Zephyr vs FreeRTOS: How to Choose the Right RTOS for Your Embedded Project

Every time I mention Zephyr, I inevitably get a few responses along the lines of “just use FreeRTOS.” I admit, FreeRTOS is amazing, but it might not be the right tool for the job. In the rest of the post, I’ll analyze each to hopefully help give you a better idea of what kinds of jobs each is better suited for.

Do You Even Need an RTOS?

The first question you should be asking is if you even need a real-time operating system (RTOS). I covered this in a previous post, but the basic answer is that if your embedded project is complex enough, it might be worth looking at using an RTOS.

At a basic level, an RTOS gives you deterministic scheduling, task isolation, synchronization primitives, and timing guarantees. Many modern embedded systems demand more than just a scheduler, especially once connectivity, security, portability, and long-term maintenance enter the picture. Connectivity, security, portability, and long-term maintenance often end up mattering just as much as real-time performance.

Assuming you’ve decided that you need an RTOS, let’s take a look at two of the most popular options.

FreeRTOS: Simple Scheduler With Optional Libraries

FreeRTOS has become ubiquitous because it does one thing extremely well: it provides a lightweight, predictable real-time kernel that you can drop into almost any project. It doesn’t try to define your system architecture or impose a particular way of organizing your code. You get tasks, queues, semaphores, timers, and little else. For many projects, that’s exactly what you want.

Because FreeRTOS is so minimal, it integrates easily with existing vendor SDKs and bare-metal codebases. It’s basically a drop-in library. You can introduce it gradually, adding multitasking where you need it without reworking your entire build system. This makes it especially appealing for smaller teams, resource-constrained hardware, or projects where you need to stay very close to the metal.

Another advantage of FreeRTOS is its approachability. It’s the first production level RTOS that I learned, and I still think it’s an amazing tool for learning RTOS concepts (I use Espressif’s fork of FreeRTOS in my Introduction to RTOS video series).

As a result of this simplicity, you can become productive quickly, and a FreeRTOS-based system is often easy to understand by simply reading the code. There’s very little “magic” happening behind the scenes, which can be a major benefit when you’re debugging timing issues or trying to reason about system behavior.

That simplicity, however, comes with tradeoffs. As projects grow, FreeRTOS doesn’t offer much help with higher-level concerns. The broader FreeRTOS ecosystem includes optional connectivity libraries (for example, networking and security components originally developed under Amazon FreeRTOS). However, you’re still expected to provide your own glue logic and integrate with the vendor-specific drivers/HAL manually. Many teams do this successfully, but it’s worth recognizing that at some point you’re effectively creating a custom operating system (whether you intend to or not).

Zephyr: Structure and Portability

Zephyr takes a fundamentally different approach. While it still provides real-time scheduling and synchronization, it positions itself as a full embedded operating system rather than just a kernel. Zephyr is opinionated, and that’s by design. It encourages a particular way of structuring projects, configuring features, and abstracting hardware.

One of Zephyr’s most significant strengths is its hardware abstraction model. Through devicetree and a standardized device API, Zephyr makes it possible to write application code that is largely decoupled from specific boards or vendors. In other words, you can create truly cross-platform, portable code that functions across a variety of microcontrollers. This is a relatively new concept in the embedded world, as most applications were traditionally written for a specific microcontroller architecture.

Zephyr also scales well as systems become more complex. When networking, storage, security, and power management all need to coexist, having a unified framework starts to pay off. The build system, configuration tools, and project layout are designed to support large codebases and multiple developers working in parallel.

The downside is that Zephyr demands more upfront investment. The learning curve is steeper, the build system is more complex, and it can feel heavy if you’re coming from a bare-metal or FreeRTOS background. You’ll also find that Zephyr typically requires more flash and RAM; that overhead is the cost of providing the structure and portability we just discussed.

When to Choose Either

A useful way to think about the difference between these two RTOSes is that FreeRTOS is a library, while Zephyr is a platform. With FreeRTOS, you decide how everything fits together. With Zephyr, you adopt an existing structure that helps manage complexity at scale.

Neither approach is inherently superior. Problems arise when the philosophy of the RTOS doesn’t match the needs of the project. Choosing Zephyr for a simple, single-purpose device can feel like unnecessary overhead, while choosing FreeRTOS for a large, long-lived product can lead to growing pains as the system expands.

FreeRTOS tends to be a better fit when you’re building a purpose-built device with well-defined behavior, especially if you’re working on constrained hardware or integrating closely with vendor-provided libraries. It’s also a strong choice when you want complete architectural control or when rapid prototyping is a priority.

Zephyr, on the other hand, shines when you expect a project to evolve over time. If hardware revisions are likely, if multiple subsystems need to coexist cleanly, or if long-term maintenance and portability are important, Zephyr’s structure can save significant effort down the road. This is particularly true for commercial products and platforms that will be supported for years.

One common mistake is committing to either RTOS too early. Picking Zephyr before requirements are clear can bog a project down in complexity. Sticking with FreeRTOS after a system has outgrown its original scope can make scaling painful. Clearly understanding the project requirements and rapidly iterating on some early concepts can help determine which is a better fit for your project.

Learning Both as an Engineer

From a learning perspective, I generally recommend starting with FreeRTOS. It’s an excellent way to understand real-time concepts without too much abstraction getting in the way. Once you’re comfortable with scheduling, synchronization, and timing, moving on to Zephyr provides valuable insight into how larger embedded systems are organized and maintained.

This progression mirrors how many engineers encounter these tools in industry, and understanding both gives you flexibility when evaluating new projects or job opportunities.

Final Thoughts

If I had to reduce the decision to a single guideline, it would be this: FreeRTOS is ideal when you want to control everything yourself, while Zephyr is ideal when you want the system to help manage complexity for you. Both are excellent tools, and either can lead to a clean, maintainable system when used in the right context.

The Zephyr versus FreeRTOS discussion isn’t really about performance or correctness. It’s about tradeoffs in structure, scalability, and workflow. When you align the RTOS with your project’s constraints and long-term goals, either choice can serve you well. When you choose based on hype or habit, you’ll feel the pain later.

Have you used FreeRTOS, Zephyr, or both? What are your thoughts, and what kinds of projects do you find each best suited for?

If you would like to dive into using FreeRTOS on the ESP32, check out my IoT Firmware Development with ESP32 and ESP-IDF course. In the course, we cover the basics of ESP-IDF, reading from sensors, connecting to the internet via WiFi, posting data via HTTP REST calls, securing connections with TLS, and interacting with MQTT brokers.