FreeFileSync.online is a free online file sync tool for comparing two local folders and applying a controlled synchronization plan from your browser. Select Folder A and Folder B, scan both directory trees, review what is missing or different, then mirror, update, or customize the planned actions. Supported scanning and file operations are designed to happen locally through browser file system access, without uploading selected file contents to the website server or installing desktop software.
Use it to compare a working folder with a backup, inspect an external drive before copying changes, or identify differences between folder versions. The guiding workflow is simple: preview first, approve second, modify files last.
Free Online File Sync Without Installing Software
FreeFileSync.online is designed for users who want to sync two folders without installing software through a supported browser based workflow.
You choose the folders yourself. After browser permission is granted, the tool can scan exposed entries, compare relative paths and file information, and build a synchronization preview. This is useful for occasional jobs such as comparing a Projects/ folder with Projects Backup/ and focusing only on new, missing, or different files.
Browser based access has limits. Folder selection and writes depend on browser support, the operating system, HTTPS, and user approval. The tool is on demand and does not continue synchronizing after the page or browser closes.
Compare Two Folders Online Before Synchronizing Files
A safe way to compare two folders and copy only changed files starts with comparison, not copying.
When Folder A and Folder B are scanned, the tool can organize results into useful categories. That gives you a chance to understand what a planned synchronization will do before it writes, replaces, or removes anything.
Only in Folder A
These files or folders exist in Folder A but do not have a corresponding item at the same relative path in Folder B. In an update or mirror workflow, they are common copy candidates from A to B.
Only in Folder B
These items exist only in Folder B. Their treatment depends strongly on the synchronization mode.
In Update Destination, destination only items can remain untouched.
In Mirror A to B, destination only items may be scheduled for removal because the purpose of a mirror is to make Folder B reflect Folder A.
That distinction is important enough to check every time you use mirror mode.
Changed and Different Files
A matching relative path does not prove that files are identical. One side may appear newer, sizes may differ, or exact content checks may find different data. Metadata provides a fast signal, while content comparison reads the files more deeply.
Identical Files
Files that satisfy the selected equality test can be shown as identical and normally require no copy action. Skipping unchanged data is one of the main reasons to compare folders before copying a backup again.
Choose the Right Folder Comparison Method
FreeFileSync.online is planned around three comparison choices so users can balance speed and certainty.
Smart Compare
Smart Compare uses relative paths, file sizes, modification information, and exact file comparison when a deeper check is needed. It is intended as the general purpose option when you want useful results without reading every byte of every matching file.
Fast Compare
Fast Compare relies on information such as relative path, filename, size, and modification data. It is appropriate when metadata is trustworthy and you want to identify likely changes quickly.
Metadata comparison is not the same as proving that two files contain identical bytes. Two files can theoretically share the same size while containing different data, and timestamps can be unreliable after some copy or export workflows.
Exact Content Compare
Exact Content Compare reads corresponding file contents to determine whether they are actually the same. This is the strongest comparison option when the question is file content rather than metadata.
It also requires more reading. For large folders, exact comparison can take substantially longer because the browser must read the relevant file data instead of checking only names, sizes, and dates.
This distinction matches a long established synchronization principle documented by the official FreeFileSync project: its desktop software also distinguishes metadata based comparison from content comparison and notes that full content comparison is slower. See the official FreeFileSync comparison documentation.
Mirror One Folder to Another Online
Mirror A to B is intended for cases where Folder A is the source of truth and Folder B should become a matching copy.
Suppose Folder A contains:
Work/Client A/Proposal.docx
Work/Client B/Assets/logo.svg
and Folder B contains those files plus an old file:
Work/Client B/Assets/old-logo.svg
A mirror plan can copy new or changed source files and can also schedule the destination only old-logo.svg for removal. That is what makes mirror different from a simple copy.
Use mirror when you genuinely want the destination to represent the current source state. Review the preview carefully, especially the removal list. Keep an independent backup of irreplaceable data before running a large destructive synchronization.
The official FreeFileSync desktop documentation uses the same core mirror concept: the target is changed until it becomes an exact copy of the source. FreeFileSync.online is independent software and does not claim the desktop product's deeper operating system features.
