How do I compare two folders to see which files are different?

Start by treating the folders as two separate inventories, which we’ll call Folder A and Folder B. Include subfolders on both sides, and compare relative paths—not just file names—because two files with the same name can be in different subfolders.

Make three lists: paths found only in A, paths found only in B, and paths present in both. The first two lists identify entries found on only one side; they do not determine which folder has the version you want. If you use a comparison utility, check that its settings include subdirectories. Microsoft describes WinDiff as a graphical option for comparing folders of ASCII text files, with an option to include subdirectories. That description does not establish that it is available by default or that it compares arbitrary file types. [S1]

Record size and modified time as metadata, not as a content check. A difference in either is a reason to review the file, but matching metadata does not establish that the contents match. Likewise, a different modified time alone does not tell you what changed in the file.

Keep the comparison columns separate: relative path, size, modified time, and content result. Mark size and time as metadata. Mark a content result only when you have compared the contents. That way, the report distinguishes what you observed from what you verified.

For a selected pair of files, Windows’ comp command compares contents byte by byte and reports file locations and names. It can check matched files, but it does not create a complete folder inventory or explain metadata-only differences. Microsoft lists it for Windows 10 and Windows 11. Review the targets and output before taking action. [S2]

Another option is PowerShell’s Get-FileHash: calculate a hash for each selected file and compare the results. Microsoft says identical hashes indicate identical contents; the default algorithm is SHA256. A hash does not compare folder trees, names, or timestamps. The cited documentation is for PowerShell 7.5, so check the help for the version you are using rather than assuming every version behaves identically. [S3]

With either method, select corresponding files deliberately. A content match does not tell you whether either file is stored at the intended path.

Yes. Work from copies or read-only views if practical, and do not merge, overwrite, or delete files just because a comparison labels them different. Treat a difference as something to review, not as an instruction about which version to keep.

Use this copyable report format:

Path | Folder A | Folder B | Size/time notes | Content check | Decision [relative path] | present/missing | present/missing | [observations] | not checked/same/different | [review later/keep A/keep B/resolve]

Example with hypothetical values: [reports/summary.txt] | present | present | same size; different modified time | different contents | review which version is intended. This records an example result; it does not determine which file is authoritative.

For many files, compare inventories first, then verify the contents of selected matched files. Record the method used, such as byte comparison or hashes, so a reviewer can distinguish a content check from metadata notes.