What Is the Difference Between NAICS and SIC Codes?

The difference between NAICS and SIC codes comes down to age, structure, and logic: NAICS is the current six-digit classification system federal agencies adopted in 1997 and groups businesses by how they produce goods and services, while SIC is the older four-digit system, last revised in 1987, that groups businesses by what they produce. NAICS is what the IRS, the SBA, and federal contracting officers require today. SIC still shows up in a few specific places, most notably SEC filings and insurance underwriting, so both systems can appear on your desk in the same week.

The Two Systems at a Glance

SIC codes are four digits. The system sorts the economy into 10 broad divisions labeled A through J, which break into major groups, industry groups, and individual industries. The first two digits identify the major group; the last two narrow it down. It was built for the mid-twentieth-century economy, where most activity fit into clean manufacturing or agricultural buckets.

NAICS codes are six digits and sit inside a deeper hierarchy. The Census Bureau organizes them into five levels: sector (two digits), subsector (three), industry group (four), NAICS industry (five), and national industry (six).¹ The first five digits are consistent across the United States, Canada, and Mexico. The sixth digit lets each country capture distinctions specific to its own economy.

NAICS also expanded coverage substantially. SIC had 10 divisions; NAICS has 20 sectors, with dedicated sectors for information technology, professional and scientific services, waste management, and accommodation and food services — industries that either didn’t exist or didn’t matter when SIC was drafted.²

The Office of Management and Budget oversees NAICS, and the Census Bureau’s Economic Classification Policy Committee handles technical review. The three North American countries collaborate on updates to keep cross-border data comparable, and the system is reviewed every five years.³⁴ SIC, meanwhile, has not been updated since 1987. No agency maintains or revises it.

Products Versus Processes

The more meaningful difference isn’t the digit count. It’s what the codes actually group together.

SIC grouped companies by their end product. Two companies selling similar goods landed in the same category regardless of how those goods were made. A factory printing newspapers and a factory printing books would sit near each other because both produced printed material, even though their equipment, workforce, and business models looked nothing alike.

NAICS reversed that logic. It groups businesses by how they operate: the equipment they use, the labor they employ, and the raw materials they consume.⁵ Companies that do similar things in similar ways end up in the same industry, even if their final products differ. A custom software firm and a mobile app developer can share a code if their development processes look alike, rather than being split by what the finished product does.

That change matters because it gives a clearer picture of where labor, capital, and technology actually sit. It also produces much more granular categories. An environmental consulting firm gets its own specific code rather than sharing a broad bucket with unrelated professional services, which is useful whenever you need to benchmark your business against genuine peers.

Which System You Actually Need

For nearly every current federal purpose, the answer is NAICS. SIC survives in a few specific corners.

Taxes

The IRS requires a six-digit NAICS code on every business tax return. Sole proprietors enter it on Schedule C, Line B; corporations enter it on Form 1120, Schedule K.⁶⁷ The IRS uses the code to compare your reported income and deductions against others in the same industry. A wrong code won’t trigger penalties by itself, but if your restaurant’s numbers look like a law firm’s, the mismatch can pull your return into closer review.

Federal Contracting

Every federal solicitation carries a NAICS code chosen by the contracting officer to reflect the principal purpose of the purchase. That code determines which SBA size standard applies, which in turn decides whether your company counts as small for that particular contract.⁸ If you think the wrong code was assigned, you can challenge it, but the window is short: generally five business days after the solicitation is posted.

SBA Size Standards

The Small Business Administration ties its definition of small business directly to NAICS codes. Each code carries its own threshold, expressed as either a maximum employee count or maximum annual revenue, and the thresholds vary enormously by industry.⁸ The code you use determines eligibility for SBA loans, set-aside contracts, and other federal small business programs.

SEC Filings

The Securities and Exchange Commission still uses SIC codes to classify publicly traded companies in EDGAR.⁹ Each company’s four-digit SIC code appears at the top of its EDGAR filing history, and the SEC uses it internally to route filings to the appropriate review office. If you file with the SEC or research companies that do, you’ll see SIC codes constantly.

Insurance

Insurance carriers also lean on SIC codes for risk classification and premium setting, especially in workers’ compensation and general liability. Insurers have decades of actuarial data organized by SIC code, and many carriers still ask for it during underwriting even though federal agencies have moved on.

Finding Your Code and Converting Between Systems

The Census Bureau maintains a free keyword search tool at census.gov/naics.³ Enter a keyword or partial code and it returns matching industries with their six-digit codes. The IRS also publishes a list of principal business activity codes based on NAICS in the instructions for Schedule C and Form 1120, so you can pull your code directly from the tax form instructions.⁶ For SIC, the SEC maintains a searchable list, and OSHA publishes the full SIC Manual.

If you need both codes, start with NAICS and map backward. The Census Bureau publishes crosswalk tables that translate between the systems.¹⁰ The conversion isn’t always one-to-one. A single SIC code sometimes splits across several NAICS codes, and several SIC codes sometimes collapse into one, so historical comparisons take some manual work.

What Happens if You Pick the Wrong Code

On the tax side, the wrong NAICS code mostly means your financials get compared to the wrong peer group, which can attract audit attention. On the contracting side, the stakes are higher.

If a business claims small business status under a NAICS code whose size standard it doesn’t actually meet, federal regulations treat that as misrepresentation. Consequences include:

Suspension or debarment from future federal work by the SBA or the contracting agency.
Civil liability under the False Claims Act and the Program Fraud Civil Remedies Act, which can carry fines beyond the value of the contract itself.
Criminal penalties under the Small Business Act and federal fraud statutes if the misrepresentation was knowing.

Federal regulations create a presumption that the government suffered a loss equal to the total contract value whenever a non-small business willfully misrepresented its size to win a set-aside.⁸ Competitors can file size protests within five business days of learning who won, triggering an SBA investigation the agency aims to resolve within 15 business days.¹¹ Even an honest miscode can disqualify a bid after the fact if a protest shows the company exceeds the size standard for the assigned code.

The safest approach is to pick your NAICS code based on your primary business activity, meaning the one that generates the most revenue, rather than shopping for whichever code offers the friendliest size standard. If your business spans multiple industries, both the IRS and SBA tell you to use the code that best describes your principal activity.

1
United States Census Bureau. Economic Census: NAICS Codes and Understanding Industry Classification Systems
2
U.S. Bureau of Labor Statistics. Industry Classification Overview
3
U.S. Census Bureau. North American Industry Classification System (NAICS)
4
U.S. Census Bureau. NAICS Update Process Fact Sheet
5
U.S. Bureau of Labor Statistics. North American Industry Classification System (NAICS)
6
Internal Revenue Service. Instructions for Schedule C (Form 1040)
7
Internal Revenue Service. 2025 Instructions for Form 1120
8
eCFR. 13 CFR Part 121 – Small Business Size Regulations
9
U.S. Securities and Exchange Commission. Standard Industrial Classification (SIC) Code List
10
United States Census Bureau. Industry and Occupation Code Lists and Crosswalks
11
eCFR. Procedures for Size Protests and Requests for Formal Size Determinations