How do I calculate sales tax when I sell in multiple states?

The first step is figuring out where you actually owe sales tax. You only collect and remit sales tax in states where you have what’s called “nexus.” Nexus used to mean physical presence like an office, warehouse, or employee in a state. After the 2018 Supreme Court ruling in South Dakota v. Wayfair, most states now also impose economic nexus. That means if you exceed a certain sales threshold in a state, typically $100,000 in sales or 200 transactions in a year, you’re required to collect sales tax there even without any physical presence.

Each state sets its own threshold, and they’re not all the same. Some states only use a dollar amount. Others use transaction count. A few use both. You need to monitor your sales by state and know when you’re approaching or crossing those lines. Once you hit the threshold, you’re obligated to register, collect, and remit.

After you identify which states require you to collect, register for a sales tax permit in each one. Don’t start collecting sales tax in a state before you’re registered because that creates its own set of problems. Registration is usually done through the state’s department of revenue website.

Calculating the actual tax on each sale depends on whether the state uses origin-based or destination-based sourcing. Origin-based means you charge the rate where your business is located. Destination-based means you charge the rate where the buyer is located. Most states are destination-based, which means you may need to apply different rates down to the city or county level for every transaction. Florida, for example, is destination-based and has varying county surtax rates on top of the 6% state rate.

This is where software becomes important. Platforms like Shopify, Amazon, and similar e-commerce tools often have built-in tax calculation features or integrate with services like TaxJar or Avalara. These tools apply the correct rate based on the buyer’s address and help track what you owe in each jurisdiction. If you’re selling through invoices or in person across state lines, you’ll need to look up rates manually or use a tax rate API.

Filing schedules vary by state and sometimes by how much you collect. High-volume sellers may file monthly while lower-volume sellers file quarterly or annually. Missing a filing deadline in any state means penalties and interest, even if the amount owed is small. Keeping track of multiple filing calendars across several states is one of the biggest headaches for multi-state sellers.

Some products and services are taxable in one state but exempt in another. Software subscriptions, digital goods, and certain services have inconsistent treatment across states. You need to know what you sell and how each state classifies it. Getting this wrong means you’re either overcharging customers or underpaying the state.

If you’re selling in more than two or three states, professional sales tax management is worth considering. The complexity scales fast and the cost of getting it wrong through audits, back taxes, and penalties usually exceeds the cost of having someone handle it properly from the start.

The bottom line is that multi-state sales tax isn’t something you figure out once and forget. Thresholds change, rates change, and your sales mix across states shifts over time. Having a system in place to track nexus, calculate rates accurately, and file on time in every state where you’re registered is essential. If you’re unsure where you stand, a firm that handles business tax preparation and compliance can review your sales data and tell you exactly where you need to be collecting and what you might owe for prior periods.