I searched internet but hard to find an easy way to get used space on Windows Server.

Following is two lines PowerShell command to get used space on  Windows 2012 R2 Server.

Get-WmiObject win32_logicaldisk | select deviceid,@{n="Size";e={[math]::Round(($_.size/1GB),2)}},@{n="Used Space";e={[math]::Round((($_.Size-$_.FreeSpace)/1GB),2)}}