What to Put on an Invoice: Mandatory and Optional Fields
The Issueable Team
Small business operations
The mandatory fields required by tax authorities in the US, Canada, and UK, plus optional fields that reduce payment friction and help your business.
What makes a legally complete invoice
Tax authorities in the US, Canada, and the UK don't require invoices to follow a specific format, but they do mandate certain information. Missing any of these fields can create problems during an audit or trigger payment delays at the buyer's end.
The good news: tax-compliant invoices and buyer-friendly invoices overlap almost completely. What speeds payment is also what auditors want to see.
Mandatory fields in the US (IRS guidance)
The IRS doesn't publish a rigid invoice template, but it expects these elements on any invoice used for business:
1. Seller information
Your business name, address, phone, and email. If you're a sole proprietor, use your business name or DBA (Doing Business As). If incorporated, use the legal entity name exactly as it appears in your Articles of Incorporation or business registration.
2. Seller's Tax ID
Your EIN (Employer Identification Number) if you have one, or your Social Security Number if you're a sole proprietor. The IRS doesn't require this to appear on the invoice itself, but it must be in your records and on your tax returns. Including it on the invoice adds credibility with large buyers.
3. Customer information
The buyer's name and mailing address. This is required so you (and the IRS) can match payments to specific customers. For business clients, include the bill-to department or named contact if possible.
4. Invoice date
The date you issued the invoice. Required to establish when payment is due and for statute-of-limitations purposes if a dispute arises.
5. Description of goods or services
What you provided. Vague descriptions ("consulting") are acceptable to the IRS but will slow payment with your buyer. Specific descriptions ("Brand strategy workshop, May 14–16, 40 hours") are better.
6. Quantity and rate
For each line item, show how much you provided (e.g., "4 hours," "1 logo design," "100 units") and the price per unit or line. The IRS needs to see that the arithmetic adds up correctly.
7. Total amount due
The sum of all line items, minus discounts, plus tax (if applicable). This is the amount the customer owes.
8. Payment terms
When payment is due (e.g., "Due on receipt," "Net 30," "Due by June 15, 2026"). The IRS doesn't mandate this, but it's critical for your accounting records and for the buyer's AP system.
Mandatory fields in Canada (CRA guidance)
The Canada Revenue Agency's invoice requirements are slightly tighter than the IRS:
1. Seller information
Your business legal name and address. If you have a GST/HST account, include your registration number (it's required if you're registered).
2. Customer name and mailing address
Required. CRA expects you to have this on file for audit purposes.
3. Invoice date
The date issued. Used to determine payment terms and reporting obligations.
4. Invoice number
Sequential numbering is standard practice. CRA doesn't mandate it but expects you to be able to track invoices.
5. Description of goods or services
Specific enough that you and the customer can match it to the underlying work or contract.
6. Quantity, rate, and amount
For each item. CRA is more strict about showing the calculation — they want to see the math.
7. GST/HST
If you're registered for GST or HST (Harmonized Sales Tax), the rate and amount are required. If you're exempt (certain services, non-residents), note it on the invoice.
8. Total amount due and payment terms
Same as the IRS.
9. PO number (if applicable)
If the customer has a purchase order system, the PO number should match the invoice. CRA doesn't require this, but buyers often do.
Mandatory fields in the UK (HMRC guidance)
HM Revenue and Customs sets the tightest requirements:
1. Unique invoice number
Sequential numbering is required. This is non-negotiable in the UK. HMRC expects invoices to be numbered and traceable.
2. Issuing date
The date you issued the invoice. Month and year are sufficient, but day is more precise and recommended.
3. Invoice period (if applicable)
If the invoice covers a period of work (e.g., "May 1–31, 2026"), show the period clearly. For one-off work, this isn't needed.
4. Seller name and address
Business legal name and full address. If you have a VAT registration number, include it.
5. Seller's VAT registration number
Required if you're VAT-registered (which you must be once your UK VAT-taxable turnover passes £90,000 in any rolling 12-month period, or if you register voluntarily below that). If not VAT-registered, note "Not VAT registered" to be explicit.
6. Customer name and address
Full details. HMRC is strict about this.
7. Description of goods or services
Specific and clear. Vague descriptions trigger HMRC inquiries.
8. Quantity and rate per unit
For each item. The calculation must be transparent.
9. Total amount due
Clear and unambiguous.
10. VAT amount and rate
If VAT applies, show the rate (standard rate is 20% in the UK) and the amount. If the supply is VAT-exempt or zero-rated, note it.
Important: If you're invoicing a customer outside the UK (e.g., a US-based customer), VAT doesn't apply. Show "0% VAT — reverse charge applies" or "VAT not charged — customer is outside the UK."
Optional fields that reduce friction
Beyond the legally mandatory fields, a few additions measurably speed payment:
1. PO number
If your customer has a purchase order system, their AP software requires this to match the invoice. Ask once: "What PO number should appear on invoices?" Then always include it.
