An Employer Identification Number works like a Social Security number for a business — a unique nine-digit identifier the IRS uses to track a business's tax filings — and getting one costs nothing if you go through the right channel. * The fastest method by far is applying directly on irs.gov. The online application walks you through a series of questions about your business, and if everything checks out, it issues your EIN immediately at the end of the session — no waiting days for a confirmation email. * There are a couple of requirements to use the online tool: your principal business has to be located in the U.S. or a U.S. territory, and the person applying — the "responsible party" — needs a valid Social Security number or Individual Taxpayer Identification Number of their own. The online session also has a hard time limit: it expires after 15 minutes of inactivity, and you can't save a partially completed application to finish later, so it helps to have your business details ready before you start. * If you can't use the online tool, the fallback is Form SS-4, submitted either by fax or mail. Faxing it in, with a return fax number included, typically gets you an EIN back in about 4 business days. Mailing the form is the slowest option, generally taking around 4 weeks to process. * One warning worth repeating clearly: the IRS never charges a fee for an EIN under any circumstance. Websites that charge money to "help" you get an EIN are charging for a service you can do yourself, free, directly on irs.gov.