks like.
Do not assign a number when you start writing. Assign it when you issue. A draft that never gets sent should not burn a number, and if your software allocates on creation you will end up explaining gaps that only ever existed in your own abandoned attempts.
Do not delete an invoice you have already sent. Cancel it instead, with a credit note or a void marker, so that the number still exists with a reason attached. Deleting leaves exactly the gap you were trying to avoid, and it removes your record of a document somebody out there is still holding.
And be careful with two devices, or two people, issuing at the same time. Both will cheerfully produce number 0104, and you will find out when an accounts department asks which of the two 0104s it is supposed to pay.
Prefixes help more than they look as though they should. EST for estimates and INV for invoices, kept in separate sequences, means a glance at a number tells you which document you are holding.
The parts nobody requires that get you paid faster
Terms in a sentence, not a paragraph of legal text nobody reads. "Payment due within 14 days by bank transfer" beats four lines of clauses that make you look as though you are expecting trouble.
A payment link or a QR code, because removing the step where somebody retypes an eight digit account number removes the evening they meant to and forgot.
And what has already been paid. If there was a deposit or a stage payment, show it, and show the balance. An invoice that appears to ask for the full amount a second time gets queried, and a query on a commercial job is a fortnight before anybody looks at it again. A job invoiced at £6,450 with £1,935 already taken as a deposit should say £4,515 to pay, with all three figures on the page.
The one that costs the most
Late payment is mostly not a decision anybody made.
On 30 day terms, an invoice that clears at 52 days has lent that customer the money for an extra three weeks, and almost never because somebody chose not to pay. It went into a query pile over a missing reference. It landed with the one person who was on leave. The due date said "30 days" and the reader did the sum wrong, or did not do it at all.
None of those are payment problems. They are document problems, and document problems are fixed once, on the template, rather than every month on the telephone. The same goes for an invoice that is the same every month: set it to repeat and check the draft, rather than retyping it and the mistakes with it.
Everything above is on the sheet the free invoice generator opens: numbered, dated, the tax as its own figure, and your payment details at the foot.