How to Write an Invoice — Complete Guide with Examples

Writing an invoice sounds simple — until a client queries a line item, a payment stalls because of a missing tax number, or you realise you've been sending documents that don't meet the legal requirements in your country. Getting it right from the start saves time, reduces disputes, and gets you paid faster.

This guide walks through exactly how to write an invoice: what to include, how to structure it, what the rules are in the US, UK, Australia, and Canada, and the most common mistakes to avoid. Whether you're invoicing for the first time or tightening up an existing process, everything you need is here.

What a Professional Invoice Looks Like

Before diving into the steps, here's a real example of a properly structured invoice — every field labelled so you know exactly what goes where.

What to Include on Every Invoice

Before worrying about formatting or software, get clear on the core fields. These apply regardless of where you're based or what industry you're in.

Your Business Details

Your full name or registered business name, your business address, your contact email or phone number, your company registration number if applicable, and your VAT, GST, or tax registration number if you're tax-registered.

Your Client's Details

Their full name or business name, their billing address, and their VAT or GST number for B2B invoices in the UK, Australia, and Canada.

Invoice Information

A unique invoice number — sequential, such as INV-001 or INV-002 — the invoice date, and the payment due date or stated payment terms such as Net 14 or Net 30.

Line Items

Each service or product on a separate line, showing a clear description of what was delivered, the quantity and unit price, and the line total.

Totals

Your subtotal before tax, the tax amount — VAT, GST, or Sales Tax — shown separately with the rate, and the total amount due including tax.

Payment Instructions

How the client actually pays you: bank transfer details, a payment link, accepted card methods. Don't make them ask.

How to Write an Invoice: Step by Step

Here's the process from blank page to sent invoice.

Step 1: Choose Your Invoicing Method

You can write an invoice in a Word or Google Doc, use a spreadsheet template, or use dedicated invoicing software like Instant Invoice. Templates work fine when you're starting out, but they don't automate numbering, send reminders, or track what's been paid. For anything beyond occasional invoicing, a purpose-built tool will save you real time.

Step 2: Add Your Business Details

Place your business name, address, and contact information prominently — typically at the top of the document. If you're VAT or GST registered, your tax number must appear here. This isn't optional once you're registered; it's a legal requirement.

Step 3: Add Your Client's Details

Include the full legal name of the person or business you're invoicing and their billing address. For B2B invoices, use the registered business name — not just a contact name — so the invoice matches their internal purchase records.

Step 4: Assign an Invoice Number and Date

Every invoice needs a unique reference number. Keep them sequential and consistent. Your invoice date is typically the date you send it, and your payment due date should be calculated from there based on your agreed terms.

Step 5: List What You Delivered

This is where most invoices fall short. Be specific. Instead of "Consulting — November," write "Brand strategy workshop — 3 sessions × 2 hours @ £150/hr." Instead of "Design work," write "Logo design and brand guidelines — as per proposal dated 3 October." Specific line items reduce the chance of a query holding up payment. If an approver can't tell what they're signing off, the invoice goes into a queue.

Step 6: Calculate and Show Your Totals

List your subtotal, apply any tax separately with the rate shown, and state the final total clearly. If you've agreed a discount, show it as a line item — don't just reduce the price without explaining why.

Step 7: State Your Payment Terms and Method

Include both your terms — such as "payment due within 30 days" — and the actual due date. Add your bank details or payment link directly on the invoice. The fewer steps between your client opening the invoice and making the payment, the better.

Step 8: Review Before Sending

Check the invoice number is unique and sequential. Check the client name and address are correct. Check the line item descriptions are specific. Check the maths. Then send it promptly — every day you delay sending is a day added to the end of your payment window.

Invoice Requirements by Country

The core fields above apply everywhere. But if you're tax-registered, additional legal requirements apply depending on where you operate.

United Kingdom (VAT)

Once you're registered for VAT — the threshold is currently £90,000 in taxable turnover — every invoice you issue to another VAT-registered business must be a full VAT invoice. This must include your VAT registration number, the VAT rate applied to each line item, the VAT amount charged shown separately, the tax point which is usually the invoice date, and the net amount payable excluding VAT.

A simplified VAT invoice is permitted for retail sales under £250 including VAT, but for B2B transactions your client needs a full VAT invoice to reclaim input tax. A proforma or a quote does not count. If you're not VAT-registered, you issue standard invoices without any VAT fields. Never show a VAT amount on an invoice if you're not registered — this is a criminal offence under UK law.

Australia (GST)

If you're registered for GST and the transaction is over $82.50 including GST, you must issue a tax invoice. The words "Tax Invoice" must appear prominently on the document. Required fields include your ABN, the date of issue, a description of what was sold, the GST amount shown separately or a statement that the total price includes GST, and for sales of $1,000 or more the buyer's ABN or name.

Your client cannot claim GST credits on their BAS without a valid tax invoice. A standard invoice or a receipt without the required fields won't qualify.

