How to Write an Invoice: What to Include and Common Mistakes to Avoid

What an invoice must include

A valid invoice needs to contain enough information for the client to identify the work, match it to their records, and process payment. The minimum required fields are:

• The word 'Invoice' clearly at the top
• A unique invoice number (e.g. INV-0047)
• Invoice date and payment due date
• Your full name or business name and contact details
• The client's name and address
• A description of each item or service provided
• The quantity and unit price for each line item
• The subtotal, any tax (VAT or sales tax), and the total amount due
• Your payment details: bank account number, sort code (UK), routing number (US), or payment link

Invoice numbers: how to do them correctly

Every invoice needs a unique reference number. This is non-negotiable - clients and accountants use it to match payments to records, and you need it to chase unpaid invoices. The simplest system is sequential: INV-0001, INV-0002, INV-0003. If you work with multiple clients, consider prefixing by client: ACME-001, NOVA-001. Never reuse a number. Never skip numbers. If you void an invoice, mark it VOID but keep the number in the sequence.

The invoice date is the date you issued it - not the date the work was completed, and not the date the client receives it. The due date is typically 14, 30, or 60 days after the invoice date. Shorter terms (14 days) are standard for freelancers and small businesses. Longer terms (30-60 days) are common in B2B where the client's accounts payable process requires more time.

Payment terms: what to write

Payment terms tell the client when you expect to be paid and what happens if they do not pay on time. Common terms include: Net 14 (payment due within 14 days), Net 30 (within 30 days), Due on receipt (immediate payment expected). Write the due date explicitly - 'Payment due by 21 July 2026' - rather than relying solely on 'Net 30', which requires the client to calculate the date themselves.

Late payment clauses are optional but worth including for larger invoices. State the interest rate you will charge on overdue amounts (e.g. 2% per month on balances outstanding after the due date) and whether you will charge a flat late fee. In many jurisdictions, you have a statutory right to charge interest on late business payments even without a prior agreement - check the rules in your country.

How to handle tax on invoices

If you are VAT-registered (UK/EU) or required to collect sales tax (US), your invoice must show the tax separately. List the net amount, the tax rate and amount, and the gross total as separate lines. Example: Services rendered 2,000.00, VAT at 20% 400.00, Total due 2,400.00. Include your VAT registration number or tax ID on every invoice once you are registered - this is a legal requirement in most jurisdictions.

If you are below the VAT registration threshold, you do not charge VAT - but you also cannot reclaim it. Simply list the total without any VAT line. If you are unsure of your obligations, consult an accountant. Never guess on tax.

Invoice vs quote vs receipt: the key differences

A quote is sent before the work begins and asks the client to approve the price. An invoice is sent after the work is done (or at an agreed milestone) and requests payment. A receipt is proof that payment has been received. A quote that is accepted becomes a binding agreement - the invoice should match it unless the scope changed. A receipt closes the transaction once the invoice is paid.

Chasing unpaid invoices: what to do

Send a polite reminder on the due date if payment has not arrived. Keep the tone professional: 'This is a reminder that invoice INV-0047 for 2,400 was due on 21 July. Please let me know if you have any questions.' If unpaid after 7 days, send a firmer follow-up. After 14 days, call rather than email. Having a unique invoice number makes every follow-up conversation faster because both parties can refer to the same reference.

Common invoicing mistakes

• Missing or unclear payment details - clients cannot pay if they do not know how. Always include account number, sort code or routing number, and IBAN for international payments.
• Vague line item descriptions - 'website work' is not a description. 'Homepage redesign including 3 revision rounds' is. Vague descriptions invite disputes.
• No due date - 'Net 30' requires the client to calculate the date. Write the actual date: 'Due: 21 July 2026'.
• Forgetting to add tax - if you are VAT or sales tax registered, every invoice to a taxable supply must include it.
• Sending one invoice at the end of a long project - break large projects into milestone invoices to protect your cash flow.

Use the Invoice Generator

The Invoice Generator on this site builds a professional invoice in your browser. Fill in your details, add line items, set the tax rate, and download as a PDF or print. Your business details save automatically so repeat invoices take under a minute. Nothing is stored on a server.