SALT compliance means meeting your business’s state and local tax obligations — registration, collection, filing, and payment — in every state and locality where your activity creates a taxable connection. For a single-location business, that may be one state return a year. For a company selling across state lines, using out-of-state fulfillment centers, or employing remote workers, it can mean dozens of registrations and returns, each on its own schedule and under its own rules. The stakes are practical: states share data with each other and the IRS, they actively look for businesses that should be filing but aren’t, and the bill for being found is almost always larger than the bill for coming forward.
Where Your Business Owes Tax
The threshold question is nexus — whether your connection to a state is strong enough to give the state authority to tax you. Before 2018, that generally required a physical presence such as an office, warehouse, or employee. The Supreme Court changed that in South Dakota v. Wayfair, Inc., holding that a state can require tax collection based on economic activity alone, even from sellers with no physical footprint in the state.1Supreme Court of the United States. South Dakota v. Wayfair, Inc.
Most states set their economic nexus threshold at $100,000 in annual sales or 200 separate transactions, matching the figures South Dakota used. It isn’t universal. A few large states use $500,000 and require a minimum transaction count. Some have dropped the transaction count and trigger nexus on revenue alone. Thresholds also differ by tax type within a single state: sales tax nexus rules don’t necessarily match income tax nexus rules, so clearing one doesn’t clear the other.
Physical nexus hasn’t gone anywhere. Storing inventory in a third-party fulfillment center, sending an employee to a trade show, or hiring one remote worker in another state can each create nexus on its own. This is where businesses stumble most often, especially after the shift to remote work that accelerated in 2020. A developer working from a different state creates a registration obligation many companies don’t discover until an audit notice arrives. If nexus exists and you aren’t registered, the state can assess back taxes plus interest and penalties covering several prior years.
The Narrow Federal Shield for Goods Sellers
A 1959 statute, 15 U.S.C. § 381, gives limited immunity from state income tax if your only in-state activity is soliciting orders for tangible personal property, with those orders approved and filled from outside the state.2Office of the Law Revision Counsel. 15 USC 381 – Imposition of Net Income Tax The protection is narrower than it sounds. It covers only physical goods, so it does nothing for software licenses, consulting, digital downloads, or subscriptions. And solicitation has to be the only activity in the state — anything beyond it, unless trivial, strips the shield entirely.3Multistate Tax Commission. Statement of Information Concerning Practices Under Public Law 86-272
Several states now treat common internet activities as exceeding mere solicitation: placing tracking cookies on in-state users’ devices, offering live chat, accepting job applications through the site, and providing post-sale help by email. If your business has an interactive website, the practical value of P.L. 86-272 has shrunk considerably. Don’t assume it protects you without checking each state’s current interpretation.
The Taxes You Have to Track
A single business activity can trigger more than one state tax at the same time. Sorting out compliance starts with knowing which apply to you.
Sales and Use Tax
Sales tax is the most visible and the biggest source of multi-state compliance work. When you sell taxable goods or services into a state where you have nexus, you collect the tax from the buyer and remit it to the state. Rates, taxable items, and exemptions vary widely — what’s taxable in one state is often exempt next door.
Use tax is the counterpart. When a buyer purchases something without paying sales tax and then uses it in a state that would have taxed the sale, the buyer owes the tax directly. For businesses, failing to self-assess use tax on untaxed purchases is a common audit finding.
Corporate Income Tax
Most states with an income tax apply it to the portion of your net income earned within their borders. Rates and calculation methods vary, and the starting point for state taxable income usually differs from federal because of state-specific additions and subtractions. How much of your income a given state can tax depends on apportionment.
Franchise and Gross Receipts Taxes
Franchise taxes are charged for the privilege of doing business in a state rather than on profits. They’re often calculated on net worth, authorized shares, or in-state capital, which means you can owe them in a year your business loses money. Minimum amounts run from a couple hundred dollars to $800 or more depending on the state.
Gross receipts taxes apply to total revenue without any deduction for expenses. A thin-margin business can face a meaningful effective rate under a gross receipts system even when the nominal rate looks low. Some states stack a gross receipts tax on top of a corporate income tax, so the same revenue is hit through two mechanisms.
How Income Gets Divided Among States
When your business earns income in more than one state, you don’t pay each state’s rate on your full profit. You apportion — divide income among the states based on a formula. Getting this wrong is one of the fastest ways to either overpay or trigger an audit.
The traditional formula weighted property, payroll, and sales equally. That has largely been replaced. As of 2026, roughly 38 states use a single sales factor formula, which bases your tax obligation entirely on the share of your total sales made into the state.4Tax Foundation. Apportionment The shift benefits businesses that manufacture or employ people in-state but sell nationally, and it increases the burden on out-of-state sellers with large in-state customer bases.
For service revenue, the key question is sourcing. Under market-based sourcing, revenue is assigned to the state where the customer receives the benefit of the service. Under cost-of-performance, it goes to the state where you did most of the work. The results can differ dramatically for the same engagement. Most states have moved to market-based sourcing, but a handful still follow cost-of-performance, and the mismatch creates both planning opportunities and traps.
