If you sell online in the US, at some point you’ll run into the word nexus. It sounds like legal jargon, but it decides something very practical: whether you’re required to register, collect, and pay sales tax in a state you don’t even live in. Here’s what it actually means, why it depends on where your sales are happening, and what to do once you’ve crossed into it.
What nexus actually means
Nexus is just the legal term for a connection between your business and a state, strong enough that the state can require you to collect its sales tax. Before 2018, that connection almost always meant a physical presence: an office, a warehouse, an employee, sometimes even a single contractor working from home. If you had none of that in a state, you generally didn’t have to worry about its sales tax.
That changed with a Supreme Court case, South Dakota v. Wayfair, which let states create nexus out of sales volume alone. That’s why online sellers who’ve never set foot in a state can still end up owing tax there.
Physical nexus vs economic nexus
Physical nexus is the older, more intuitive version: you have people or property in the state. Economic nexus is newer and applies to almost every online seller, since it’s triggered by how much you sell into a state, not whether you’re present there.
Most states set their economic nexus threshold around a certain dollar amount of sales into that state within a year, and some also count the number of separate transactions. Cross either one, and you have nexus, even if every order shipped from a warehouse a thousand miles away.
What counts toward your total
This is where a lot of sellers get caught out. States usually count gross sales into that state, not net revenue and not just taxable sales. Refunds, exempt sales, and marketplace sales (if a platform like Amazon or Etsy already collects for you) are handled differently depending on the state, so the same $80,000 in orders can put you over the line in one state and nowhere near it in another.
The practical takeaway is that you can’t just watch your total revenue. You need to know what you sold into each state specifically, which is a much harder number to keep in your head than it sounds.
What happens once you cross a threshold
Crossing a state’s threshold doesn’t trigger anything automatically. No email arrives, no warning light goes on. It just means you’re now required to register with that state, start collecting its sales tax on future orders, file returns on whatever schedule it assigns you, usually monthly, quarterly, or annually, and remit what you’ve collected.
Skip that step and the tax doesn’t go away. It usually just gets discovered later, during an audit or a marketplace data request, at which point you can owe back tax, penalties, and interest on sales you already made and can’t go back and add tax to.
How to keep track without guessing
The honest answer for most small stores is that nobody is checking this by hand every week. You either wait until an accountant flags it at tax time, which can be months after you crossed a threshold, or you build the habit of looking at a state-by-state breakdown of your sales regularly enough to catch it early.
That’s the exact gap sales by state is built to close. It shows you gross and net sales by state, so instead of guessing where you stand, you can see it directly from your own store data and act before a threshold turns into a filing you weren’t expecting.
This isn’t tax advice, and thresholds change from state to state and year to year, so always confirm the current rules with your accountant or a sales tax professional before making a filing decision. But knowing where you stand is the part you can control yourself, and that’s where this starts.
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