Calculating the FileStream Data Columns Length matching used FS Drive used space
Space for each file on the file stream volume is allocated in units of the file system allocation unit (blocksize). When the actual data size is not an exact multiple of the allocation unit size, allocated space is rounded up to the next allocation unit size multiple.

The PowerShell command below will show allocation unit size for each volume on the server:

Get-WmiObject `
    -Query "SELECT Name, Label, Blocksize FROM Win32_Volume WHERE FileSystem='NTFS'" `
    | Select-Object Name, Label, Blocksize;


Using the returned blocksize value from the above command, you can determine the actual space allocated for each file stream file with a query like:

DECLARE @AllocationUnitSize decimal(10,0) = 65536;
SELECT
      CEILING(DATALENGTH(file_stream)/@AllocationUnitSize)*@AllocationUnitSize AS AllocatedSpaceBytes
    , DATALENGTH(file_stream) AS UsedSpaceBytes
FROM dbo.YourFileStreamTable;


The amount of unused allocated space will be significant for small files and a large allocation unit size. For example, consider that an actual file size of 1K will require 64K with that AU size, wasting 63K of storage per file.