What is EIN (Employer Identification Number)?

How it works

An EIN (Employer Identification Number) is the IRS's tax-only identifier for entities. Format: XX-XXXXXXX (nine digits, 9 in the middle distinguishes from SSN). Used to identify the entity on every federal tax filing, every payment-processor account, and every bank-account opening.

Required by:

• LLCs — single-member or multi-member, US or foreign owned.
• Corporations — C-corp, S-corp, even shell companies.
• Partnerships (LP, LLP, GP).
• Trusts and estates in many cases.
• Sole proprietors with employees or pension plans.

Three application channels:

For non-US founders without SSN/ITIN, the fax route is the only viable option — the online application requires a US-issued personal TIN to identify the responsible party. Fax SS-4 to IRS Cincinnati at +1 855-641-6935 (international applicants).

Responsible party

The Form SS-4 requires identification of the responsible party — the natural person who controls the entity (signatory, owner, managing member). Cannot be a service provider, registered agent, or another entity.

For non-US founders: identification by passport + foreign address. Responsibility person's tax ID line can be left blank or marked "foreign" if no SSN/ITIN exists.

Where the EIN is needed

• Banking: Mercury, Brex, Wise, Relay all require an EIN before opening business accounts.
• Payment processing: Stripe, PayPal Business, Square — won't onboard without one.
• Tax filings: every Form 1120, 1065, 5472, 1042 references the EIN.
• State registrations: most US states require the federal EIN before issuing state IDs (sales tax permits, unemployment IDs).
• Vendor onboarding: US clients fill in the EIN on W-9 (US-owned LLC) or in alternate fields on W-8BEN-E (foreign-owned).

EIN vs ITIN

The two are commonly confused:

A non-resident founder with a US LLC needs EIN to operate the LLC. They may also need an ITIN — separately — to file personal Form 1040-NR if the LLC has US-source ECI flowing to them.

Common mistakes

• Reusing your old SSN as the responsible party for multiple entities. The IRS limits each responsible party to one EIN application per business day. Founders forming multiple entities at once must space the SS-4 filings.
• Listing a registered agent or formation service as the responsible party. Wrong by the rules — the responsible party must be a natural person with control over the entity.
• Treating fax as immediate. It is not. Fax processing has been 4-12 weeks for non-resident applicants. Plan around it for time-sensitive bank-account openings.
• Forgetting the EIN is permanent. EINs don't expire, ever. When an entity dissolves, the IRS closes the EIN but never reissues the number.