Smartwatches and Trackers: What They Measure and What to Believe

These devices display everything with the same confident precision, and the numbers behind them vary enormously in reliability. Some are measured directly. Some are estimated from an assumption. Some are effectively a guess with a decimal point attached.

Knowing which is which is the difference between a useful device and a source of unnecessary worry.

Nothing here is health advice. These are consumer devices, and questions about your health belong with your doctor rather than with a watch or an article.

What is measured reasonably well

Steps. Accurate enough for the purpose. It will miscount if you push a trolley or a walker, and it may count arm movement while sitting. As a rough daily comparison against yourself, fine.

Heart rate at rest. Generally close to accurate on a well-fitted device.

Time spent moving versus sitting. Broad and useful.

Sleep duration, approximately. It knows when you stopped moving and when you started again.

Location and distance, if the device has GPS.

What is estimated

Calories burned. An estimate built on your age, weight, height and heart rate, using a general formula. Treat it as a rough relative figure — useful for comparing today with yesterday, not as a real number.

Heart rate during exercise. Less reliable than at rest, particularly with wrist movement, cold hands, or a loose strap.

Sleep stages. Deep, light, REM. These are inferred from movement and heart rate, not measured. Sleep laboratories use brain activity. The overall duration is roughly right; the breakdown into stages should not be taken seriously.

Blood oxygen. Varies considerably by device and by fit. A single low reading on a consumer device is far more likely to be a positioning issue than anything else.

Stress scores. Derived from heart rate variability and presented as a number. Interesting; not a measurement of anything definite.

What to be careful with

Anything presented as a health assessment. These devices are consumer electronics, not diagnostic equipment. A reading that concerns you is a reason to speak to a doctor, not a reason to conclude anything.

Some devices have regulatory clearance for specific functions, such as irregular rhythm notifications, and those are different from general readings. Even so, they are designed to prompt a conversation with a clinician, not to replace one.

Numbers that make you anxious. Some people find continuous measurement reassuring, and some find it produces worry that did not previously exist. Both are common. If it is the second, that is a reason to turn features off, and doing so is not a failure of anything.

What they are genuinely good for

Less dramatic than the marketing, and more useful.

Noticing changes in your own patterns. The absolute numbers matter less than whether this week resembles last week. The device is a reasonable comparison against yourself and a poor comparison against anyone else.

A prompt to move. The reminder to stand up is simple and effective for a great many people.

Fall detection and emergency calling. Genuinely valuable, particularly for people living alone. Worth setting up properly, along with emergency contacts.

Not carrying a phone. Calls, messages and directions on the wrist, for a walk or a shop.

Finding a mislaid phone. Trivial and used constantly.

Choosing one

Start with what you would actually use. Most people use two or three functions. The rest is a reason for the price.

Battery life varies enormously — from a day to two weeks. This determines whether it gets worn or ends up in a drawer, more than any other specification.

Check the screen in daylight, and check whether you can read it without glasses.

Check the strap. Comfort determines whether it is worn at night, which determines whether sleep data exists at all.

A simpler band may suit better than a full watch. Lighter, longer battery, fewer things to learn.

Consider whether it needs a phone nearby, and which phone. Some watches only work with one system.

Practical setup

Wear it snugly, slightly above the wrist bone. Loose fit is the single largest cause of unreliable readings.

Set up emergency features first — fall detection, emergency contacts, medical information.

Turn off notifications you do not want. A watch that buzzes at every email is a watch that gets removed.

Do not enable every measurement. Continuous readings cost battery and, for some people, generate worry without providing anything actionable.

The framing worth keeping

A tracker is a rough diary of your own activity, useful for comparing you with you.

It is not a medical device, its most confident-looking numbers are frequently its least reliable, and no reading on it should be the basis for a decision about your health. That conversation belongs with a doctor, who can also tell you whether anything the watch reported is worth attention.

For additional public information on health and aging, see National Institute on Aging.