What must an invoice legally include?

A legally compliant invoice needs: your business identity and registration numbers, your customer's identity, a unique sequential invoice number, the issue date and the date of supply, an itemised description of what was supplied with quantities and unit prices, the amount payable, and — if you are registered for a sales tax — that tax broken down by rate.

The exact list varies by jurisdiction, and the differences are real. What follows is the common core plus the places countries most often diverge. It is a starting point for a conversation with your accountant, not a substitute for one.

The common core

Almost every tax authority requires these:

1. The word "Invoice" — or "Tax invoice" where you are registered for a sales tax. This sounds pedantic and is genuinely required in several jurisdictions.
2. A unique, sequential number. Sequential matters as much as unique: an auditor reads a gap as a document you did not disclose.
3. The issue date.
4. The date of supply (the "tax point"), if it differs from the issue date.
5. Your full legal name and address, and your trading name if it differs.
6. Your tax registration number, if you have one.
7. Your company registration number, if you are incorporated.
8. The customer's name and address — the legal entity, not the contact.
9. The customer's tax registration number, for cross-border business-to-business supply.
10. A description of what was supplied, itemised, with quantity and unit price.
11. The amount excluding tax, per rate.
12. The tax rate and amount, per rate.
13. The total payable.

Where the rules diverge

Sales tax registration thresholds. In most countries you must register once turnover crosses a threshold, and once registered you must issue tax invoices. Below it, you must not show a tax line at all.

Reverse charge. For cross-border business-to-business supply within some tax areas, the customer accounts for the tax rather than you. The invoice must say so explicitly and show no tax — usually with a specific form of words.

Simplified invoices. Many jurisdictions allow a shortened invoice below a value threshold — typically dropping the customer's details and the per-line tax breakdown.

Language and currency. Some tax authorities require the invoice in a local language, or require the tax amount to be restated in the local currency even when the invoice is in another. See how to invoice in another currency.

Retention periods. Six years is common; some jurisdictions require ten. This applies to the invoice as issued, which is one of several reasons not to edit a sent invoice.

What is a "tax point" and why does it matter?

The tax point is the date that determines which tax period a supply falls into, and which rate applies if rates changed. It is usually the date of supply rather than the date you got round to invoicing — so an invoice issued in April for work delivered in March generally belongs to March.

Getting this wrong shifts revenue between periods. It is the single most common correction an accountant makes to a small business's invoicing.

What is not legally required but should be there anyway

• The due date as a calendar date. Terms are shorthand; dates are not.
• A purchase order reference, if your customer uses them. Without it, a large organisation's accounts payable system may have no way to match your invoice at all.
• How to pay, in one obvious place.
• A contact for queries. An invoice with nobody to ask is an invoice that waits.

Does an electronic invoice count?

Yes, essentially everywhere, and in a growing number of countries a structured electronic format is becoming mandatory for business-to-government and some business-to-business supply. A PDF emailed to a customer is an electronic invoice; it is not the same thing as a structured e-invoice in a prescribed format, and the two are increasingly being legislated separately.

Frequently asked questions

What must a legally valid invoice include? Your legal name, address and registration numbers, the customer's name and address, a unique sequential invoice number, the issue date and date of supply, an itemised description with quantities and unit prices, the net amount, the tax per rate, and the total payable.

Do I have to number invoices sequentially? Yes, in most jurisdictions. Uniqueness alone is not enough — the sequence is what demonstrates that no invoice has been omitted from your records.

Can I issue an invoice if I am not registered for VAT or GST? Yes. You issue an ordinary invoice with no tax line and no tax registration number. Do not show a 0% tax line; that implies a registration you do not have.

Does an invoice need a signature? Almost never. A few jurisdictions require one for specific document types, but for ordinary commercial invoices a signature is not a general requirement.

How long must I keep copies of invoices? Commonly six years, and ten in some jurisdictions. Keep the invoice as it was issued — which is why corrections are made with a credit note rather than by editing the original.