An invoice is a request for payment that identifies both parties, itemises what was supplied, states the amount owed, and says how and when to pay it. Everything else — logos, colours, thank-you notes — is decoration. Get the required fields right and you get paid; get them wrong and you spend three weeks chasing an accounts-payable inbox that rejected your document on a technicality.
What every invoice needs
The word "Invoice." Sounds trivial. Many AP systems route documents by keyword, and a page headed "Statement" or "Summary" can sit unprocessed.
A unique invoice number. More on this below — it's the field people most often get wrong.
Issue date and due date. "Net 30" alone isn't enough; write the actual date the payment is due. Ambiguity here is the single most common cause of late payment.
Seller details. Your legal or trading name, address, contact email, and any tax registration number you're required to display.
Buyer details. Their legal entity name — not the name of your contact — plus their address. If they gave you a purchase order number, put it on the invoice. Missing PO numbers are the most common reason a large company's AP system bounces an invoice.
Line items. Description, quantity, unit price, line total. Be specific: "Consulting" invites questions; "Website redesign — phase 2, 12 hours @ $95" does not.
Tax, clearly separated. Show the rate and the tax amount as their own lines, not baked into the totals. What you're required to charge and display varies by country and by what you sell — check your local rules or ask an accountant rather than copying another business's invoice.
Subtotal, tax, and grand total. In that order, with the total visually dominant.
Payment details and terms. Bank details, payment link, accepted methods, and any late-payment terms you intend to enforce.
How to number invoices
Invoice numbers must be unique and should be sequential with no gaps — auditors read a gap as a deleted invoice, and in some jurisdictions gaps in a sequence require an explanation.
A format like INV-2026-0041 works well: a fixed prefix, the year, and a zero-padded counter. Per-customer prefixes (ACME-0041) are fine too, as long as each series is itself gapless. What to avoid: restarting the counter at 1 whenever it suits you, reusing a number after cancelling an invoice (issue a credit note instead), and using a raw date as the number, which collides the moment you bill twice in one day.
Three ways to create one
A free invoice generator. Fill a form, download a PDF. Best when you invoice occasionally and don't want a subscription. Our free invoice generator needs no signup, adds no watermark, and stores nothing.
A spreadsheet template. Total control, works offline, and the maths is visible. The failure mode is human: broken formulas after a copy-paste, and numbering that drifts because it's manual.
An API. When invoices should appear without anyone opening a form — a subscription renews, an order ships, a project milestone closes — generate them from data:
curl -X POST https://www.ashdocs.com/api/v1/tools/invoice-pdf \
-H "X-API-Key: ash_live_..." \
-H "Content-Type: application/json" \
-d '{
"invoice": {
"number": "INV-2026-0041",
"issue_date": "2026-08-14",
"due_date": "2026-09-13",
"currency": "USD",
"seller": { "name": "Acme Studio", "email": "billing@acme.test" },
"buyer": { "name": "Globex Ltd" },
"line_items": [
{ "description": "Website redesign — phase 2", "quantity": 12, "unit_price": 95, "tax_rate": 0 }
],
"payment_terms": "Net 30"
}
}'
The Invoice PDF API computes line totals, tax subtotals and the grand total from the numbers you send, and returns a finished PDF.
Automating the whole loop
The pattern that removes invoicing from your week entirely: a trigger (Stripe payment, Shopify order, project marked complete, monthly schedule) fires a Make scenario, Zapier Zap or n8n workflow; the workflow pulls customer and line-item data; it calls the invoice endpoint; the returned PDF is emailed to the customer and filed to Drive or Dropbox. No code, no forms, and the invoice number comes from a counter rather than someone's memory.
What goes wrong
Sending the invoice to a person rather than the AP address. Omitting the PO number. Vague line descriptions that trigger a query. A due date expressed only as "Net 30". Editing an invoice after sending it instead of issuing a credit note — this breaks your audit trail and confuses their ledger. And invoicing late: an invoice sent three weeks after the work is a low-priority item in someone else's queue.
FAQ
Is an invoice from a free generator legally valid? The tool doesn't determine validity — the content does. An invoice containing the required fields for your jurisdiction is valid however it was produced. Requirements differ by country, so confirm what yours expects.
What should an invoice number look like? Unique, sequential and gapless. A prefix-year-counter format such as INV-2026-0041 is easy to read, sorts correctly, and survives an audit.
Can I generate invoices automatically? Yes. Send your invoice data to an API from a Make, Zapier or n8n workflow triggered by a payment, an order or a schedule, and the PDF is created without anyone filling a form.
Should I include tax on my invoice? Only what you're actually registered to charge, shown as its own line with the rate stated. Rules vary by country and by product — check your local requirements.
Create your first invoice free — no signup required — with the invoice generator, or get an API key and automate it.