The question that comes before "how much tax do I charge?"

Most owners learn how to put sales tax on an invoice before they learn the harder question underneath it: which states am I even supposed to collect for? The answer turns on a concept called nexus — a connection between your business and a state strong enough that the state can require you to register, collect its sales tax, and remit it. No nexus, no obligation. Cross the line into nexus, and you are on the hook whether you charged the tax or not. (General education, not tax advice — sales-tax rules are state-specific and change often, so confirm your own situation with a professional.)

Two kinds of nexus

There are two main ways a business gets nexus in a state:

Physical nexus is the old, intuitive kind. You have an office, a warehouse, inventory, an employee, or a contractor in the state — some tangible footprint. Storing goods in a fulfillment warehouse in a state you have never visited can be enough on its own.
Economic nexus is the newer kind, and the one that surprises people. Since a 2018 U.S. Supreme Court decision, a state can require you to collect its tax based purely on how much you sell into the state, with no physical presence at all. Cross the state's sales threshold and you have nexus.

The economic-nexus thresholds vary by state, but a very common shape is a certain dollar amount of sales into the state in a year — often around one hundred thousand dollars — or a certain number of separate transactions. Some states use both, some have dropped the transaction count, and the figures move, so the only safe move is to check each state where you sell meaningfully rather than assume.

Why this matters before you invoice, not after

Nexus is a before problem because the obligation attaches to the sale, not to whether you remembered to charge tax. If you had nexus in a state and did not collect, the tax is still owed — and it typically comes out of your pocket as an assessment, plus interest and penalties, because you can rarely go back and bill old customers for it. That is the quiet danger of economic nexus: a growing online business can trip a threshold in a state it has never thought about and not realize it until an audit or a notice arrives.

The flip side is just as real: collecting tax in a state where you have no nexus and no registration is also a problem, because you are holding money you are not authorized to collect. The goal is not "collect everywhere to be safe" — it is to collect in exactly the states where you have nexus, and nowhere else.

A practical way to keep track

You do not need to monitor fifty states by hand. The workable routine for a small business is:

Know your physical footprint. List every state where you have people, property, or inventory. Those are automatic nexus states from day one.
Watch your sales by state. Run an income or sales report grouped by the customer's state, and watch which states are approaching the common threshold levels. A state where you are doing thirty thousand dollars of business is not urgent; one closing on a hundred thousand deserves a look at that state's specific rule.
Register before you collect. When you do cross into nexus, register with that state first, then start collecting. Collecting before you are registered creates its own mess.

This is the same instinct behind broader sales-tax compliance for a growing business: the work is not the arithmetic on any one invoice, it is knowing the rules of engagement before the invoices go out.

Once you have nexus, keep the bookkeeping honest

Sales tax you collect is never your revenue — it is money you are holding in trust for the state until you remit it. On the books it sits as a liability, not income, which is why a clean profit-and-loss statement never counts collected tax as part of your sales. Keeping the collected tax separated and tied to the right state is what makes filing a return a lookup instead of a reconstruction, and it is the foundation that makes you audit-ready if a state ever asks.

The takeaway

Nexus decides where you owe; the tax rate decides how much. Get the where wrong and the how-much never matters. Map your physical presence, watch your sales by state against the economic thresholds, register before you collect, and keep collected tax booked as a liability you owe — not as money you earned.

Hosting Books lets you set tax on invoices, books collected sales tax as a liability rather than revenue, and reports sales and income by customer so you can see where your business is concentrated — the raw material for spotting a state that is creeping toward a nexus threshold.

This article is general educational information about sales-tax concepts and is not tax advice for your specific situation. Sales-tax nexus rules are set by each state and change frequently — confirm your obligations with a qualified professional.

Sales Tax Nexus: Figuring Out Where You Actually Owe Sales Tax

The question that comes before "how much tax do I charge?"

Two kinds of nexus

Why this matters before you invoice, not after

A practical way to keep track

Once you have nexus, keep the bookkeeping honest

The takeaway

See Hosting Books in action

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