Employer identification numbers, also called tax identification numbers, are nine-digit numbers the Internal Revenue Service uses to identify businesses for federal tax purposes. Just about any business, other than a single-person sole proprietorship, needs a tax ID number. You may be able to look up a business's tax ID number, but it depends on whether the company is required or chooses to make its number public.

Public Companies and Nonprofits

Finding the tax ID number of a public company is relatively easy if you have the right tools at your disposal. Publicly traded companies include their tax ID number in many public filings with the Securities and Exchange Commission (SEC), including forms 10-K and 20-F, on a database is referred to as the EDGAR Search Database on the SEC site. Many nonprofit companies include their tax ID number on IRS Form 990, which is publicly available. You can request the form directly from the nonprofit or use a service that compiles such information.

The IRS also maintains a database specifically referred to as the Exempt Organization Select Check. Every registered tax-exempt entity must file proper documentation with the IRS. The database provides the TIN as well as current tax-exempt status.

Private Companies

Finding the tax ID number of a private company is more difficult, and may not be possible at all, because private companies are not required to disclose their tax ID numbers. Some private companies may include their tax ID numbers on their annual reports, and if they make those reports public, such as by posting them on the company website, that's one way to find the tax ID number.

You also might be able to find a business's tax ID number in public documents, such as a business filing with the secretary of state's office, bankruptcy filings or lawsuits. One other place to look, according to the Small Business Administration, is business and public records databases. Employees have a record of the tax ID number on payroll documents. Paystubs and W2 forms have this information often referred to as an Employer Identification Number (EIN).

Reverse Look-up

If you have a business's tax ID number but don't know the name of the business, and plugging the number into an Internet search engine doesn't get you the business's name, you may be able find out by using a reverse look-up service such as Search Bug. Keep in mind, though, that such services usually charge a fee for each search.