In relation to the IRS and businesses, a “45 number” most commonly refers to an Employer Identification Number (EIN) that begins with the prefix 45.
An EIN (also called a Federal Employer Identification Number or FEIN) is a unique nine-digit number (formatted as XX-XXXXXXX) assigned by the IRS to businesses, trusts, estates, and certain other entities for tax purposes. It’s similar to a Social Security Number but for businesses, used for filing taxes, hiring employees, opening bank accounts, and other official business activities.
The first two digits of an EIN are a prefix indicating where or how it was assigned (e.g., which IRS campus processed it or if it was applied for online). The prefix 45 is one of the valid prefixes specifically assigned for EINs requested online through the IRS website (along with others like 20, 26, 27, 46, 47, etc.).
For example:
- An EIN like 45-1234567 would be a “45 number” — valid and commonly issued for internet/online applications.
People sometimes casually refer to their EIN as a “45 number” (or similar prefix-based term) when discussing applying for one via Form SS-4 (Application for Employer Identification Number), especially in online forums or groups where applicants mention receiving a “45” prefix after faxing or applying electronically.
Other less likely interpretations in tax/business contexts include:
- Line 45 on various IRS forms (e.g., calculations or counts like number of employees, business components, or days in penalty computations), but that’s not typically called “a 45 number.”
- References to Section 45 of the Internal Revenue Code (related to the renewable electricity production tax credit), but again, not phrased as “a 45 number.”