How to file an extension with the IRS
You can choose whether to file for your IRS Form 4868 extension online or through the mail. If you file online, you can use 1040.com, an official provider of IRS e-file services for 1040 returns and 4868 extensions.
If you choose to file through the mail, fill out Form 4868 and send to the address shown on page 4 of the form.
Remember: Extension applications are due by April 17, 2018, and any taxes you owe are also due on that date. With an extension, your return will be due October 16, 2018. For those in military or living outside the U.S, special rules may apply.