Looking up an employer identification number works or fails on one question: has this company ever had to put the number in a public document? If it has, the number is findable in minutes. If it has not, no amount of searching will produce it, because there is no index behind the search. This page sets out which record types carry the number and how to read them.
Securities filings carry it on the cover
Any company registered with the Securities and Exchange Commission prints its employer identification number on the cover page of its periodic filings, in the block with the state of incorporation and the principal executive offices. It is labelled as the IRS employer identification number. This is the most reliable public source that exists for the companies it covers, and it covers only companies with registered securities.
Exempt organisations are published by design
A tax-exempt organisation's employer identification number is public, because exempt status itself is public and donors need to identify the organisation they are giving to. The number appears in the organisation's annual information return and in the published lists of exempt organisations. If the entity you are checking is a charity, a foundation or a non-profit corporation, this route usually works.
Doing several at once, and where it stops
There is no bulk source that maps names to federal numbers across all US companies. What can be done at scale is the state layer: several states publish their entire business register as a downloadable dataset, so entity names, file numbers, statuses and agents can be matched in bulk. That gives you identification and standing for every entity in the state, without the federal number. It is the honest ceiling on this task.
Questions people ask about look up ein
Where is the most reliable public EIN?
The cover page of a Securities and Exchange Commission filing, for any company that files them. It is labelled as the IRS employer identification number.
Can I get EINs for many companies at once?
Not federally. Some states publish their whole business register as a dataset, which gives names, file numbers, statuses and agents in bulk, but not federal numbers.
Is a paid EIN lookup service worth it?
It can only resell what is already public. If the company has never disclosed its number, no service has it either.