Income outside your regular operations, like gains from selling a building or royalties from intellectual property, may be allocated entirely to one state rather than apportioned. Misclassifying business income as nonbusiness income (or the reverse) can either inflate or understate your liability in a given state.
Selling Through Marketplaces
If you sell through Amazon, Walmart Marketplace, eBay, or Etsy, you may not need to collect sales tax yourself in most states. After Wayfair, nearly every state with a sales tax enacted marketplace facilitator laws that shift collection and remittance from third-party sellers to the platform.1Supreme Court of the United States. South Dakota v. Wayfair, Inc. If the platform lists your products, processes payments, and helps with fulfillment, the platform carries the sales tax burden on those transactions.
That doesn’t clear you entirely. Sales through your own website or other direct channels remain your responsibility. Income tax obligations stay with you regardless of who collects the sales tax. And if you store inventory in a marketplace’s fulfillment centers across multiple states, that physical presence can create nexus for other tax types the platform doesn’t handle. Marketplace facilitator laws solve the sales tax collection problem, not the rest of it.
Registering in Each State
Once you know where you have nexus, you register with each state. This is more involved than creating an online account. Most states want your Federal Employer Identification Number, your NAICS code, information about corporate officers or owners including Social Security numbers and addresses, and records showing when your taxable activity in the state began.
Tax registration and foreign qualification are two different things. Foreign qualification is authority from the Secretary of State to conduct business in the state. Tax registration sets up accounts with the department of revenue so you can file returns and pay. You may need both, and one doesn’t substitute for the other. Some states won’t process tax registration until the Secretary of State filing is done; others won’t process the Secretary of State filing until any outstanding tax obligations are cleared.
Get the effective date right. States care about when your nexus began, not just when you registered. If you’ve been selling into a state for two years before registering, the state may require returns covering the whole period. Registering as soon as you hit a threshold avoids the back-filing problem and the penalties that ride with it.
Filing and Paying Across Multiple States
States assign filing frequencies — monthly, quarterly, or annually — based on the volume of tax you collect or owe. High-volume sellers typically file monthly, with returns due around the 20th of the following month. Lower-volume filers may qualify for quarterly or annual filing. The state notifies you of your frequency at registration and can change it if your volume shifts.
Electronic filing is standard in nearly every state, and many require it. Each state runs its own portal for returns and payment, and payment is typically by electronic funds transfer at the time of filing. Save every confirmation number the system generates. Those receipts are your proof of timely filing if the state later says you were late. Acknowledgments confirming receipt usually arrive within 48 hours.
The real challenge isn’t filing one return. It’s managing 15, 30, or 45 returns with different due dates, different portals, different rate structures, and different rules for what’s taxable. This is where SALT compliance turns into a genuine operational burden, and it’s why many multi-state businesses invest in tax automation software or outsource filing entirely. The cost of compliance is real, but it’s smaller than the cost of missed filings across multiple states.
Cleaning Up Past Noncompliance
If your business should have been collecting and remitting tax in a state but hasn’t been, you have a choice. Wait and hope the state doesn’t notice, or come forward through a voluntary disclosure agreement (VDA).
A VDA is a negotiated arrangement. You disclose your past-due liability, file returns for a limited number of prior years, and pay the back taxes plus interest. In exchange, the state waives penalties and agrees not to assess taxes for periods before the look-back window.5Multistate Tax Commission. FAQ Look-back periods vary by state but commonly cover three to four years, extending to six years in some states for certain tax types.
You still owe the tax and interest under a VDA, but the penalty waiver and the capped look-back can save substantial money against what happens if the state finds you first. States that discover you through an audit typically assess the full statute of limitations, which for unfiled returns often has no limit at all, add penalties, and charge interest from the original due date. The math almost always favors coming forward.
States discover unregistered businesses through federal data sharing, audits of in-state customers, shipping records, and marketplace facilitator data. Flying under the radar has become increasingly unreliable as a strategy. If you have unfiled obligations, a VDA through the Multistate Tax Commission or directly with the state is the more defensible path.
What Noncompliance Actually Costs
State penalties for late filing, late payment, and failure to register follow patterns similar to federal ones but vary in specifics. A common structure charges a percentage of the unpaid tax for each month or part of a month the return or payment is late, capped at a stated ceiling. At the federal level for comparison, the failure-to-pay penalty is 0.5% per month up to 25%.6Internal Revenue Service. Failure to Pay Penalty Many states use a similar framework for their own taxes.
Beyond late-filing penalties, some states impose separate penalties for negligence (understating liability without reasonable cause) and fraud (deliberately underreporting). Fraud penalties are dramatically higher, often double the underpayment, and there is no statute of limitations on fraudulent returns. Interest runs on top of all penalties from the original due date, compounding the total over time.
The cost that catches businesses most by surprise isn’t a fine. It’s the back-assessment itself. A state that discovers you had nexus five years ago can demand returns for every year you should have been filing. If you were supposed to be collecting sales tax from customers and weren’t, you owe the uncollected tax out of your own pocket. There’s no going back to those customers to collect retroactively. It is the single most expensive SALT compliance failure, and it is entirely avoidable with timely registration.