Zephyr vs FreeRTOS: which RTOS should your product use?
A practical comparison for product teams choosing firmware foundations.
Short answer: FreeRTOS is a small, simple real-time kernel: a scheduler, tasks, queues and timers that you build your own drivers around. Zephyr is a full embedded operating system with a driver model, device tree, Bluetooth and networking stacks, and built-in update support. Choose FreeRTOS for small, focused firmware or when your vendor SDK already uses it (ESP-IDF, STM32Cube). Choose Zephyr for connected products that need a portable, feature-rich platform, especially on Nordic nRF chips.
Side-by-side comparison
| FreeRTOS | Zephyr | |
|---|---|---|
| What it is | Real-time kernel | Full RTOS with drivers, stacks and tooling |
| Licence | MIT | Apache 2.0 |
| Governance | Maintained by AWS | Linux Foundation project |
| Footprint | Very small, a few KB for the kernel | Larger, depends heavily on configuration |
| Drivers | Come from the chip vendor's SDK | Unified driver model and device tree across many chips |
| Bluetooth and networking | Added separately | Built-in Bluetooth LE, IP networking, Thread and more |
| Firmware updates | Vendor or custom bootloader | MCUboot integration |
| Where you meet it | ESP-IDF (ESP32), STM32Cube, many vendor SDKs | nRF Connect SDK (Nordic), many NXP, ST and Intel boards |
| Learning curve | Gentle | Steeper: Kconfig, device tree and the west tool |
When FreeRTOS is the better choice
- Small devices with a handful of tasks and tight memory.
- ESP32 projects, where ESP-IDF already runs FreeRTOS underneath.
- Teams that want to keep using a chip vendor's familiar SDK and drivers.
- Projects where certification of a small, well-understood kernel matters.
When Zephyr is the better choice
- Bluetooth LE products on Nordic nRF52 and nRF53, where the official SDK is Zephyr-based.
- Products that may move between chip vendors; device tree and the driver model make porting easier.
- Connected devices that need networking, file systems, shell and secure updates without assembling them yourself.
What it means for your project cost
FreeRTOS projects often start faster, because there is less framework to learn. Zephyr projects take longer to set up but save time later when you add connectivity, updates or a second hardware variant. The right answer depends more on your chip and product roadmap than on the kernel itself. See our nRF52 vs ESP32 comparison for the chip side of the decision.
Frequently asked questions
Is Zephyr better than FreeRTOS?
Neither is better in general. Zephyr is a fuller platform for connected products; FreeRTOS is a lighter kernel for focused firmware.
Does ESP32 use FreeRTOS?
Yes. Espressif's ESP-IDF framework is built on a modified FreeRTOS, so ESP32 firmware normally uses FreeRTOS tasks and queues.
Does nRF52 use Zephyr?
Nordic's current nRF Connect SDK is built on Zephyr, so most new nRF52 and nRF53 projects use it.
Can SixSol work with both?
Yes. We develop firmware on ESP-IDF (FreeRTOS) and nRF Connect SDK (Zephyr), as well as bare-metal code where an RTOS is not needed.