An invoice is the document that turns finished work into money owed. It tells your client what you did, how much it cost, and when and how to pay. A good invoice is clear enough that the person paying it never has to email you to ask what a line means — and complete enough that your records, and your tax return, hold up later. Writing one is not complicated, but there are a few details that first-time invoicers usually miss, and those details are the ones that cause late payments.
This guide walks through what every invoice must contain, how to number invoices, how to set payment terms, and how tax works — with a worked example you can copy for your own work.
What every invoice must contain
Start with the word “Invoice” at the top, so nobody mistakes it for a quote or a receipt. Then add your business name, address, email, and phone number, followed by your client’s name and address. If you are registered for tax, include your tax or VAT registration number — in many countries it is legally required on invoices.
Next comes the invoice number and the dates: the date you issued the invoice and the date payment is due. Then the line items — each product or service on its own line, with a short description, the quantity, the unit price, and the line total. Below the items, show the subtotal, any discount, the tax amount, and the grand total. Finally, add your payment details: bank details, a payment link, or whatever method you accept, plus any notes about late fees or terms.
Keep descriptions specific. “Design work — $400” is vague; “Website homepage design, 1 page — $400” is clear. If a client ever questions a charge, a specific description ends the conversation in your favour.
How to number your invoices
Give every invoice a unique number, and never reuse one: start at INV-001 and count up, or use the date, such as INV-20261109-001. Sequential numbering matters because gaps look suspicious to a tax authority, and because a client who wants to query a payment needs a number to quote. Pick one format and stick to it — changing formats halfway through the year makes your records harder to reconcile.
Setting payment terms
Payment terms are the deadline you set for payment, written on the invoice. Common terms are “due on receipt,” Net 7 (seven days), Net 14, and Net 30 (thirty days). Net 30 is the most common for business clients; freelancers often use Net 14 or Net 7, because waiting a month for payment is a long time when you are the whole business.
Whatever you choose, write it plainly — “Payment due within 14 days of the invoice date” — and state what happens if payment is late. A small late fee, such as 1.5% per month on overdue amounts, is standard in many industries, but check the rules in your country first: some places limit late fees, and some require them to be agreed in advance. Put the terms on the invoice itself, not only in an email, so the client cannot say they never saw them.
Tax on an invoice
Whether you add tax depends on where you are and whether you are registered for it. If you are registered for VAT, GST, or sales tax, you must charge it on taxable supplies and show it as a separate line — the rate, the amount, and the total including tax. If you are not registered, you generally do not charge tax, and you should not show a tax line at all, because charging tax you are not registered to collect can create a real problem.
Apply tax to the subtotal after any discount, not before it: if you give a 10% discount, tax is calculated on the discounted price, because that is what the client actually pays. The order is items first, then discount, then tax on what remains.
If you are unsure whether you should be charging tax, ask your accountant before you send your next invoice. Getting it wrong in either direction — charging tax you shouldn’t, or not charging tax you should — is more expensive to fix later than a short conversation now.
Worked example: a three-item invoice with 10% tax
Suppose you are a freelance designer invoicing a client for a small website job. Your line items are:
- Website homepage design — 1 × $400.00 = $400.00
- Logo design — 2 × $150.00 = $300.00
- Hosting setup — 1 × $120.00 = $120.00
Add the line totals to get the subtotal: $400.00 + $300.00 + $120.00 = $820.00. There is no discount on this invoice, so the taxable amount is the full $820.00.
Tax at 10% is $820.00 × 0.10 = $82.00. The grand total is the subtotal plus the tax: $820.00 + $82.00 = $902.00. So the invoice shows a subtotal of $820.00, tax of $82.00, and a grand total of $902.00, due within the payment terms you set — for example, Net 14, fourteen days from the invoice date.
How to use the invoice generator
Open the invoice generator, enter your business and client details, and add your line items. The tool totals the items, applies any discount before tax, works out the due date from your payment terms, and shows a live print-ready preview as you type. When it looks right, print it or save it as a PDF. For a simpler job, the invoice maker does the same job with fewer fields, and the invoice templates page offers five ready-made styles.
Common mistakes to avoid
The most common mistake is sending an invoice with no number, no due date, or no payment details — any one of those can delay payment by weeks. The second is calculating tax before discount, which overcharges the client. The third is vague line descriptions that invite questions. And the fourth is waiting too long to send the invoice at all: send it the day the work is finished, while the value is fresh in the client’s mind.
Frequently asked questions
What is the difference between an invoice and a bill?
They are the same thing from opposite sides. The seller sends an invoice; the buyer receives a bill. The document, the numbers, and the payment terms are identical — only the point of view changes.
Do I need to send a paper invoice?
No. A PDF sent by email is a valid invoice in most countries, as long as it contains all the required details: your business information, the client’s details, a unique invoice number, the dates, itemised charges, tax, and the total. Keep a copy for your own records.
What should I do if a client doesn’t pay on time?
Send a polite reminder the day after the due date, restating the invoice number, the amount, and the due date. If there is still no payment after a week, follow up again and mention the late fee if your terms include one. Keep every reminder in writing, and if the amount is large, talk to a professional about your options before the debt gets old.