To add a NAICS code to your business, pick the six-digit code that matches your primary activity and enter it on your next federal tax return, and, if you sell to the government, in your SAM.gov profile. There is no separate application, no registration form, and no fee for the code itself. The code is a self-reported classification, and the “adding” happens inside filings you’re already doing.
Pick the Right Code First
Use the Census Bureau’s free search tool at census.gov/naics. Type in a keyword that describes what your business actually does, such as “residential construction” or “mobile app development,” and the tool returns matching codes from broad sectors down to specific six-digit national industries.1U.S. Census Bureau. North American Industry Classification System (NAICS) The 2022 NAICS is the current version.2Bureau of Labor Statistics. Industry Classification Overview
The numbering is hierarchical. The first two digits identify the broad sector (23 for Construction, 54 for Professional Services), and each additional digit narrows the classification down to a specific national industry.3United States Census Bureau. NAICS Codes and Understanding Industry Classification Systems If a keyword search isn’t producing good results, browse the two-digit sectors and drill down.
Your primary code should reflect the activity that generates the largest share of your annual revenue. The IRS instructions define principal business activity as the one producing the “highest percentage of total receipts.”4Internal Revenue Service. 2025 Instructions for Form 1065 U.S. Return of Partnership Income A landscaping company that also sells garden supplies uses the landscaping code, because that’s where most of the money comes from. Picking a code that doesn’t match your actual operations can make your deductions look out of line with peers in that industry and draw closer IRS review.
Add It to Your Tax Return
For most business owners, the tax return is where the code actually gets recorded. The exact box depends on how the business is organized.
- Sole proprietors enter the six-digit code on Line B of Schedule C (Form 1040). The instructions include a list of Principal Business Activity codes at the back.5Internal Revenue Service. 2025 Instructions for Schedule C (Form 1040)
- Partnerships enter the code in Item C on page 1 of Form 1065, labeled “Business code number.” Item A describes the business activity and Item B names the principal product or service.4Internal Revenue Service. 2025 Instructions for Form 1065 U.S. Return of Partnership Income
- S corporations enter the code in Item B on page 1 of Form 1120-S, with the corresponding business activity and product description on Schedule B, Item 2.6Internal Revenue Service. 2025 Instructions for Form 1120-S U.S. Income Tax Return for an S Corporation
- C corporations report the code in Schedule K of Form 1120, Question 2: the code on line 2a, the business activity on line 2b, and the principal product or service on line 2c.7Internal Revenue Service. 2025 Instructions for Form 1120 U.S. Corporation Income Tax Return
Each of these forms carries its own reference chart of Principal Business Activity codes at the end of the instructions, based on NAICS.5Internal Revenue Service. 2025 Instructions for Schedule C (Form 1040) If the code you used last year is still accurate, enter it again. If your primary activity has shifted, enter the new code on this year’s return. You don’t file a separate notice with the IRS to change your code, and there’s no mid-year update. The next annual return is the update.
Add It in SAM.gov if You Want Federal Contracts
If you plan to bid on federal contracts, you need an active entity registration in the System for Award Management, and that registration includes your NAICS codes.8SAM.gov. Entity Information SAM.gov issues a Unique Entity Identifier during registration, so you don’t need to get a separate identifier from a third party first.
NAICS codes go in the Assertions section of the profile, under Goods and Services. You select one code as primary and can add secondary codes for other lines of business. After submitting, validation can take several business days. SAM.gov registration has to be renewed annually, and renewal is a natural time to update the codes if the business has moved in a new direction.
When to List More Than One Code
The Census Bureau assigns only one NAICS code per establishment, based on primary activity. SAM.gov works differently: it lets you list multiple codes, and doing so broadens the range of federal opportunities you appear eligible for. Three or four codes that genuinely describe your capabilities is a common pattern for contractors.
Keep the primary and secondary distinction honest. Primary is always the largest revenue source. Secondary codes cover real additional services you can actually deliver. A civil engineering firm that also offers environmental consulting and surveying might legitimately carry three codes. Don’t add codes for work you can’t perform. Each NAICS code has its own SBA size standard, and misrepresenting eligibility under a code you claim can carry serious consequences, including potential criminal penalties.9U.S. Small Business Administration. Size Standards
Why the Code Choice Matters
The SBA sets a size standard for every NAICS code to determine whether a business counts as small for loan programs and set-aside contracting.10U.S. Small Business Administration. Table of Size Standards Thresholds vary widely by industry, so the code you claim controls which size standard applies to you.
The IRS uses your principal business activity code to benchmark your return against industry averages. A wrong code isn’t itself a penalty, but if it leads you to claim deductions that don’t fit the industry you reported, the IRS may assess a 20% accuracy-related penalty on any resulting underpayment.11Internal Revenue Service. Accuracy-Related Penalty Insurance carriers and commercial lenders also lean on industry classifications when they price coverage and evaluate credit, so an outdated code on those files can quietly cost money.
Keeping the Code Current
Business activities drift over time. When yours does, update the code the next time you file and, if you’re registered, the next time you renew SAM.gov. The classification system itself also changes. NAICS is revised every five years, and the next update is already in motion. The Office of Management and Budget is expected to publish final decisions on changes by March 2026, with the updated 2027 NAICS manual submitted by June 2026 and the full revised structure available on the Census Bureau website by January 2027.12U.S. Census Bureau. NAICS Update Process Fact Sheet When the 2027 codes go live, check whether your current code has been split, merged, or reclassified. Federal agencies and the IRS typically update their Principal Business Activity lists within a filing season or two after a revision takes effect.
State Filings Usually Don’t Use NAICS
NAICS is a federal classification system. State corporate filings, such as articles of incorporation, annual reports, or statements of information, generally ask for a plain-English description of your business purpose rather than a NAICS code. If a state form has a business-activity field, it’s usually free text. You don’t ordinarily need to amend state filings because your NAICS code changed. A small number of state licensing or regulatory programs do reference NAICS for specific industries, but most small business owners won’t run into that. Keep the focus on the federal tax return and, if it applies, the SAM.gov profile.