Update a Destination Folder Without Removing Existing Files
Update Destination is the more conservative choice when you want to copy missing or changed files from Folder A into Folder B while preserving unrelated files already present in Folder B.
Consider a photo workflow. Folder A contains this year's edited images, while Folder B is a long term archive that also contains older years. You may want to copy new material into the archive without deleting files that no longer exist in the current source folder.
Update Destination is designed for that situation. It can add or replace files according to the comparison result without treating every destination only file as something that must be removed.
If you are unsure whether you need Mirror or Update, start by asking one question: Should a file that exists only in Folder B remain there? If the answer is yes, Update Destination is usually the safer starting point.
Create a Custom Folder Synchronization Plan
Not every folder pair fits a standard mirror or update pattern.
Custom Sync is intended to let you decide what should happen for comparison categories such as:
- Files only in Folder A
- Files only in Folder B
- Files where A appears newer
- Files where B appears newer
- Files with different contents
A custom rule can be useful when you are consolidating work from two locations but still want one direction to be treated as primary. It can also help when you want to skip a category rather than copying it automatically.
Custom Sync is not presented as full history based Two Way synchronization. The planned browser tool does not claim the persistent synchronization database and historical change tracking used by some desktop sync applications.
Sync Only the Files You Need
FreeFileSync.online is planned to support selective synchronization plus include and exclude rules so temporary files, caches, generated assets, or unrelated media can stay outside a job. Filters may use file extension and other exposed attributes such as size or date.
A project backup might include source code while excluding generated output. A photo copy might include common image formats while skipping previews. Review filtered results because an overly broad exclusion can hide files you expected to process.
Verify Files After Synchronization
Copy completion and content verification answer two different questions.
A successful copy operation tells you that the requested file write completed without an immediate reported error. Verification goes further by reading source and destination data and comparing the results.
Exact post copy verification can increase confidence that the copied bytes match the source, but it requires extra reads and therefore adds work. The official FreeFileSync desktop documentation similarly notes that binary verification after copying can significantly increase total copy time and cannot prove that the original data was never corrupted before the copy.
For valuable archives, verification is useful, but it should be combined with sensible backups rather than treated as a complete data protection strategy.
Private Local Folder Synchronization
FreeFileSync.online is designed around local browser processing for supported operations. When you grant folder access, the browser exposes handles that web applications can use to enumerate files and, with permission, write changes.
MDN documents the File System API and directory handles used for this class of workflow. The browser still loads normal website resources such as HTML, CSS, JavaScript, and fonts, but your selected file contents do not need to be transferred to the site server simply to compare or synchronize them.
Browser support is not identical everywhere. The showDirectoryPicker() capability requires a secure context and is still classified by MDN as having limited availability across widely used browsers. See MDN showDirectoryPicker documentation and the broader MDN File System API reference.
FreeFileSync Alternative Online
FreeFileSync.online is an independent browser based alternative for supported local folder comparison and synchronization workflows. It is not the FreeFileSync desktop application, is not made by that project's developer, and is not an official FreeFileSync web edition.
Desktop synchronization software can provide deeper operating system integration. This browser tool instead focuses on user initiated local folder jobs without installation. If you need an always running service, scheduled tasks, SFTP, or other desktop level integration, use software designed for those requirements.
How to Sync Two Folders Online
- Select Folder A.
- Select Folder B.
- Choose Smart Compare, Fast Compare, or Exact Content Compare.
- Run the comparison.
- Review files shown as only in A, only in B, newer, different, identical, or excluded.
- Choose Mirror A to B, Update Destination, or Custom Sync.
- Preview the planned file operations.
- Confirm the synchronization.
- Review progress, errors, verification results, and the final activity report.
Do not skip the preview when using Mirror or any custom rule that can remove or replace files.
Common Folder Sync Use Cases
Common uses include updating an external drive, maintaining a secondary project copy, comparing photo archives, checking a USB drive against a primary folder, updating document archives, and inspecting old versus new folder versions before choosing which side should supply changed files.
If your goal is to combine several source folders into one destination rather than maintain two folder states, use the Free Online Folder Merger instead.