Your software company is headquartered in Texas, your developers work remotely from Colorado and Tennessee, your sales team covers all 50 states, and roughly half your revenue comes from customers in California, New York, and Illinois. Where is your income taxed—and how much of it?
The answer is rarely intuitive, and it has become dramatically more expensive to guess wrong. Since 2010, eighteen states have abandoned the traditional way of dividing up corporate income among jurisdictions, and a parallel shift in how services are sourced has rewritten the tax map for any business that sells across state lines. Two companies with identical revenue, identical expenses, and identical customers can owe wildly different state taxes simply because of where their offices are pinned on a map.
This guide walks through the three big apportionment formulas you will encounter, explains the market-based sourcing revolution, demystifies the throwback and throwout traps, and shows you what to track so a state auditor never catches you flat-footed.
What Apportionment Actually Is
When a business operates in more than one state, every state with jurisdiction wants its share of the income. Apportionment is the mathematical formula each state uses to decide what percentage of your total business income it gets to tax. The states do not coordinate, so you can—and frequently will—have apportionment percentages that add up to more or less than 100 percent of your income.
Apportionment kicks in only after a separate question is answered first: nexus. Nexus is the legal connection that gives a state the right to tax you at all. Historically nexus meant physical presence—an office, a warehouse, an employee. Today, after the Supreme Court's 2018 Wayfair decision, most states also enforce economic nexus based on sales thresholds (commonly $100,000 of sales or 200 transactions per year), and many extend that standard from sales tax over to income tax as well.
Once you have nexus, the state pulls out its apportionment formula and starts carving up your income.
The Three Classic Apportionment Methods
Equal-Weighted Three-Factor Formula
The traditional formula, codified in the Uniform Division of Income for Tax Purposes Act (UDITPA) in 1957, weighs three factors equally:
- Property factor: in-state property divided by total property
- Payroll factor: in-state payroll divided by total payroll
- Sales factor: in-state sales divided by total sales
You average the three percentages, and the result is the share of your income that state can tax. A retailer with 30 percent of its property, 25 percent of its payroll, and 40 percent of its sales in State A would apportion (30 + 25 + 40) ÷ 3 = 31.67 percent of income to State A.
This balanced formula made sense in a manufacturing economy where factories and warehouses were the dominant value drivers. Only a handful of states still use the equal-weighted three-factor formula, and most of those have carve-outs for specific industries.
Double-Weighted Sales Formula
Many states transitioned from the equal-weighted formula to one that doubles the sales factor. The denominator becomes 4 instead of 3, and the sales factor is counted twice:
(Property % + Payroll % + Sales % + Sales %) ÷ 4
The reasoning: states wanted to encourage in-state property investment and employment by reducing the tax penalty for being headquartered there. Doubling the sales factor shifts more of the tax burden onto out-of-state sellers and less onto local employers.
Single Sales Factor (SSF) Formula
The big trend of the last two decades is the single sales factor. Property and payroll drop out of the calculation entirely, leaving only:
Sales in the state ÷ total sales everywhere = apportionment percentage
For a multistate business with a single headquarters, this formula is transformative. A Texas-based software company that sells nationwide pays a much smaller share of California's tax under SSF than under three-factor, because none of its California-bound product comes from California-located property or payroll—it is the customer's location that drives the math.
States adopting SSF generally argue two things: (1) it rewards in-state job creation and capital investment by removing the property and payroll penalty, and (2) it more accurately reflects where economic activity occurs in a digital, remote-work economy where physical presence is no longer the proxy it once was. Since 2010, eighteen states have moved to single sales factor, and the count keeps climbing—Montana and Tennessee being among the most recent.
Sourcing Rules: Where Does a Sale "Happen"?
The apportionment formula is only half the story. To know your sales factor, you have to assign every dollar of revenue to a state. For tangible goods, this is easy: revenue is sourced to the destination state where the buyer takes delivery. For services and intangibles, it gets murky—and this is where the second great state tax revolution has been playing out.
Cost-of-Performance (Origin Sourcing)
Under the older cost-of-performance method, service revenue is sourced to the state where the seller incurs the most expense in producing the service. If your developers in Texas build the software, Texas gets the revenue regardless of which customer pays for it.
Cost-of-performance favors states that house the productive activity. It also creates an obvious planning opportunity: by clustering high-cost activities in a low-tax state, a service business can shift its apportionment footprint toward favorable jurisdictions.
Market-Based Sourcing
Market-based sourcing reverses the rule. Service revenue is sourced to the state where the customer is located, or where the benefit of the service is received, regardless of where the work was performed. A consultant in Texas billing a client in California assigns that revenue to California for apportionment purposes.
Market-based sourcing matches well with single sales factor: together they treat services and intangibles like tangible goods sold to the customer's state. The combination is sometimes called the "destination-based" approach.
As of 2026, the majority of states with corporate income tax use market-based sourcing for services, and the trend has accelerated rapidly. Kansas and Arkansas both adopted market-based sourcing effective in 2025, and Kansas's H.B. 2231 will broaden the rule to additional revenue categories starting after December 31, 2026.
