How to know the available free space in sql server?

select name,filename, convert(decimal(12,2),round(a.size/128.000,2)) as FileSizeMB, convert(decimal(12,2),round(fileproperty(a.name,'SpaceUsed')/128.000,2)) as SpaceUsedMB,convert(decimal(12,2),round((a.size-fileproperty(a.name,'SpaceUsed'))/128.000,2)) as FreeSpaceMB from dbo.sysfiles a