The short answer: an invoice is "proper" because of what's written on it, not because of the software that made it. A Word document, a spreadsheet and a $40-a-month app all produce equally valid invoices if the right fields are there. So here's the checklist I wish I'd had when I sent my first one.

This is general information, not legal or tax advice. Rules change, and your country, state and business type matter. When in doubt, check your tax authority's own page (I link the main ones below) or ask an accountant.

The fields almost everyone needs

Whatever country you're in, an invoice someone can actually pay and file has:

1. The word "Invoice" at the top. It sounds silly, but a document titled "Statement" or with no title can get routed to the wrong place in a client's finance team.
2. A unique invoice number. Each invoice gets its own number, and you never reuse one. Sequential is safest (2026-001, 2026-002…). Gaps raise questions in some countries, so don't skip numbers for vanity.
3. The date you issued it.
4. Your legal name or business name, and your address. If you trade under a name that isn't your legal name, put both.
5. The client's name and address. Use their legal entity name, not just the name of the person you talk to. "Acme Outdoor Co." beats "Sarah from marketing".
6. What you supplied. A description specific enough that someone who wasn't on the project understands it: "Website redesign, phase 1, as per quote Q-114" rather than "Services".
7. Quantity and unit price for each line, and the line total.
8. The total amount due, with the currency written out or clearly symbolised (USD, GBP, EUR). If you work internationally, "$" alone is ambiguous.
9. Payment terms and a due date. "Net 14" or "Due 2 November 2026".
10. How to pay you. Bank details, a payment link, or both.

If you only do these ten things, you're already ahead of most first invoices I've seen.

United States

There is no single federal invoice format in the US. What the IRS cares about is that you can back up the income and expenses on your return, so your invoices are part of your records. In practice that means the ten fields above, plus:

• Sales tax, if it applies. Sales tax is set by states (and sometimes cities), not federally. Many services aren't taxable in many states, but some are, and physical goods usually are. If you charge it, show it as a separate line with the rate.
• Keep copies. The IRS's general guidance is to keep records for at least three years after you file, longer in some situations. A folder of PDFs named by invoice number is fine.

Things people often think belong on an invoice but don't: your Social Security number (never put it there) and a W-9. If a US client needs your taxpayer details for a 1099, they'll ask for a W-9 separately. Sending it securely and separately is better than printing tax IDs on every invoice.

Official reading: IRS: Recordkeeping for small businesses.

United Kingdom

The UK is more specific, and it depends on whether you're VAT-registered.

Every UK invoice, according to GOV.UK, must include a unique identification number, your business name, address and contact information, the customer's name and address, a clear description of what you're charging for, the date the goods or service were provided (the supply date), the date of the invoice, the amounts charged, VAT if applicable, and the total owed. If you aren't VAT-registered, you can't charge VAT, so leave it off.

Sole traders also need their own name next to any business name, and if you trade under a business name, an address where legal documents can be delivered to you.

Limited companies must use the full company name exactly as it appears on the certificate of incorporation. If you list directors on the invoice, you have to list all of them, so it is simpler to list none.

If you're VAT-registered and your customer is too, you need a full VAT invoice. It adds your VAT registration number, the time of supply (tax point) if it differs from the invoice date, and for each line the unit price, quantity, VAT rate and amount excluding VAT, followed by the total excluding VAT and the total VAT. Show any discount.

Official reading: GOV.UK: Invoicing and taking payment from customers and GOV.UK: Record keeping for VAT (Notice 700/21).

European Union

EU VAT invoices follow Article 226 of the VAT Directive, which each country implements in its own law. The common core is close to the UK VAT list: sequential number, issue date, supplier and customer names and addresses, supplier VAT number, the customer's VAT number for cross-border B2B, quantity and nature of goods or extent of services, the supply date if different, taxable amount per rate, the VAT rate, VAT payable, and the unit price excluding VAT.

Two EU cases that trip up freelancers:

• Reverse charge. If you sell services B2B to a business in another EU country, you usually don't charge VAT; the customer accounts for it. The invoice should show both VAT numbers and a note such as "Reverse charge". Countries have their own preferred wording, so check yours.
• Language and currency. Some countries have rules about the language of the invoice or require the VAT amount in their local currency. If you invoice in a foreign currency, check.

Official reading: Your Europe: Invoicing rules.

Mistakes that get invoices bounced

From my own invoices and from people who've emailed me:

• No PO number. Larger clients often won't pay without their purchase order number on the invoice. Ask for it before you send. Add it as a line under the client's address.
• Wrong legal entity. Billing "Acme" when the contract is with "Acme Holdings Ltd" means accounts payable sends it back.
• Totals that don't add up. If you calculate tax per line and round each one, you can end up a cent away from the total the client's system calculates. Rounding once per tax rate on the subtotal is the usual approach, and it's what most accounting software expects.
• Editing an invoice after sending it. Don't change a sent invoice. Issue a credit note for the old one and a new invoice with a new number. That keeps your numbering clean.
• No due date. "Payable on receipt" is legal but in practice gets you paid last. A specific date gets you paid sooner.

Making one for free

You don't need paid software for this. Three options that work:

A spreadsheet. Make a template once with the fields above, then "Save as" for each new invoice. The risk is formulas: test your totals with awkward numbers like 12.5 hours at $85.

A plain HTML template. If you're comfortable with a bit of code, an HTML page with a print stylesheet prints to a clean PDF from any browser. I published the template I use as open source, with the totals done in integer cents so rounding is predictable.

A browser generator. If you just want to fill in fields and download a PDF, I made a free invoice generator that runs entirely in your browser. Nothing you type is uploaded, there's no account, and the free PDFs are unlimited. (Full disclosure: it's my tool. There's an optional one-time Pro upgrade that removes a small footer line, but the free version has every field on this list.)

Whichever you use, run through the checklist before you hit send. It takes thirty seconds and saves the awkward "can you reissue this?" email two weeks later.

A copy-paste checklist

[ ] "Invoice" title
[ ] Unique, sequential invoice number
[ ] Issue date (and supply date if different)
[ ] My legal/business name and address
[ ] Client's legal entity name and address
[ ] Client PO number (if they use them)
[ ] Clear description per line, quantity, unit price, line total
[ ] Tax shown separately with rate (if I charge tax)
[ ] My tax/VAT number (if registered)
[ ] Client's VAT number + reverse charge note (EU/UK cross-border B2B)
[ ] Total due with currency code
[ ] Due date and payment terms
[ ] How to pay me
[ ] Full registered company name (UK/EU companies)

Gary builds small, privacy-first browser tools at Gaiya365.