Syncthing is a peer-to-peer sync tool that keeps folders in sync in real time by connecting your own devices directly, without going through the cloud. There is no central server: authenticated devices communicate with each other directly, propagating updates efficiently through block-level delta transfer. Because your data is never stored anywhere other than your own devices, it is well suited to syncing files you would rather not entrust to the cloud. First released in 2013, it is an open-source project (MPL-2.0) led by the Sweden-based Syncthing Foundation. The major 2.0 series was released in 2025, and development remains active today.
Key Features
- Cloud-free P2P sync: No data is stored on a central server — your devices talk to each other directly. Files never leave your control, making it a good fit when privacy is the top priority
- Block-level delta transfer: Files are split into blocks and only the changed parts are transferred. Even when you edit a small part of a large file, the transfer volume stays minimal
- TLS encryption and certificate authentication: Communication between devices is encrypted with TLS, and devices mutually authenticate with cryptographic certificates. Even when traffic passes through relay servers, the content is never decrypted
- Browser-based admin interface: Sync folders and connected devices are managed through a web GUI. In principle there is no need to specify IP addresses or fiddle with network settings — exchanging device IDs is enough to connect
- Operable via REST API / CLI: In addition to the web UI, a REST API and CLI are available, making it easy to script automation or integrate into server operations
- Broad platform support: Runs on Mac / Windows / Linux as well as FreeBSD and others. It is also commonly deployed on NAS devices (Synology / QNAP, etc.)
Pricing
| Plan | Price | Highlights |
|---|---|---|
| Free (open source) | $0 | All features available. No limits on devices, folders, or storage |
Syncthing is open-source software and every feature is free to use. Commercial support for enterprises is offered separately by the affiliated company Kastelo. Pricing information is as of August 2026. Please check the official website for the latest details.
Pros & Cons
✅ Pros
- Data is never stored in the cloud, keeping privacy and data sovereignty entirely in your own hands
- Completely free with no storage limits — no monthly cloud storage fees
- Delta transfer and direct LAN communication make syncing fast on the same network
- Open source with a documented protocol, offering a high level of transparency
- File versioning lets you recover files that were accidentally overwritten or deleted
⚠️ Cons
- Devices must be online at the same time to sync (there is no cloud-style “always available somewhere” copy)
- The official Android app has been discontinued; on mobile you rely on community forks (distributed via F-Droid and elsewhere) or, on iOS, third-party apps
- The initial concepts (device IDs, folder sharing, conflict handling) come with a slight learning curve
- Not suited to distributing files to the general public, such as issuing share links
Comparison with Similar Services
| Comparison | Syncthing | Resilio Sync | Dropbox | Nextcloud |
|---|---|---|---|---|
| Model | P2P (serverless) | P2P (serverless) | Cloud | Self-hosted cloud |
| Pricing | Completely free | Free tier / paid Pro | Free tier / paid plans | Free (you pay for the server) |
| Open source | ✅ (MPL-2.0) | ─ (proprietary) | ─ | ✅ |
| Where data lives | Your devices only | Your devices only | The cloud | Your own server |
| Sync to offline devices | Only while both are online | Only while both are online | Via the cloud | Via the server |
| Mobile support | Forks / third-party | Official apps | Official apps | Official apps |
Who Is It For
- People who want to sync sensitive files only between their own devices, without entrusting them to the cloud
- People who want to automatically sync work folders, photos, or music libraries across multiple PCs and NAS devices
- People who want to sync large amounts of data for free, avoiding cloud storage capacity fees
- Developers and server administrators who want to script and automate syncing via the REST API / CLI
- People who prefer open-source tools whose internals can be inspected
Summary
Syncthing is an open-source P2P file sync tool built around the principle of never letting your data leave your own hands. Completely free with unlimited storage, it syncs multiple devices safely and efficiently through TLS encryption and block-level delta transfer. It is not for every use case — there is no cloud-style always-on access or link sharing — but if you value privacy and data sovereignty, it is a first-choice candidate. A good way to start is to install it on two PCs, exchange device IDs, and try syncing a single folder.