Monitoring free space in SQL Server data files

I have manually resized mdf/ndf files to a big size to avoid autogrow operations on SQL Server databases.
Since the files are bigger there is very little free space on disk partitions and the sysadmins keep alerting me that I'm running out of space.

Because I resized them, there is a lot of free space in the data files but one can't notice it looking at file sizes/disk free space.

How can I monitor the real % usage of data files? I would prefer using perfmon counters. I am conerned that when the file really runs out of space SQL Server won't be able to allocate enough space and will crash.

Not sure why you want to use performance counters for this when you can get it from a simple query. And in fact while you can get this information about log files from performance counters (Log File(s) Size (KB) / Log File(s) Used Size (KB)), there is no such counter for how much space is used in a data file.

;WITH f AS 
(
  SELECT name, size = size/128.0 FROM sys.database_files
),
s AS
(
  SELECT name, size, free = size-CONVERT(INT,FILEPROPERTY(name,'SpaceUsed'))/128.0
  FROM f
)
SELECT name, size, free, percent_free = free * 100.0 / size
FROM s;

StackExchange Database Administrators Q#97412, answer score: 11