The Double-Taxation Trap
The shift is not coordinated. While most states have moved to market-based sourcing, several still use cost-of-performance, and the definitions of "market" and "performance" vary state-by-state. A service business can find that the same dollar of revenue is sourced to two states under their respective rules—or, conversely, that revenue falls through the cracks and is sourced to no state at all. This is why multistate businesses with significant service revenue often need a state-by-state revenue mapping for their sales factor, not a single national number.
Throwback and Throwout: The Nowhere-Income Traps
The federal Interstate Income Act of 1959 (commonly called P.L. 86-272) prohibits a state from taxing a company's income if the company's only activity in that state is soliciting orders for tangible goods that are shipped from outside the state. Combined with the destination-based sales factor, this protection can create "nowhere income"—revenue that, under the strict math, gets apportioned to a state that has no jurisdiction to tax it.
States hate giving up that revenue, and 22 of them plus D.C. have adopted one of two responses.
Throwback Rule
Under a throwback rule, sales that would otherwise be nowhere income are "thrown back" into the numerator of the origin state's sales factor. The origin state taxes the income that the destination state cannot. A California-based manufacturer selling protected goods to a customer in Nevada (which has no corporate income tax) and a customer in Iowa (where P.L. 86-272 protects the manufacturer) ends up apportioning those sales to California under California's throwback rule.
Throwout Rule
The throwout rule attacks the same problem from the denominator side. Instead of adding nowhere income to the home state, the throwout rule removes it from total sales entirely. The home state's apportionment percentage rises because the denominator shrinks, even though the numerator stays the same.
Throwback is more common than throwout, and a meaningful body of research suggests both approaches actually shrink state tax revenue over time by chasing businesses to states without these rules. Several states have repealed throwback rules in recent years.
Worked Example: Why the Formula Matters
Imagine a SaaS company with $10 million of taxable income, headquartered in a state with a 7 percent corporate income tax rate. The business has:
- 80 percent of its property and payroll in the home state
- 20 percent of its sales to home-state customers, 80 percent to out-of-state customers
Under an equal-weighted three-factor formula:
(80 + 80 + 20) ÷ 3 = 60 percent apportioned to the home state. Tax owed: $10M × 60% × 7% = $420,000.
Under double-weighted sales:
(80 + 80 + 20 + 20) ÷ 4 = 50 percent apportioned. Tax: $350,000.
Under single sales factor:
20 percent apportioned. Tax: $140,000.
The home-state tax bill drops by two-thirds simply by changing the formula—without moving a single employee, customer, or server. This is precisely why states aggressively recruit headquarters with SSF adoption: the in-state tax burden on a national or global seller becomes minimal.
The flip side: the out-of-state customer states pick up the difference, and now require the SaaS company to file, register, and remit there. Filing complexity scales with every new market state.
What Multistate Businesses Should Track
To navigate apportionment with confidence, your books need to capture enough granularity to support the math. At minimum:
- Revenue by customer state, broken out between tangible goods, services, and intangibles, since each category may follow different sourcing rules
- Property by location, including owned and rented real estate, equipment, and inventory, valued consistently across states
- Payroll by work location, including remote employees whose tax home may differ from the office address on file
- Nexus triggers: dates and amounts that crossed economic nexus thresholds in each state
- P.L. 86-272 protection status, especially in states with throwback rules
Accurate bookkeeping that tags every transaction with the necessary location metadata makes apportionment a query rather than a forensic project. The alternative—reconstructing this data at year-end—is where mistakes, penalties, and missed planning opportunities pile up.
Common Planning Pitfalls
Remote employees breaking your apportionment story. A single developer who relocates from your home state to a market state can shift payroll factors materially in three-factor states and, more importantly, often creates income tax nexus in the new state. Track remote work locations as carefully as you track office leases.
Inconsistent sourcing across states. If you use one sales spreadsheet for the home state and a different one for filings elsewhere, you will eventually be questioned. Build a single source of truth for revenue-by-state and apply each state's sourcing rule to it.
Ignoring the "fixed place of business" doctrine. Some cost-of-performance states require you to source service revenue based on where you perform a plurality of work for a specific contract, not your overall cost mix. The unit of analysis matters.
Assuming P.L. 86-272 still protects you. The Multistate Tax Commission has revised its guidance to treat many web-based activities—chatbots, cookies, online support—as activities that exceed mere solicitation, stripping the protection. Software businesses in particular cannot rely on the historical safe harbor.
Filing late in throwback states. If your home state has a throwback rule and you have not filed in a market state because you believed P.L. 86-272 applied, you can owe home-state tax on those sales. Audits often surface these in batches.
Keep Your Financial Records Audit-Ready
State apportionment audits frequently turn on the quality of your records: which sales went to which state, where employees were physically working in which months, and how revenue was sourced contemporaneously rather than reconstructed after the fact. The businesses that come through audits cleanly are the ones whose books were structured to answer state tax questions before the questions were asked.
Beancount.io offers plain-text accounting that gives you complete transparency and control over your financial data—every transaction is human-readable, version-controlled with Git, and ready for the kind of dimensional queries that multistate compliance demands. No black boxes, no vendor lock-in. Get started for free and see why developers and finance professionals are switching to plain-text accounting for exactly the kinds of cross-jurisdictional challenges that state apportionment creates.