Founding Mobile Engineer

Founding Mobile Engineer

New York City · In person near Grand Central · Full time

Competitive base + bonus + equity

Compensation range: $125k - $240k

Build the intelligence behind every workout

Calibrex is building an AI voice coach for strength training, powered by a magnetic wearable that understands a workout as it happens and tells the athlete what to do next. Like a GPS for the gym: no thinking required, just train and listen.

We’ve shipped thousands of units through a sold-out pilot, built a backlog of customer demand, and earned a core group of athletes who use Calibrex every session. Our next challenge is turning that early traction into a product that can serve one million athletes.

We’re hiring a Founding Mobile Engineer to lead the technical evolution of our hardware-connected mobile experience.

What you’ll do

You’ll own our Expo and React Native application and help shape the systems around it, including Firebase, payments, release infrastructure, and the native integrations that connect the app to our hardware.

Your work will include:

• Building reliable, responsive mobile experiences around real-time sensor data
• Improving the performance of a 200 Hz data pipeline
• Developing and debugging native iOS and Android modules

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• Evolving our Firebase and Firestore architecture as usage grows
• Owning payments, testing, observability, and mobile release workflows
• Making architectural decisions with the founding team
• Talking with athletes, understanding their problems, and carrying solutions through to production

This is a hands-on role with a broad surface area. You might optimize sensor processing one week and improve checkout or deployment infrastructure the next.

What success looks like

During your first year, you will help us:

• Make the app and its hardware connection exceptionally reliable
• Identify and remove the largest technical barriers to growth
• Establish architecture and engineering practices that can support a much larger user base
• Increase our speed of experimentation without sacrificing product quality
• Build a mobile experience athletes trust during every training session

What we’re looking for

You’ll likely thrive here if you have:

• Shipped and maintained a production React Native application

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• Strong mobile and full-stack engineering judgment
• Experience with Firestore, Firebase Authentication, and Cloud Functions or comparable systems
• Built or integrated mobile payment flows
• Worked with native iOS or Android code
• Diagnosed and resolved meaningful performance or reliability problems
• Used AI-assisted development tools thoughtfully in your daily workflow
• Taken ambiguous problems from discovery through release and measurement

You don’t need to have personally scaled an application to one million users. We care more about whether you understand the engineering decisions, failure modes, and tradeoffs involved.

Helpful, but not required

• Bluetooth or BLE experience
• Experience with connected hardware or wearable products
• Sensor data, time-series data, or signal-processing experience
• Personal familiarity with strength training

How we work

Calibrex is a small, mission-driven team of athletes and builders. We value ownership, candid communication, sound judgment, and speed with purpose.

This is an in-person role based one block from Grand Central. At our current stage, we believe working side by side helps us learn and build faster. If you’re not currently based in New York City, we’re open to providing relocation support.

You’ll have meaningful influence over our architecture, roadmap, engineering culture, and product. Not simply a queue of specifications to execute.

How to apply

Send us your résumé or LinkedIn profile, along with a brief note about a mobile product or technical problem you’ve owned.

To stand out, include a 3-4 minute video recording (gdrive/loom link) introducing yourself and answering:

"You’ve joined Calibrex with the goal of preparing our product for one million users. What are the first three things you would examine - in the codebase, infrastructure, or product - to assess our readiness, and why did you prioritize those three?"

Submitting a video recording is not required, but it will significantly increase your likelihood of receiving an interview. We prioritize candidates who take this step because it gives us a much better understanding of how they frame problems, establish priorities, and communicate technical decisions.

Don’t worry about production quality or delivering a perfectly polished answer. Thinking out loud is welcome - we want to understand how you think, not how well you present.

Learn more at calibrex.com.

From the team

“Building with the team at Calibrex has been exceptional. From day one, we’ve combined strong technical execution with a deep understanding of customer needs, building production-ready systems rather than just prototypes. We move quickly, iterate constantly, and tackle complex problems with a high degree of ownership. As Calibrex’s founding machine-learning engineer, I’ve had a meaningful role in shaping the technical foundation of the platform and turning advanced research into reliable, scalable systems that deliver real value. If you’re excited by strength training, technology, and the opportunity to shape a product from an early stage, Calibrex is the place for you.”

— Ross B., PhD, Founding Machine-Learning Engineer at Calibrex