Whether a tax line belongs on your invoice depends on registration, jurisdiction, and what you sell. The rules differ; the underlying logic is the same nearly everywhere.
You charge tax on an invoice when, and only when, you are registered to collect it. Unregistered businesses invoice without a tax line - no rate, no tax registration number, no "including tax" wording. Charging tax you are not registered for is not a harmless rounding-up: you are collecting money on behalf of a tax authority that has no record of you, which creates a liability, a refund problem, and in most places an offence.
Registration is usually triggered by turnover crossing a threshold over a rolling period, and the thresholds vary enormously - some countries register businesses from the first sale, others only well into six figures of revenue. The obligation typically arrives before you feel large, so the practical advice for a growing business is to know your jurisdiction's threshold now and watch your rolling total, because registering late means paying tax you never collected.
Everything else in this guide is jurisdiction-dependent, and none of it is advice for your specific situation - an hour with an accountant when you approach registration is one of the better purchases a small business makes.
In VAT and GST systems - the UK, the EU, Australia, Canada, and most of the world - the invoice itself is a legal tax document. Once registered, your invoices must show your tax registration number, the tax rate applied to each line or category, the tax amount as its own figure, and the totals with and without tax. Business customers reclaim the VAT they pay you, and your invoice is their evidence; an invoice missing the required fields can be rejected by their accountants for exactly that reason.
The tax point - the date the tax is owed to the authority - is generally the invoice date or payment date depending on the scheme, which is why deposits matter: in many VAT systems, taking a deposit creates a tax point for that amount immediately, not when the work completes.
Cross-border services add the reverse charge: for many business-to-business services between countries, the supplier invoices without tax and the customer accounts for it locally, with wording on the invoice saying so. If you sell services abroad, this is the mechanism you will meet first, and getting the wording right matters.
US sales tax works differently in almost every respect. It is levied by states (and often cities and counties on top), applies primarily to goods and only selectively to services, and is owed where the customer is, not where you are. The registration trigger is "nexus" - a connection to a state through physical presence or, since 2018, simply enough sales into it. A service business in one state can acquire obligations in another purely by selling there.
The saving grace for most small service businesses is that many US states tax few or no professional services - but "many" is not "all", the lists are specific and change, and digital products are increasingly taxed. The invoice mechanics are simpler than VAT - a tax line at the applicable combined rate, no tax number required on the document in most states - but knowing whether to charge at all is the hard part.
If you sell into multiple states or countries, this is the point where automating tax lookup, or handing the question to an accountant, stops being optional. The invoice is the easy part; the determination behind it is not.
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