Save a file in a shared folder on your laptop and Syncthing copies it to your connected desktop automatically. You don't need a cloud account or a subscription. Both machines have to be awake and able to reach each other at the same time. That's about it.
Syncthing is free and open source. Every machine gets a long identity code the first time it runs. Paste your laptop's code into your desktop and accept the device request on your laptop. Share a folder with the other device, then accept that folder there and choose where to save it.
From there the two machines talk over an encrypted connection, directly when the networks between them allow it. On the same network they can usually find each other by themselves. Across the internet, Syncthing's default discovery servers make the introduction. When a direct link is impossible, public relays pass the traffic along without being able to read it, and the transfer slows down.
Syncthing FAQ and what the relays do
What it looks like running
The screenshots show Syncthing 2.1.3, with devices named Laptop and Desktop. Each side has its own identity code. Pairing devices means adding one code to the other and accepting the connection request.
The control panel shows 11 files across three subfolders, about 1.4 MB. The shared folder and Desktop both read Up to Date. That's the state to look for after your devices finish syncing; the screenshot doesn't measure transfer speed.
Syncthing puts its controls in a web page it serves at localhost:8384 by default, and that page carries the settings that matter. Send only mode suits the machine holding the copy you trust, so a phone pushes photos out and ignores anything coming back. Receive only mode suits a mirror. Edits you make on the receiving device aren't sent elsewhere. They stay put unless the same file changes on another device, in which case that version wins.
Phones are the weak spot
Syncthing runs on Windows, macOS, Linux and BSD. Its official Android app has been retired, but community apps remain available. The maintainer of the Android app announced its retirement in October 2024, saying Google had made publishing on the Play store "something between hard and impossible" and that the code had gone unmaintained. Syncthing's community page lists Syncthing-Fork and BasicSync, and replies in the retirement thread pointed users to Syncthing-Fork.
Syncthing has no official iPhone app either, and outside apps fill the gap. Synctrain is free and open source, listed on Syncthing's community contributions page, and on the App Store. Möbius Sync is free up to 20 MB of files stored inside the app, and one purchase removes the limit.
iOS restricts background syncing. Möbius Sync describes short background sessions whose timing depends on iOS. Its FAQ says they may take a day to get going. These iOS apps remain subject to Apple's background limits, so don't expect the unrestricted syncing a desktop gives you.
discontinuing the Android app, community apps and Möbius Sync background limits
It isn't a backup
Sync means the machines agree, and that includes agreeing about deletion. With the default send-and-receive mode, deleting a file on one device also deletes it on the other after they sync. That's the whole point, right up to the moment you delete something by mistake. Enable file versioning on the devices receiving changes. It keeps older copies when another device edits or deletes a file. It doesn't preserve local changes on the device where you made them, so keep a real backup too.
You get a folder synced between devices you choose, without requiring cloud storage. If you use iCloud Backup, Möbius Sync currently includes its synced folders in that backup. Once paired, Syncthing handles transfers automatically while the devices are running and connected.
