What Is an EFI File?

A file with the EFI file extension is an Extensible Firmware Interface file.

EFI files are boot loader executables, exist on UEFI (Unified Extensible Firmware Interface) based computer systems, and contain data on how the boot process should proceed.

EFI files can be opened with EFI Developer Kit and Microsoft EFI Utilities but frankly, unless you're a hardware developer, there's little use in "opening" an EFI file.

On a system with an installed operating system, the boot manager that exists as part of the motherboard UEFI firmware will have an EFI file location stored in the BootOrder variable. This might actually be another boot manager if you have an installed multi-boot tool but is usually just the EFI boot loader for your operating system.

Most of the time, this EFI file is stored on a special EFI system partition. This partition is usually hidden and does not have a drive letter.

On a UEFI system with Windows 10 installed, for example, the EFI file will be located at the following location, on that hidden partition:

\EFI\boot\bootx64.efi

or

\EFI\boot\bootia32.efi

On some Windows computers, the winload.efi file acts as the boot loader and is usually stored in the following location:

C:\Windows\System32\Boot\winload.efi

On a system without an installed operating system, with a blank BootOrder variable, the motherboard's boot manager looks in predefined places for an EFI file, like on discs in optical drives and on other connected media. This occurs because, if that field is empty, you don't have a working OS installed and so you're likely going to install one next.

For example, on a Windows 10 installation DVD or ISO image, the following two files exist, which your computer's UEFI boot manager will quickly locate:

D:\efi\boot\bootx64.efi

and

D:\efi\boot\bootia32.efi

Note: Like with the Windows installation drive and path from above, the drive here will be different depending on the media source. In this case, D is the letter assigned to my optical drive. Additionally, as you may have noticed, both 64-bit and 32-bit EFI boot loaders are included on the installation media. This is because the install disc contains both architecture types as installation options.

Here are some of the default EFI file locations for some non-Windows operating systems:

macOS uses the following EFI file as its boot loader but not in all situations:

\System\Library\CoreServices\boot.efi

The EFI boot loader for Linux will differ depending on the distribution you have installed, but here are a few:

\EFI\SuSE\elilo.efi
\EFI\RedHat\elilo.efi
\EFI\ubuntu\elilo.efi

You get the idea.

Take note that there are some file types that are spelled very much like ".EFI" that you might actually have and can therefore open with a regular software program. This is most likely the case if you've simply misread the file extension.

For example, you might really have an EFX eFax Fax Document file that has nothing to do with Extensible Firmware Interface files and is instead a document that opens with a fax service. Or maybe your file uses the .EFL file extension and is an External Format Language file or an Encryptafile Encrypted file.

If you are sure that you can open the file you have, then it's most likely not in the same format that's described on this page. Instead, double-check the file extension for your file and research the program that can open it or convert it to a new format.

You might even try uploading it to a file converter service like Zamzar to see if it will recognize the file type and suggest a conversion format.