What Counts as a Wearable
A wearable is more than a fitness band counting steps — the category runs from consumer gadgets to medical-grade devices a doctor prescribes. Where a device sits on that spectrum changes what its numbers are actually worth.
Say “wearable” and most people picture a fitness band counting steps. That is the least consequential end of the category. The same word covers a device a doctor actually prescribes to catch a heart condition — and the two ends of that spectrum are worth telling apart before you trust either one.
Further than a fitness band on your wrist
A smartwatch counting your steps is a wearable. So is a continuous glucose monitor taped to a diabetic patient's arm, a heart-rhythm patch a cardiologist has you wear for two weeks, and a smart ring tracking your sleep overnight. What ties them together is not the shape of the hardware — it is that a sensor is on or in your body, continuously, generating a stream of readings nobody had access to before you put it on.
Consumer-grade versus medical-grade
The two ends of that spectrum are not just different in quality — they are built, tested, and regulated completely differently, and knowing which one you are holding changes what its number is actually worth.
Consumer-grade
Optimised for comfort and battery life. Accuracy is good enough for trends, not for a diagnosis. No clinical trial required before it ships.
Example: a fitness band's heart-rate estimate during a workout.
Medical-grade
Validated against a clinical gold-standard measurement, cleared by a regulator, and built to a specific accuracy tolerance a doctor can rely on for an actual decision.
Example: a hospital-grade pulse oximeter used during surgery.
What a wearable is actually measuring
Almost nothing on your wrist measures what you think it measures directly. A smartwatch does not have a direct line to your heart — it shines light through your skin and infers your pulse from how much of that light bounces back, a technique called photoplethysmography. A step counter does not count steps; it infers them from acceleration patterns that look like a stride. Every reading you see is the output of a model guessing at the real number from an indirect signal, which is exactly the subject of the next two chapters.
The number on the screen is an estimate, not a direct readout — true of both ends of the spectrum, though medical-grade devices are held to a far tighter standard for how close that estimate has to be.
Key takeaways
- A wearable is any sensor on or in your body generating a continuous stream of readings — the category runs from a fitness band to a prescribed heart-rhythm patch.
- Consumer-grade devices optimise for comfort and trends; medical-grade devices are validated against a clinical gold standard before a doctor can act on them.
- Neither kind measures your body directly — a wrist-worn heart-rate sensor infers your pulse from reflected light, not a direct electrical reading.
- Every reading on screen is an estimate produced by a model, which is why the next two chapters look at how much noise sits underneath that estimate.