What is the VMDK file format?
A .vmdk file is a VMware virtual machine disk file. It contains the files and data that comprise a virtual disk created using VMware software (now part of Broadcom). A single .vmdk can hold information for one or more virtual disks, or act as a plain-text descriptor that points to separate binary extent files.
.vmdk files are mainly used as a means of storing data and creating backup copies of important content, and are essentially the software equivalent of a physical disk drive. They also enable users to run multiple operating systems or virtual servers on a single machine.
Each virtual disk is a separate .vmdk file (or set of files) that can contain one or more extents, depending on the disk contents and size. .vmdk files can be configured for fixed (thick-provisioned, pre-allocated) or variable (thin-provisioned, sparse) image size.
Standard .vmdk sets are typically named starting from vmname.vmdk (alternative naming schemes are also common):
vmname-s###.vmdk- split-sparse extents configured to support file-size expansion as data is writtenvmname-f###.vmdk- split-flat extents with disk space fully allocated during the file creation processvmname-flat.vmdk- monolithic flat extent holding the raw sector image; used by ESXi-style deployments
Sparse hosted extents begin with the 4-byte ASCII magic KDMV at offset 0, while flat extents are raw sector images with no header signature. .vmdk images can be converted to other formats such as VHD or VDI using qemu-img. For portability, .vmdk files are frequently bundled inside an OVA package or paired with an OVF descriptor.
Security & safety
RISK: MEDIUMA .vmdk is a disk image, not directly executable on the host - but it usually contains a full bootable OS, so a downloaded VMDK can harbour malware inside the guest. Run untrusted VMs isolated (NAT/host-only network, no shared folders or clipboard) until you trust them. Mounting a VMDK (7-Zip or as a host drive) exposes its filesystem to the host - scan unknown images before opening files. VMDKs also contain whatever sensitive data the guest had (credentials, personal files), so treat them as confidential, and remember to keep the descriptor and its '-flat'/'-s00N' extents together or the disk is unusable.
Format details
in a nutshellPrograms that open VMDK files
Windows4 apps
7-Zip
Open-source Right-click > 7-Zip > Open archive to browse and extract files from inside the VMDK read-only, without mounting or booting. 
VMware Workstation Pro
Free Native VMware disk. Create/open a VM that uses the .vmdk and power it on; or VM Settings > Add disk to attach it. Free for personal use since 2024 (download via a free Broadcom account).
Oracle VirtualBox
Open-source Create a VM and attach the existing .vmdk as its hard disk (Storage > Add disk), then boot it. VirtualBox reads/writes VMDK natively.
qemu-img
Open-source Command line: 'qemu-img info disk.vmdk' to inspect, or convert to VHD/VDI/raw/qcow2 for other hypervisors. 
macOS3 apps
Parallels Desktop
Paid Import/attach a VMware .vmdk (Parallels can convert it to its own format on import). Paid product.
VMware Fusion Pro
Free Open or create a VM that uses the .vmdk and run it. Free for personal use since 2024.
Oracle VirtualBox
Open-source Attach the .vmdk to a VM and boot it. On Apple-silicon Macs you need the ARM build and an ARM guest. 
Linux3 apps
Oracle VirtualBox
Open-source Attach the .vmdk to a VM and boot it, or convert with 'VBoxManage clonemedium'.
qemu-img / libguestfs (guestmount)
Open-source Convert with qemu-img, or mount the guest filesystem to copy files out: 'guestmount -a disk.vmdk -i --ro /mnt'.
VMware Workstation Pro
Free Linux build runs VMs from .vmdk disks; free for personal use since 2024. Technical details
deep spec| Format type | Virtual hard-disk container holding an entire guest disk - partition table, filesystem, OS, and files |
| Layout variants | Monolithic (single file) or split (multiple 2 GB extents); chosen at creation time |
| Provisioning modes | Sparse/thin (grows on demand) or pre-allocated/thick (full space reserved at creation) |
| Descriptor file | Plain-text file beginning with the line "# Disk DescriptorFile"; lists disk geometry, type, and extent paths |
| Magic bytes (sparse extents) | 0x4B 0x44 0x4D 0x56 (ASCII "KDMV") at byte offset 0 in sparse hosted extents |
| Flat extent header | Raw sector images with no magic signature at offset 0; descriptor is external or embedded separately |
| Maximum virtual disk size | Up to 62 TB per virtual disk (VMware vSphere 6.x and later) |
| Sector size | 512-byte sectors by default; 4 KB native sector support added in ESXi 6.5+ |
| MIME type | application/x-vmdk |
| Sparse grain size | Default 128 sectors (64 KB) per grain; configurable in the descriptor file |
| Developer | VMware, Inc.; acquired by Broadcom in 2023 |
| Hypervisor compatibility | Native to VMware products; also supported by Oracle VirtualBox, QEMU/KVM, and Parallels Desktop |
| Conversion | qemu-img convert translates .vmdk to/from QCOW2, VHD, RAW, and other disk image formats |
| OVF/OVA integration | .vmdk is the standard disk format inside OVA/OVF virtual appliance packages per the DMTF OVF standard |
| Snapshot delta files | Incremental changes are tracked in separate vmname-######.vmdk redo-log (delta) files alongside the base disk |
| Filesystem independence | The .vmdk container is filesystem-agnostic; the guest OS formats the virtual disk independently of the host |
| Released | 2003 (VMware Workstation / GSX); VMDK spec published by VMware for the OVF effort |
| Open standard | Yes · royalty-free |
| Specification | www.dmtf.org |
VMDK conversions
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