2. Contract or statement of work reference
Example: "Per SOW dated May 5, 2026." This ties the invoice to the original agreement and survives AP review.
3. Invoice due date (not just terms)
Instead of "Net 30," write "Due: May 31, 2026." Specific dates move faster than relative terms.
4. Payment instructions
Bank account, Stripe link, PayPal, or a pay link. If the buyer has to ask how to pay, you've lost time.
5. Late-fee clause
Example: "Late fees of 1.5% per month apply to unpaid balances after 30 days." This is legally enforceable in most jurisdictions (check your local law) and signals seriousness to the buyer.
6. Early-payment discount (optional)
Example: "2% discount if paid within 10 days." Only offer this if you need immediate cash; the cost-benefit rarely works out for small businesses.
7. Business logo or branding
A professional touch that signals reliability. Optional but high-impact.
8. Thank-you message or brief note
"Thanks for the opportunity to work on this project" adds warmth without appearing unprofessional.
9. Discount or promo code
If you offered a discount, show it as a line item: "15% early-bird discount: -$750.00." This helps the buyer understand the invoice amount.
10. Project name or work order number
If the work was part of a larger project, include the project name or internal reference number. Large buyers track work across projects; this helps AP match it to the right cost center.
The complete invoice checklist
Legally required:
- Your business name and address
- Customer name and address
- Invoice date
- Description of goods or services provided
- Quantity and rate or total amount
- Total due
- VAT/GST/Sales Tax (if applicable, or note if not applicable)
Required in your region:
- Invoice number (required in UK; required everywhere for your own records)
- Tax ID or VAT registration (required in Canada if registered; recommended elsewhere)
- Payment terms (required everywhere by practice, even if not by law)
Highly recommended for faster payment:
- Specific invoice due date (not just "Net 30")
- PO number (if customer provided one)
- Payment instructions
- Professional layout and branding
- Late-fee clause (legal in most jurisdictions)
Nice to have:
- Early-payment discount (if you need immediate cash)
- Contract or SOW reference
- Thank-you message
- Project or cost-center reference
Regional edge cases
Invoicing a US customer from Canada: Show GST/HST if you're registered, but note that the US customer may be exempt. Ask them: "Do you need a GST/HST number shown on invoices?" Many US companies are registered for GST to claim refunds.
Invoicing a Canadian customer from the US: You won't charge GST/HST (you're not registered in Canada). Show the invoice in USD or CAD per your agreement, but don't show Canadian tax.
Invoicing a UK customer from the US or Canada: You won't charge UK VAT. Note "VAT not charged — customer is outside the UK" if clarity is needed.
Invoicing as a non-resident (e.g., a Canadian freelancer invoicing a US customer): You're not required to show a US tax ID. Show your Canadian business info and currency (CAD). For large invoices, US buyers often ask for a W-8BEN form (US tax exemption for non-residents); have one ready.
Ready to create a compliant invoice?
Issueable's invoice generator auto-calculates tax by location, shows all mandatory fields, and lets you customize optional fields. Start now.
Frequently asked questions
- Is it legal to invoice without an invoice number?
- In the US, no formal requirement. The IRS doesn't mandate invoice numbers. However, you must have a way to track and identify each invoice for your own accounting records. Using sequential numbers (INV-001, INV-002) is standard practice and makes audits easier. Without numbering, you lose track of which payment corresponds to which work.
- What if I don't collect sales tax — do I still need to mention it on the invoice?
- If you don't collect sales tax (you're exempt, or you're selling services that aren't taxable), you don't need to show a tax line at all. If the buyer asks, you can add a line that says 'Tax exempt' or simply omit it. But if you do charge tax, showing the rate and amount explicitly is required in most jurisdictions.
- Can I invoice without the client's address?
- In the US, no. The IRS expects you to show the customer's name and address (or at minimum, their mailing address or billing address). For Canada, CRA rules require the customer's name; address is good practice. In the UK, the customer's name is required, but the address is optional if you have it on file. Always include the address if you have it — it reduces AP friction.
- Do I need to include my tax ID on every invoice?
- In the US, a Tax ID (EIN or Social Security Number) is not required on invoices, but you must have it on file with your business. In Canada, a GST/HST registration number should be shown if you're registered. In the UK, your VAT registration number is required if you're VAT-registered. Include it if you have it; it signals legitimacy to the buyer.
- What if my client asks me to invoice a different company than the one they work for?
- Invoice the entity that signed the contract or purchase order. If they ask you to bill a parent company or subsidiary instead, get written confirmation (email is fine) of the new bill-to address and entity name. Update your records and use that entity name going forward to avoid AP confusion.
- Is there a difference between a 'tax ID' and a 'business number' on an invoice?
- Functionally, no. A tax ID is a unique number issued by tax authorities to identify your business (EIN in the US, BN in Canada, UTR in the UK). Include it if you're registered, but it's not legally required on the invoice itself in most jurisdictions — only in your records. Including it adds legitimacy.