# Folder Compare > Compare two folders in the browser: only in A, only in B, or contents differ. Size first, so a 4 GB video of unequal length is never hashed. Nothing uploaded. - URL: https://bytescope.dev/en/folder-compare/ - Locale: en · Also: [ja](https://bytescope.dev/ja/folder-compare.md) · [zh-Hant](https://bytescope.dev/zh-Hant/folder-compare.md) - Runs: Both folders are opened by the browser on your own machine. Comparison and hashing run here and are never sent anywhere — no upload, no account, no copy on a server. - Requires: Full mode needs Chrome, Edge or Opera on desktop (File System Access API) to read a folder and write results next to it. Firefox and Safari get a reduced mode: pick the folder through a file input and download the results. - Updated: 2026-09-08 ## What it does Pick folder A and folder B. This page walks both trees and tells you which paths exist on only one side, and which exist on both but are not the same bytes. Nothing is uploaded: the walk, the size check and the hashing all run in this tab. The comparison itself is `compareFolders` from a shared engine — this page does not keep a second copy of the rules. ### Size first, then hash Two files of different sizes cannot be the same, so a size mismatch is a verdict, not a hint. Only equal-size pairs are streamed through `SHA-256`. A 4 GB video that is 4 GB on A and 3 GB on B is never read. That is the difference between a backup check that finishes and one that hashes a disk image you already know is the wrong length. ### A tree, not a wall of paths The result is a directory tree. Each folder node shows how many descendants are only in A, only in B, different, or the same, so you can collapse the identical subtrees and open the ones that actually drifted. Tables under the tree list each bucket with a cap, because drawing 200,000 rows would freeze the tab. ### What this is not This is exact bytes, not a rename detector and not a duplicate finder. A file that moved is only-in-A plus only-in-B. Two copies of the same photo at different sizes will not match. For a checksum file against one folder, use Folder Hash. For byte-identical copies you want to delete, use Duplicate File Finder. Binary Diff can show where two files diverge, but it cannot receive these files automatically — open it and pick the A and B copies yourself. ## Limits Does not sync or merge folders and does not show text diffs of changed files; it reports added, removed and changed files. ## FAQ ### Are my folders uploaded to compare them? No. Both walks, the size checks and any `SHA-256` hashing run in your browser. Hashing streams each file in chunks through a local worker, and the bytes go nowhere. The page is a static file with no server behind it, so there is no upload path even in principle. ### Why doesn't it hash every file? Size is cheaper than a digest and already proves two files differ. Hashing is reserved for paths that exist on both sides with the same size. If you stop the run, remaining equal-size pairs stay listed as not yet hashed rather than guessed as matching. ### What does unreadable mean? The walk found the path but the bytes could not be sized or hashed (gone, locked, or changed under the scan). That is not the same as only in A or only in B — those mean the other folder has no such path. Unreadable files are listed on their own so a backup check cannot report a present file as missing. ### Can I jump into Binary Diff from a differing file? You can open Binary Diff from this page, but it has no way to receive the two files. There is no URL parameter and no hand-off. The link is a reminder to pick the A copy and the B copy of that path yourself on Binary Diff. ### Which browsers does it work in? Chromium desktop browsers (Chrome, Edge, Brave) can pick two folders with the File System Access API and remember them for next time. Firefox and Safari can still compare through the file input (`webkitdirectory`); they just cannot remember the folders. The support table on the page reports what this browser actually has. ## Related tools - [Folder Size](https://bytescope.dev/en/folder-size.md) — Pick a folder and see where the bytes went: a treemap, the largest files, and an extension breakdown. - [Folder Hash](https://bytescope.dev/en/folder-hash.md) — Walk one folder, write SHA256SUMS, or drop a checksum file and verify every path. - [Duplicate File Finder](https://bytescope.dev/en/duplicate-finder.md) — Find exact duplicate files in a folder by size, then SHA-256. - [Binary Diff](https://bytescope.dev/en/binary-diff.md) — Compare two files byte by byte. This page cannot hand the files over — pick them there.