Fitness Trackers and Watches: Which Numbers Are Reliable

A wrist device is useful for spotting trends and staying consistent. It is less useful as a source of precise numbers, and knowing which is which prevents both disappointment and bad decisions.

What they measure reasonably well

• Steps and movement — good enough for comparing your own days against each other.
• Heart rate at rest and during steady effort — generally close, and resting heart rate trends over weeks are genuinely informative.
• Distance and pace outdoors with GPS — reliable on a device with its own GPS, less so when it relies on your phone.
• Consistency — the most valuable function is showing up on your wrist as a reminder.

What is an estimate

• Calories burned. A model, not a measurement, and it can be substantially off. Fine as a relative signal, poor as a basis for eating decisions.
• Heart rate during high-intensity intervals and weight training, where wrist optical sensors struggle. A chest strap is far more accurate for this.
• Sleep stages. Total sleep time is reasonable; the breakdown into stages is an estimate and should not be treated as a diagnosis.
• Recovery and readiness scores. Proprietary, unpublished formulas. Useful as a rough trend, not as an instruction.
• Blood oxygen and similar — consumer wellness features, not medical devices, unless specifically cleared as one.

Anything health-related that concerns you is a question for a doctor, not for a wrist device. These are not diagnostic tools and are not regulated as such.

Choosing one you will keep wearing

• Battery life. The biggest practical difference between devices. Something needing daily charging will not track your sleep, because it will be on the charger at night.
• Comfort and size. You wear it constantly. A device too bulky for sleep is a device that only tracks half your life.
• Phone compatibility, and whether all features work with your phone's operating system. Some are noticeably better on one platform.
• Water rating, if you swim. Ratings differ between splash resistance and actual swimming.
• Built-in GPS, if you run or cycle without carrying a phone.
• Whether it exports your data, so you are not locked in if you switch brands.

The subscription question

Several brands put the useful analysis behind a monthly fee. Check before buying whether the features you want are included or subscription-only, and add the annual cost to the purchase price when comparing. A cheaper device with everything included can cost less over three years than an expensive one with a subscription.

Using it well

Pick one or two metrics and ignore the rest. Resting heart rate trending down over weeks, or simply moving on more days than not, is more useful than a dashboard of numbers you cannot act on.

If tracking makes you anxious or turns exercise into an obligation, taking it off is a legitimate option. The device is meant to serve the habit, not replace it.

Before you order

Check battery life against how often you are willing to charge. Check full feature compatibility with your phone. Check whether the analysis you want needs a subscription and price that over a year. And check the water rating properly if you swim, since "water resistant" and "swim rated" are different things.