What is the IMG file format?
Files with the .img extension are raw disc images - sector-by-sector binary copies of a disk or disc in which the structure of data is not changed. The resulting file is an exact duplicate of the source medium and can be mounted in a virtual drive and read as the original disc, or written back to physical media to reproduce an identical copy. An .img file may represent a floppy diskette image, an external hard drive or USB image, or a CD/DVD image.
Although .img has long been associated with Mac OS - it was the standard disc image format in Mac OS 9 and its older versions - it is in fact a generic, cross-platform format with no single owner. With the arrival of Mac OS X, .img files were largely superseded by the .DMG format, though .img files remain supported in macOS to this day. On Windows 10 and later, File Explorer can mount .img files directly without third-party software.
Unlike some optical formats, .img carries no universal file signature. An optical-disc .img is often structurally identical to an .ISO image and may expose the CD001 identifier at byte offset 32,769; a raw FAT floppy or USB .img typically shows the 55 AA boot-sector marker at offset 510. Identification depends on the embedded filesystem's own signatures rather than a fixed header.
.img can be converted to other image formats using dedicated software. In many cases a raw optical .img is completely interchangeable with .iso and users can simply rename the file - however, not every program will correctly interpret a manually renamed file, so conversion using dedicated software is the safer approach.
Security & safety
RISK: MEDIUMThe .img format is passive data, but disk images are a known malware-delivery method: like ISOs, a mounted image can present an autorun-style drive and hide an executable that sidesteps some 'mark of the web' download warnings. Download OS/recovery images only from the official vendor and verify the published SHA-256 checksum. When FLASHING an .img with dd/Rufus/Etcher, the target drive is overwritten entirely - pick the right device or you can wipe the wrong disk.
Format details
in a nutshell- Android system/partition image - Firmware partitions such as system.img or boot.img, flashed to a device with fastboot.
- Garmin map image - GPS map data (e.g. gmapsupp.img) loaded onto Garmin devices or via BaseCamp.
- GEM raster bitmap - A legacy planar picture format from Digital Research's GEM environment (Atari ST, old DTP).
Programs that open IMG files
Windows4 apps
7-Zip
Open-source Right-click > 7-Zip > Open archive to browse/extract files from a disk-image .img without mounting. 
Windows File Explorer (built-in mount)
Built-in Right-click the .img > Mount (Windows 8+). It appears as a virtual drive. Works for ISO-style images; some raw/odd images may need a third-party mounter.
Rufus
Open-source Write (flash) an OS .img to a USB stick to make a bootable drive: pick the USB, select the .img, Start.
balenaEtcher
Open-source Flash an .img to a USB stick or SD card (the standard tool for Raspberry Pi and Linux images). 
macOS2 apps
Finder / DiskImageMounter (built-in)
Built-in Double-click an ISO-style .img to mount it; or use Disk Utility. (Apple Disk Utility - keep from db.) 
Linux2 apps
GNOME Disks / loop mount (built-in)
Built-in Right-click > Open With Disk Image Mounter, or 'sudo mount -o loop file.img /mnt'. Write with 'dd if=file.img of=/dev/sdX bs=4M' (carefully). Technical details
deep spec| File structure | Raw binary sector-by-sector copy; no proprietary container or header wrapper |
| Magic bytes / signature | No universal header. Optical-disc images expose `CD001` at offset 32,769 (ISO 9660); FAT floppy/USB images expose `55 AA` at offset 510 in the boot sector |
| MIME type | `application/octet-stream` |
| Compression | None - uncompressed raw data; every byte maps directly to a sector byte on the source medium |
| Sector size | 512 bytes for HDD, floppy, and USB images; 2,048 bytes for optical-disc images |
| Bootable media support | Yes - boot sectors and partition tables are copied verbatim, preserving bootability |
| Filesystem independence | Container-agnostic; the inner filesystem may be FAT12/16/32, NTFS, ext2/3/4, ISO 9660, UDF, HFS+, or any other |
| Native OS mount support | macOS (DiskImageMounter / Finder), Windows 10 and later (File Explorer), Linux (loop device via `losetup` or `mount -o loop`) |
| Relation to .ISO | A raw optical `.img` is often byte-for-byte identical to an `.iso`; many tools and operating systems treat the two extensions as interchangeable |
| Write-back / flashing | Can be written directly to physical media with tools such as `dd` (Linux/macOS), Rufus, or balenaEtcher |
| Maximum image size | Determined by source medium and host filesystem; NTFS and ext4 support multi-terabyte images, FAT32 caps at ~4 GB |
| Error correction (ECC) | Not included in raw `.img`; subchannel and ECC data from optical discs are not captured unless a specialised raw-rip format is used |
| Historical floppy use | Standard container for 1.44 MB 3.5-inch floppy images (FAT12, 80 tracks × 18 sectors × 512 bytes) |
| Mac OS legacy | Was the default disc image format on Mac OS 9 and earlier; superseded by `.dmg` with the introduction of Mac OS X |
| Released | Raw disk images predate standardization (used since the floppy/optical era) |
| Open standard | Yes · royalty-free |
| Specification | en.wikipedia.org |
IMG conversions
Community Q&A
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