Canada (GST/HST)

GST/HST-registered businesses must include their registration number on invoices. For transactions under $30, no formal invoice is required. Between $30 and $149.99, a simplified invoice is permitted showing supplier name, date, total, and tax rate. Between $150 and $999.99, you must include your GST/HST number and a breakdown of the tax charged. For transactions of $1,000 and over, you must also include the buyer's name or trading name. Your client needs this information to claim input tax credits — a missing GST/HST number means they can't.

United States (Sales Tax)

There's no federal VAT or GST system in the US, and no single federal invoicing standard. However, if your business collects Sales Tax, you must show the applicable rate, the jurisdiction it applies to — state, county, or city — and the dollar amount of tax charged separately from the subtotal. Sales tax rules vary significantly by state and in some states by product or service type. Beyond tax, there are no federally mandated invoice fields, but the core elements remain best practice and are expected by most professional buyers.

Invoice Examples by Business Type

The same invoice structure applies across industries, but the line item descriptions differ. A freelance designer might list: brand identity package — logo, colour palette, and typography guidelines; 50% deposit deducted — balance due. A trades contractor might list: labour — bathroom retile, 2 days @ £280/day; materials — tiles, adhesive, grout with itemised receipt attached; call-out and site preparation — flat fee. A consultant or agency might list: brand strategy workshop — 3 sessions; competitor analysis report; monthly retainer — as per agreement. A retailer might list: handmade ceramic mugs × 24 units @ £18.00 each; standard tracked delivery; wholesale 10% discount applied.

In every case, the principle is the same: enough detail that anyone picking up this invoice — an accountant, an auditor, a new accounts payable clerk — knows exactly what it refers to without needing to make a call.

Common Invoicing Mistakes

Sending to the Wrong Person

Many businesses have a dedicated accounts payable contact or an invoicing email address that's different from your day-to-day point of contact. Invoices sent to the wrong person can sit unread for weeks. Confirm the correct invoicing contact before your first invoice goes out.

Missing or Wrong Purchase Order Numbers

Larger businesses often won't process an invoice without a matching PO number. If your client uses a purchase order system, get the number before you start work. An invoice without a valid PO will be queried or returned — adding days or weeks to your payment cycle.

Vague Descriptions

"Services — October" doesn't tell an accounts payable approver what they're signing off. Specific, complete descriptions reduce queries and speed up approval. Write line items as if the reader has no context for the project.

Sending a Proforma as a Final Invoice

A proforma invoice is a preliminary document used to confirm scope or request a deposit, but it is not a valid tax invoice in any jurisdiction. Once work is complete, a proper invoice must follow. Neither party has a legally compliant record of the transaction until a real invoice is issued.

Not Following Up

An overdue invoice won't resolve itself. A structured follow-up process — a reminder on the due date, a firmer nudge at 7 days overdue, a formal notice at 14 days — is not aggressive, it's professional. Most late payments are the result of invoices getting buried, not bad intent.

Frequently Asked Questions

What is the correct format for an invoice?

There's no single legally mandated invoice format in most countries — but every invoice should include your business details, your client's details, a unique invoice number, an invoice date, an itemised description of goods or services, the total amount due with tax shown separately if applicable, and your payment terms and banking details. Tax-registered businesses in the UK, Australia, and Canada have additional mandatory fields.

Do I need accounting software to write an invoice?

No. You can write a valid invoice in a Word document, a Google Doc, or a spreadsheet. That said, manual invoices don't track payment status, generate reminders, or maintain sequential numbering automatically. For anything beyond occasional use, purpose-built invoicing software reduces errors and saves time.

How do I number my invoices?

Use a sequential numbering system and never reuse a number. Common formats include INV-001, 2026-001, or client-specific prefixes like SMITH-001. The format doesn't matter — consistency does. Gaps in your invoice number sequence can raise questions during a tax audit.

Can I send an invoice by email?

Yes. Email is the standard delivery method for invoices in all four jurisdictions covered here. PDF is the most widely used format — it's not easily edited and renders consistently across devices. Some businesses use online invoicing platforms that send a payment link rather than a PDF attachment, which can improve payment speed.

What should I do if I make an error on an invoice?

Do not edit and resend the same invoice number. Instead, either issue a corrected invoice with a new number and a note referencing the original, or issue a credit note against the original invoice and reissue. Editing an already-sent invoice creates discrepancies in both parties' records and can cause tax reporting issues.

Key Takeaways

Every invoice needs your details, your client's details, a unique invoice number, specific line item descriptions, the total due with tax shown separately, and clear payment terms.

Tax-registered businesses in the UK, Australia, and Canada must meet additional legal requirements — specific fields are mandatory, and missing them means your client may not be able to reclaim tax.

Specific line item descriptions are one of the most effective ways to speed up payment approval. Vague descriptions create queries; queries create delays.

Send invoices promptly. Every day between completing work and issuing the invoice is a day added to the end of your payment window.

A proforma is not a final invoice. Once work is complete, a proper invoice must follow regardless of what preliminary documents were exchanged.