What is an invoice?
An invoice is a dated request for payment — not a quote, not a receipt. The eight things every invoice must carry, and the documents it gets confused with.
An invoice is a dated request for payment. It names who is asking, who is being asked, what was supplied, what it costs and when the money is due — and it carries a unique number so that both sides, and everybody's accountant, can refer to the same document later without describing it.
That is the whole definition. Everything else is a consequence of it: an invoice creates a debt, which is why it has to be precise, why it cannot be edited after it is sent, and why three other documents that look similar are not invoices at all.
What an invoice has to contain
Requirements vary by country, but eight things appear in essentially every version of the rules, and a missing one is the usual reason an invoice is sent back rather than paid.
1. The word "Invoice". Written on it, prominently. It is what separates a demand for payment from a quote, an estimate or a statement, and some accounts systems will not accept a document that does not say what it is.
2. A unique invoice number. Never reused, and running in sequence with no gaps. This is the single most common reason an invoice is rejected, and the gap rule matters more than people expect — a missing number looks, to an auditor, exactly like a deleted invoice. Invoice numbers covers the rules and three schemes that work.
3. Your details. Your trading name, address, and tax registration number if you have one. If you are a registered company, the registered name and number usually have to appear too — the name on the invoice must match the name on the bank account, or payment stalls.
4. The client's legal name and address. Their registered entity, not the brand on the door and not your contact's personal name. An invoice addressed to the wrong entity cannot be entered in their system, and in most tax regimes cannot be reclaimed against.
5. The date issued, and the date due. Two separate dates. "Payment on receipt" is not a date and cannot be late; "due 5 September" can. Where nothing is agreed, many jurisdictions default to 30 days.
6. A line for each thing you are charging for. Description, quantity, unit price, line total. Written so that somebody who was not in the room can tell what was bought — "design work" invites a question, "brand identity: logo, type and colour system, August" does not.
7. Tax, shown separately. The rate, the amount, and the subtotal it applies to. A total with tax buried inside it cannot be reclaimed by the buyer, which turns your invoice into their problem.
8. The total, and how to pay it. The amount due in a stated currency, and the bank details or payment link. Currency matters more than it looks the moment two countries are involved.
Bank transfer · GE00TB0000000000000000 · reference INV-014
- 1
The word “Invoice”
So it is not mistaken for a quote, a receipt or a statement. A quote is an offer, a receipt is proof of payment; only an invoice is a request for money.
- 2
A unique invoice number
The single most common reason an invoice is rejected. It has to be unique and it should run in sequence — INV-001, INV-002 — so both sides can refer to it later.
- 3
Your details
Trading name, address, and your tax or company registration number if you have one. If you are VAT-registered, your VAT number is not optional.
- 4
The client's details
Their legal name, not the name you call them, plus their address. An invoice made out to the wrong entity is one their accountant cannot pay.
- 5
Two dates
The date issued and the date payment is due. “Due on receipt” and “Net 30” both count, but something must be there — an invoice with no due date is never late, which is exactly the problem.
- 6
What you are charging for
One line per thing, with quantity and unit price where it makes sense. “Consulting — 2 days × 600” tells a story a client can approve. “Services rendered” does not.
- 7
Tax, shown separately
If you charge VAT or sales tax, the rate and the amount belong on their own line. A total that silently contains tax is the kind of thing that gets queried.
- 8
The total, and how to pay it
One unmissable number, and the bank details or payment link to settle it. Every extra step between reading and paying is a day added to how long you wait.
Invoice or bill — is there a difference?
Not really, and the distinction is about who is speaking. The same document is an invoice to the person sending it and a bill to the person receiving it. "Bill" is more common in consumer contexts — a phone bill, a restaurant bill — and "invoice" in business ones, but no accounting or legal difference hangs on the word.
Where a real difference does hang on a word is invoice versus receipt, and invoice versus proforma invoice. Those are different documents, not different vocabulary.
Who issues an invoice, and when
The party who supplied something issues it, to the party who received it, after the supply and before the payment. That order is what makes it an invoice at all: issued before the supply it is a proforma, issued after the payment it is a receipt.
One common exception: in some industries the buyer raises the document, under an arrangement called self-billing, and the supplier agrees in advance not to issue their own. It is normal in agriculture and in some large-retailer supply chains, and it requires a written agreement — because otherwise two invoices exist for one supply.
What an invoice is not
Four documents get called invoices and are not. The difference is not pedantry — each one has a different effect on your accounts.
| Document | What it says | Creates a debt |
|---|---|---|
| Quote or estimate | This is what it would cost | No |
| Proforma invoice | This is what it will cost, formally, before the supply | No |
| Invoice | This is what you owe, and by when | Yes |
| Receipt | This was paid | No — it closes one |
| Statement | These are all your open invoices | No |
The statement is the one that causes real damage. It is a summary, not a demand, and paying "against a statement" rather than against each invoice number is how the same invoice gets paid twice.
When you have to issue one
If you are selling to a business, essentially always — they need a document to enter, and in most tax systems they cannot reclaim tax without one. If you are selling to consumers the rules are looser, but a request from the customer is usually enough to make it obligatory.
Timing is regulated more often than people realise. Many countries require the invoice within a set window of the supply — commonly thirty days — and some now require electronic invoices in a defined format for business-to-business or public-sector sales. It is worth checking your own rules once; they change rarely.
An invoice for work not yet done is not an invoice. If you need money in advance, that is a proforma invoice, and it has its own rules.
How long you have to keep them
Longer than feels necessary. Most tax authorities require several years of invoice records — commonly five to seven, sometimes ten — and the obligation runs from the end of the accounting period, not from the invoice date. Electronic copies are accepted almost everywhere, provided they cannot be altered and can still be produced on request.
This applies to invoices you receive as well as ones you send. See how to pay an invoice for the other side of the same record-keeping.
What makes an invoice invalid
Rarely anything dramatic. In practice it is one of five things:
- No invoice number, or one that duplicates an earlier invoice.
- Addressed to a person or a brand rather than the legal entity.
- Tax buried in the total rather than shown as its own line.
- No due date, or a due date that contradicts the agreed terms.
- Missing the purchase order or reference number the client asked for — which is not a legal defect at all, but stops it being matched, which has the same effect.
None of these makes the debt disappear. They make it unpayable until fixed, which in cash-flow terms is the same thing.
Making one
An invoice is a short list of facts in a fixed order, which is why it has been a form to fill in for a hundred years and why filling it in is still the part people avoid. How to write an invoice goes field by field; how to send one covers getting it to the person who actually pays.
Or describe what you want on it in one sentence and let the free invoice generator write, number, calculate and lay it out — no account, no card, no watermark.