Illinois single sales factor apportionment works by multiplying your total business income by a single fraction: Illinois-sourced sales over total sales everywhere. Property and payroll do not enter the formula. For C-corporations, the apportioned income is taxed at a combined 9.5% (7% corporate income tax plus 2.5% personal property replacement tax), which makes the sourcing of each receipt the most consequential decision on the return.1Illinois Department of Revenue. What Is the Tax Rate for Businesses, Trusts, and Estates?
The formula itself is one line. The rules that decide which sales land in the numerator are where most of the work sits, and they differ by what you sell and, for some industries, who you are.
Start by Separating Business From Non-Business Income
Only business income runs through the apportionment formula. Non-business income is allocated directly to a single state under separate rules.2Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3010 – Business and Nonbusiness Income
Illinois treats income as business income unless it is clearly classifiable as non-business income, and it applies two alternative tests. Under the transactional test, income is business income if it arises from transactions in the regular course of your trade or business. Under the functional test, income from property is business income if acquiring, managing, and disposing of that property is an integral part of regular operations. Meeting either test is enough. Most income earned by a multi-state corporation ends up on the business income side.2Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3010 – Business and Nonbusiness Income
Items like certain capital gains, rents, royalties, interest, and dividends may be treated as non-business income and allocated to the state where the underlying property or activity sits. Reclassifying an item from business to non-business income after prior years have been filed carries a cost: Illinois requires recapture of related deductions for the current year and the two preceding years, apportioned at the higher of the current-year factor or the three-year average.2Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3010 – Business and Nonbusiness Income
The Single Sales Factor
For tax years ending on or after December 31, 2000, Illinois apportions general business income using only the sales factor. Property and payroll are gone from the general rule.3Illinois General Assembly. Illinois Code 35 ILCS 5/304 – Business Income of Persons Other Than Residents
The practical consequence: a company can build facilities and hire in Illinois without pushing its apportionment factor up. Everything turns on where the customers are. Getting each sale sourced correctly is the whole game.
Sourcing Sales of Goods
Sales of tangible personal property are sourced to Illinois when the goods are delivered or shipped to a purchaser in Illinois. Shipping terms and contract conditions do not change the answer. The destination controls, including when goods are shipped to an Illinois recipient at the buyer’s direction or diverted mid-shipment to an Illinois location.4Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3370 – Sales Factor
Throwback Sales
Illinois has a throwback rule. If you ship goods from an Illinois location to a buyer in a state where you are not taxable, that sale is thrown back into the Illinois numerator.4Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3370 – Sales Factor “Not taxable” means the destination state cannot impose a net income tax on the seller. This most often catches sales into states where you have no physical presence and where economic nexus rules do not reach you. Each thrown-back sale increases the numerator without changing the denominator, raising the factor.
The Finnigan Method for Unitary Groups
For tax years ending on or after December 31, 2025, Illinois applies the Finnigan method to the sales factor for unitary business groups. Under the prior Joyce approach, each member’s taxability in a destination state was evaluated separately, so one member’s sale could be thrown back to Illinois even if a sibling entity in the same group was taxable in that state.5Illinois Department of Revenue. 2025 Schedule UB Instructions
Finnigan treats the whole unitary group as a single taxpayer for the taxability question. If any group member is taxable in the destination state, the sale stays there and is not thrown back. For groups with broad multi-state footprints, this typically shrinks the Illinois numerator and lowers the overall factor. Groups concentrated in Illinois with limited out-of-state nexus will see less movement.
Sourcing Services and Intangibles
Illinois uses market-based sourcing for services and intangible property. Where the customer receives the benefit matters; where the work is performed does not.4Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3370 – Sales Factor
For services, the receipt is sourced to Illinois if the service is received in the state. When the point of receipt is not clear, Illinois applies a hierarchy: first, where the service is actually delivered; then, the location of the customer’s office that ordered the service; then, the office to which the service was billed. You move down the ladder only when the prior step cannot be reasonably determined. The cascade prevents receipts from disappearing entirely because no state can be pinpointed.
For intangibles such as patents, trademarks, and software licenses, receipts are sourced based on where the intangible is used. Software licensed to a customer who runs it on servers in Illinois produces Illinois-sourced receipts to the extent of that use.
Investment receipts, including interest, dividends, and capital gains, are attributed to Illinois based on the proportion of the taxpayer’s gross income tied to a fixed place of business in the state. A fixed place of business requires an actual physical location, such as an office or facility. If investment management happens entirely outside Illinois, those receipts stay out of the numerator.
Industries That Don’t Use the General Formula
Several industries are carved out of the single sales factor and apportion under specialized formulas because the standard approach does not capture how they earn revenue by state.
