A new generation of wearable technology has quietly moved a once-clinical ritual — the measurement of what our bodies are made of — onto the wrist of anyone who can afford a smartwatch. The Samsung Galaxy Watch 4, released in mid-2021, uses bioelectric impedance analysis to estimate body fat, muscle mass, bone density, and hydration in seconds, democratizing a form of self-knowledge that once required a doctor's office or specialized equipment. Yet as with many tools that promise intimacy with our own biology, the watch raises as many questions as it answers — about accuracy, about safety, and
How to safely use Samsung Galaxy Watch 4's body composition feature
Trust the trend, not the absolute number.
Mark
Why does the watch need you to hold your arms away from your body? That seems oddly specific.
Mimi
The electrical current needs a clear path through your torso to get an accurate reading. If your arms are touching your sides, you're creating an alternate route for the current, which throws off the measurement.
Mark
So it's measuring the resistance of electricity moving through you?
Mimi
Exactly. Different tissues—fat, muscle, bone, water—all conduct electricity differently. By measuring how much resistance the current encounters, the watch can estimate what percentage of your body is each type of tissue.
Mark
And that's why pregnant people need to check with a doctor first?
Mimi
Right. There's no evidence the current harms a pregnancy, but it's an electrical stimulus going through your body during a vulnerable time. Better to get clearance than to wonder later.
Mark
What about the accuracy issue? Should I trust the number it gives me?
Mimi
Trust the trend, not the absolute number. If it says you're 28 percent body fat today and 27 percent next week, that direction matters. But comparing your watch reading to a doctor's measurement might show a difference of several percentage points.
Mark
So it's more useful for motivation than diagnosis?
Mimi
Precisely. It's a tool for staying aware and engaged with your health. Just don't let it replace the conversations you need to have with actual doctors.
The Pulse
- The Galaxy Watch 4 puts body composition analysis on your wrist, but the readings carry real limitations that users risk overlooking in the excitement of instant feedback.
- People with pacemakers or those who are pregnant face genuine safety considerations, and the watch offers no automatic safeguards — the burden of caution falls entirely on the user.
- Accuracy fluctuates enough that a single reading can mislead; the technology earns its value only through consistent, same-time-daily measurements tracked as trends rather than absolute truths.
- The measurement process demands an awkward, precise posture — arms extended, fingers resting on crown buttons, hands apart — without which the electrical current cannot travel correctly through the body.
- Samsung Health delivers the full data picture on your phone after each reading, but the watch itself is clear that no wrist-worn sensor can replace the contextual judgment of a physician.
A new generation of wearable technology has quietly moved a once-clinical ritual — the measurement of what our bodies are made of — onto the wrist of anyone who can afford a smartwatch. The Samsung Galaxy Watch 4, released in mid-2021, uses bioelectric impedance analysis to estimate body fat, muscle mass, bone density, and hydration in seconds, democratizing a form of self-knowledge that once required a doctor's office or specialized equipment. Yet as with many tools that promise intimacy with our own biology, the watch raises as many questions as it answers — about accuracy, about safety, and about the difference between data and understanding.
The Samsung Galaxy Watch 4 arrived carrying a promise once belonging almost exclusively to clinical settings: the ability to measure your body composition in seconds. Using bioelectric impedance analysis — a method that sends a faint electrical current through the body to estimate fat, muscle, bone, and water content — the watch can surface body fat percentage, BMI, muscle mass, bone density, and hydration levels without an appointment or a waiting room.
Before reaching for that convenience, safety deserves attention. Those with pacemakers are advised to consult a cardiologist before using the feature, and pregnant users should speak with their doctor as well — the watch, unlike some smart scales, has no pregnancy mode to disable the function automatically. Samsung also flags that readings may be unreliable for users under 20, though no age restriction is enforced by the device itself.
Accuracy is a more nuanced matter. Independent testing found the Galaxy Watch 4's readings diverge from dedicated smart scales by varying degrees — not useless, but not clinical-grade either. The feature earns its worth through consistency: measurements taken at the same time each morning, before eating or exercising, create a baseline against which meaningful trends can emerge over weeks and months.
The measurement itself requires a specific, somewhat awkward posture. After stepping on a scale and entering your weight into the watch's body composition app, you reposition the watch higher on your forearm, rest your middle and ring fingers lightly against the crown buttons with your palm facing up, raise your opposite hand so the two don't touch, and extend both arms away from your torso. Hold that pose for roughly 15 seconds while the watch works.
Results appear on the watch face immediately and expand into richer detail through the Samsung Health app. The convenience is genuine — but so is the caveat. A wrist reading can track patterns and sustain engagement with your fitness, yet it cannot replace the judgment of a doctor who can interpret what those numbers mean for your particular life and health.
The Samsung Galaxy Watch 4 arrived with a feature that promised to do something previously reserved for doctor's offices or expensive home scales: measure your body composition in seconds. Using bioelectric impedance analysis—a technology that sends a weak electrical current through your body to estimate fat, muscle, bone, and water content—the watch can deliver readings on body fat percentage, BMI, muscle mass, bone density, and hydration levels. It's convenient, it's fast, and it sits right on your wrist. But before you start taking daily measurements, there are some important things to understand about whether it's right for you and how to actually use it correctly.
The safety question comes first. People with pacemakers should be cautious, though research suggests the electrical current used in bioelectric impedance analysis poses minimal risk. Still, consulting your cardiologist or electrophysiologist before using the feature makes sense. Pregnant people face a similar situation: experts generally consider the technology safe during pregnancy, but a conversation with your doctor is warranted. Notably, the Galaxy Watch 4 lacks a pregnancy mode that some smart scales include to disable the feature automatically. Samsung also notes that measurements may not be reliable for anyone under 20 years old, though the watch doesn't enforce an age restriction.
Accuracy itself is a moving target. Reviewers testing the Galaxy Watch 4 against dedicated smart scales found varying degrees of agreement between the devices. The readings aren't worthless, but they're not clinical-grade either. This matters less if your goal is tracking trends over time rather than getting a single precise number. Taking measurements at the same time each day—ideally in the morning before eating or exercising—creates consistency that makes week-to-week or month-to-month changes meaningful.
The process itself requires some preparation and an awkward pose. First, you need to know your weight, so step on any scale beforehand. Open the body composition app on your watch, enter that weight figure, and adjust the watch higher on your wrist so it sits more on your forearm than across your wrist bones. Then comes the positioning: place your middle and ring fingers against the watch's crown buttons with your palm facing up—don't press, just make contact. Raise the back of your non-watch hand so your hands don't touch each other. Extend both arms away from your body, keeping them from touching your torso. It's an unnatural stance, but it's necessary for the electrical current to travel properly through your body.
Once you've assumed this position, press start and hold steady. The watch needs about 15 seconds to complete the measurement. When it finishes, you'll see a summary right on your wrist, with the option to dive deeper into the data through the Samsung Health app on your phone. The convenience is real—no appointment, no waiting room, no cost beyond the watch itself. But this convenience comes with an important caveat: the Galaxy Watch 4 body composition feature should complement your health routine, not replace it. A quick reading on your wrist can't substitute for regular check-ups with a doctor who can contextualize what those numbers mean for your individual health. Use the watch to track patterns and stay engaged with your fitness, but treat professional medical guidance as irreplaceable.
Notable Quotes
Don't use the Galaxy Watch 4 body composition feature as a replacement for regular check-ups— Tom's Guide guidance
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