Box 15 on your W-2 shows two things: the two-letter abbreviation for the state where you worked and your employer’s state tax identification number assigned by that state’s tax agency. The state uses that number to match the state wages in Box 16 and the state income tax withheld in Box 17 to your employer’s withholding account, so you get credit for taxes already paid on your behalf. If you work in a state with an income tax, your tax software will ask for this number when you file your state return.
What the Box Actually Contains
The official label is “State/Employer’s state ID number.” The state abbreviation sits on the left (CA, NY, OH, and so on), and the employer’s state-assigned ID sits next to it.1Internal Revenue Service. General Instructions for Forms W-2 and W-3 (2026) Boxes 16 and 17 are laid out the same way, so each line of wages and withholding ties back to a specific state in Box 15.
The box has room for two states, separated by a broken line. If you worked in more than two states during the year, your employer must issue a second W-2 to cover the additional ones.1Internal Revenue Service. General Instructions for Forms W-2 and W-3 (2026)
Why Box 15 Is Not the Same as the EIN
The number in Box 15 is not the Employer Identification Number shown in Box b. The EIN is a nine-digit federal number issued by the IRS and used only for federal tax reporting.2U.S. Small Business Administration. Get Federal and State Tax ID Numbers The Box 15 number comes from the state’s tax agency (names vary by state: Department of Revenue, Franchise Tax Board, Department of Taxation, and others) and is used only for state tax reporting. Different issuer, different purpose, different format.
Formats vary by state. If you want to sanity-check what’s in your Box 15, pull a prior-year W-2 from the same employer and compare. The number should match unless the employer re-registered with the state.
Some states issue a single number that covers both income tax withholding and unemployment insurance; others assign separate numbers for each. The one in Box 15 is specifically the withholding account number.
When Box 15 Is Blank
Eight states have no individual income tax: Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, and Wyoming.3Tax Foundation. State Individual Income Tax Rates and Brackets, 2026 If you worked only in one of these, expect Box 15 to be blank along with Boxes 16 and 17. Nothing is wrong with the form; there is simply no state withholding to report and no account to report it under.
If your employer withheld something like state disability insurance, that amount shows up in Box 14 (“Other”), not in Boxes 15 through 17.4Internal Revenue Service. 2026 General Instructions for Forms W-2 and W-3
If You Worked in More Than One State
When you live in one state and work in another, what appears in Box 15 depends on whether those states have a reciprocity agreement. About half the states with an income tax participate in at least one. Under a reciprocity agreement, you owe income tax only to your home state, your employer withholds for the home state, and the home state’s ID appears in Box 15.
Reciprocity isn’t automatic. You usually have to file an exemption form with your employer so they stop withholding for the work state. Skip that form and the employer withholds for the work state by default; the work state’s ID shows up in Box 15, and you’d need to file in the work state to recover the money while separately paying your home state.
When no reciprocity applies and you worked in multiple states, each state is reported separately. Box 15 holds two; anything beyond that goes on a second W-2.1Internal Revenue Service. General Instructions for Forms W-2 and W-3 (2026) You’ll likely owe a return in each state listed.
If Box 15 Is Wrong, Blank, or Shows the Wrong State
Start with your employer’s payroll department. Most Box 15 problems are data-entry errors that payroll can fix in a few minutes. Once corrected, the employer issues a Form W-2c, the corrected version of the original W-2.5Internal Revenue Service. About Form W-2 C, Corrected Wage and Tax Statements Employers must furnish W-2s by January 31, so if you haven’t received yours or spot the error early, you have some runway.6Social Security Administration. Deadline Dates to File W-2s
If payroll doesn’t respond or hasn’t fixed the error by the end of February, the IRS recommends calling 800-829-1040 or visiting a Taxpayer Assistance Center. The IRS will send your employer a letter asking for a corrected W-2 within 10 days.7Internal Revenue Service. If You Don’t Get a W-2 or Your W-2 Is Wrong
If that still doesn’t produce a correct W-2, or the employer has gone out of business, you can file your federal return using Form 4852 as a substitute for the W-2.8Internal Revenue Service. What If My Employer Goes Out of Business or Into Bankruptcy? Form 4852 has a line (7f) for state income tax withheld and asks you to explain how you came up with the figures, usually from your final pay stubs.9IRS. Form 4852, Substitute for Form W-2, Wage and Tax Statement You attach it to your Form 1040. For your state return, check with the state tax agency; many accept Form 4852, but some require their own substitute form.
You can still file without a correct Box 15, but the return is more likely to be pulled for manual review. Include your employer’s full legal name and address so the state can try to match your return to the employer’s records, and expect any refund to take longer.