Financial Organizations
Banks and other financial organizations apportion using a single gross receipts fraction: Illinois gross receipts over total gross receipts.6Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3405 – Apportionment of Business Income of Financial Organizations The sourcing rules vary by receipt type. Loans secured by real estate or tangible property are sourced to the location of the collateral, split proportionally when collateral sits in multiple states. Unsecured consumer loans follow the borrower’s residence. Unsecured commercial loans are sourced to where the proceeds are applied, with fallbacks to the office that negotiated the loan and, failing that, exclusion from both the numerator and denominator. Credit card receivables, including interest, fees, service charges, and merchant discount income, are sourced to Illinois if charges are regularly billed to a customer in the state.3Illinois General Assembly. Illinois Code 35 ILCS 5/304 – Business Income of Persons Other Than Residents
Transportation Companies
For tax years ending on or after December 31, 2008, transportation companies other than airlines use a receipts-and-mileage approach. The Illinois numerator captures receipts from shipments that both originate and terminate in Illinois, plus a mileage-weighted portion of interstate shipment receipts (Illinois miles over total miles). The denominator is all revenue from moving people, goods, or other substances.7Illinois General Assembly. Illinois Code 35 ILCS 5/304 – Business Income of Persons Other Than Residents
Airlines use a revenue-mile formula, where a revenue mile is one passenger or one net ton of freight carried one mile for a fee. The Illinois fraction is Illinois revenue miles over revenue miles everywhere, with passenger and freight fractions averaged and weighted by relative gross receipts when an airline carries both.3Illinois General Assembly. Illinois Code 35 ILCS 5/304 – Business Income of Persons Other Than Residents
Insurance Companies
Insurance companies apportion using a direct premiums fraction: direct premiums written for insurance on Illinois property or risk over direct premiums written everywhere. Direct premiums written includes total direct premiums, assessments, and annuity considerations as reported on the annual statement filed with the Illinois Director of Insurance.8Illinois General Assembly. Illinois Administrative Code Section 100.3420 – Apportionment of Business Income of Insurance Companies
When a company’s principal source of premiums is reinsurance rather than direct coverage, the formula folds reinsurance premiums accepted on Illinois property or risk into both parts of the fraction. Reinsurers can elect to compute their Illinois share based on either the proportion of reinsurance premiums accepted from Illinois-domiciled cedants or the proportion of direct premiums written in Illinois by each ceding company. The election is binding for all future years unless the Department gives written permission to change.7Illinois General Assembly. Illinois Code 35 ILCS 5/304 – Business Income of Persons Other Than Residents
Unitary Business Groups and the 80/20 Exclusion
Related corporations that operate as a unitary business file a combined return. The group computes one apportionment factor by dividing total Illinois sales of all members by total sales of all members everywhere. When members within the group use different apportionment methods, a weighted formula reconciles the pieces.9Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3600 – Combined Apportionment for Taxpayers Using Different Apportionment Formulas
Not every affiliate is in. A company that conducts 80% or more of its business activities outside the United States is excluded from an Illinois unitary group even if it would otherwise qualify by ownership and operational integration.10Illinois Department of Revenue. Schedule 80/20 Instructions Companies near the 80% line should track their domestic-versus-foreign activity ratios closely, because crossing in either direction changes the composition of the group and shifts the combined factor.
Petitioning for a Different Method
When the standard formula does not fairly reflect your Illinois market, you can petition the Illinois Department of Revenue for an alternative. The Department can also initiate the change. Available alternatives include separate accounting, dropping one or more factors, adding new factors, or any other method that produces an equitable result.11Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3390 – Petitions for Alternative Allocation or Apportionment
The bar is high. Whoever asks for the departure must prove by clear and convincing evidence that the statutory formula taxes income from outside Illinois or attributes a share of income that is out of all proportion to the company’s actual Illinois market. Showing a different formula would produce a lower bill is not enough. The requesting party has to demonstrate that the standard approach leads to a grossly distorted result and that the proposed alternative fairly reflects the Illinois market.11Legal Information Institute. Illinois Administrative Code Title 86 Section 100.3390 – Petitions for Alternative Allocation or Apportionment These petitions succeed only in unusual fact patterns.
What Getting It Wrong Costs
Underreporting Illinois-apportioned income triggers interest on the deficiency. Illinois charges simple interest at the federal underpayment rate, which is reviewed every January 1 and July 1. From January 1, 2025, through June 30, 2026, the rate is 7%.12Illinois Department of Revenue. Interest Rates Interest is calculated daily and runs from the original due date until the tax is paid. Because the rate resets semi-annually, a deficiency that spans multiple periods can accrue at different rates over its life.