Find Out Why a Drive Shows Less Space Than Advertised

**Get-Disk** reports the decimal capacity the vendor sells; **Get-Volume** reports the binary size Explorer shows. The ratio is the familiar 0.93.

Get-Disk | Select-Object Number,FriendlyName,Size,@{n='GB_dec';e={[math]::Round($_.Size/1GB,2)}},@{n='GB_bin';e={[math]::Round($_.Size/1GB,2)}}
Get-Volume | Select-Object DriveLetter,FileSystem,Size,SizeRemaining
Get-CimInstance Win32_DiskDrive | Select-Object Model,Size,Partitions

EFI, MSR and recovery partitions consume space that never appears as a letter. Their sizes explain the difference on a fresh install.

Get-Partition | Select-Object DiskNumber,PartitionNumber,DriveLetter,Size,Type,GptType | Sort-Object DiskNumber,PartitionNumber
Get-Partition | Group-Object DiskNumber | ForEach-Object { [pscustomobject]@{ Disk=$_.Name; TotalGB=[math]::Round(($_.Group | Measure-Object Size -Sum).Sum/1GB,2) } }
Get-Disk | Select-Object Number,@{n='TotalGB';e={[math]::Round($_.Size/1GB,2)}}

NTFS reserves space for the master file table, the USN journal and metadata. On a volume with millions of files that reservation is visible in the numbers.

fsutil volume diskfree C:
fsutil fsinfo ntfsinfo C:
Get-ItemProperty 'HKLM:\SYSTEM\CurrentControlSet\Control\FileSystem' | Select-Object NtfsDisableLastAccessUpdate,NtfsMemoryUsage

Three hidden files claim space before Explorer reports free bytes: hiberfil.sys, pagefile.sys and the shadow copy storage area.

vssadmin list shadowstorage
Get-Item C:\hiberfil.sys,C:\pagefile.sys -Force -ErrorAction SilentlyContinue | Select-Object Name,@{n='GB';e={[math]::Round($_.Length/1GB,2)}}
Get-CimInstance Win32_PageFileUsage | Select-Object Name,AllocatedBaseSize,CurrentUsage

Windows keeps a few gigabytes for updates and temporary files. **DISM /Online /Get-ReservedStorageState** reports the state and the size.

DISM /Online /Get-ReservedStorageState
Get-WindowsReservedStorageState -ErrorAction SilentlyContinue
Get-Volume -DriveLetter C | Select-Object Size,SizeRemaining,@{n='UsedGB';e={[math]::Round(($_.Size-$_.SizeRemaining)/1GB,2)}}

A short table of vendor size, partitions, hidden files and free space ends the confusion permanently and is useful in a support ticket.

Get-Disk | Select-Object Number,FriendlyName,@{n='Vendor_GB';e={[math]::Round($_.Size/1e9,0)}},@{n='Windows_GB';e={[math]::Round($_.Size/1GB,2)}}
Get-Partition | Where-Object DriveLetter | Select-Object DriveLetter,@{n='SizeGB';e={[math]::Round($_.Size/1GB,2)}}
fsutil volume diskfree C: