How to Write an Invoice
An invoice is a formal payment request you send to a client after delivering a product or service. Getting it right matters — a clear, complete invoice gets paid faster, reduces disputes, and makes your business look professional.
This guide covers every field that belongs on an invoice, how to number them, what payment terms to use, and the most common mistakes to avoid.
1. Required Fields on an Invoice
Every professional invoice needs these core elements:
- The word "Invoice" — clearly labeled at the top
- Your business name, address, email, and phone number
- Client name and billing address
- Invoice number — unique sequential ID (e.g., INV-2026-001)
- Invoice date — when the invoice was issued
- Due date — when payment is expected
- Itemized list of services or products with descriptions, quantities, and unit rates
- Subtotal, applicable taxes, and total amount due
- Payment instructions — how you accept payment (bank transfer, check, PayPal, etc.)
2. How to Number Invoices
Invoice numbers must be unique. Use a consistent sequential system:
- Simple sequence: INV-001, INV-002, INV-003
- With year: INV-2026-001 (resets each year)
- With client code: SMITH-001, SMITH-002 (useful if you have many repeat clients)
Never reuse an invoice number. Gaps are fine — if you cancel INV-005, skip it and use INV-006 next. Accounting software and tax auditors use invoice numbers to trace transactions.
3. Itemizing Your Services
A good line item includes:
- Description — be specific. "Website design — homepage mockup, 3 revisions" is better than "Design work."
- Quantity — hours worked, units delivered, or days on-site
- Rate — per hour, per unit, or flat fee
- Line total — Qty × Rate
Specific descriptions reduce client questions and disputes. If the client can't tell what they're paying for, they're more likely to push back.
4. Payment Terms
Payment terms define when you expect to be paid. Common options:
| Term | Meaning | Best for |
|---|---|---|
| Net 30 | Payment due 30 days after invoice date | Established client relationships |
| Net 15 | Payment due 15 days after invoice date | Shorter-cycle work |
| Net 7 | Payment due 7 days after invoice date | Small jobs or new clients |
| Due on Receipt | Payment due immediately | One-time clients, retail, events |
| 2/10 Net 30 | 2% discount if paid within 10 days; otherwise Net 30 | Incentivizing early payment |
If you're just starting out, Net 15 or Net 7 reduces your cash flow risk. You can offer Net 30 as trust builds.
5. Late Fees
Adding a late fee clause is one of the most effective ways to get paid on time. Include it on every invoice:
"A late fee of 1.5% per month (18% annually) will be applied to invoices unpaid after the due date."
Important: state the late fee policy before the job starts — in your contract or proposal — so it's agreed to in advance. Clients are less likely to dispute a late fee they knew about.
Check your state's maximum allowable late fee rate — some states cap it. 1.5%/month (18%/year) is commonly within legal limits.
6. Taxes on Invoices
Whether you charge sales tax depends on:
- Your location — each state (and country) has different rules
- What you're selling — services may be taxable or exempt depending on the state
- Your client's status — businesses with resale or exemption certificates may not owe tax
If you're unsure whether to charge sales tax on a service, consult an accountant or your state's revenue department. Incorrectly charging (or not charging) tax creates liability.
If you're registered for VAT (UK, EU, Canada GST/HST), include your VAT/GST number and break out the tax rate and amount on the invoice — it's required by law for B2B transactions.
7. Payment Methods
Tell the client exactly how to pay. List your accepted methods clearly:
- Bank transfer (ACH): Include routing number and account number (or use a secure payment link)
- Check: Provide your business name and mailing address
- Online: PayPal, Venmo Business, Stripe, Square, Wave
- Credit card: Link to an online payment page or note your processing details
The fewer friction points between the client and paying, the faster you'll get paid. Offering online payment options typically reduces time-to-payment by 30-50%.
8. Common Invoice Mistakes to Avoid
- No due date — "Net 30" is not obvious to everyone. Write the actual date: "Due: April 5, 2026."
- Vague descriptions — "Consulting" doesn't tell the client what work was done. Be specific.
- Wrong client name or address — this causes delays at accounts payable. Confirm billing details before sending.
- Sending by wrong channel — ask the client their preferred invoice delivery method. Some require invoices via a vendor portal, not email.
- Not following up — a polite reminder on the due date (or a day before) significantly improves on-time payment rates.
- No invoice number — every invoice needs a unique number for tracking and accounting.
9. When and How to Send an Invoice
- For project work: Send immediately upon project completion, or on the agreed milestone date.
- For hourly work: Weekly or bi-weekly billing is common. Monthly billing can extend your cash flow gap.
- For retainers: Send on the same day each billing period (1st of the month is standard).
- Format: PDF via email is the standard. Some clients use vendor portals — ask first.