Tracker accuracy

How accurate are step counts on phones and wearables?

Step counts are a rough estimate, not an exact figure. Wearables mostly undercount steps, with errors from about −9% to 12%¹. Some overcount instead, including for older adults, so the direction of error isn't consistent^2,3.

The brand and model matter a lot. Some models count steps closely, while others can be far off². Your walking speed matters too: trackers underestimate steps during slow walking, by anywhere from 0.94% to 60%⁴. Differences from a hand count ranged from 0.0 to 26.4%, depending on the tracker and the walking speed⁵.

What to do with this:

• Treat your step count as a rough guide rather than an exact score.
• Don't compare totals between two different devices. This is why your phone and your watch can show different numbers.
• If you walk slowly, expect your tracker to miss some steps.
• Judge progress from how your own days compare on the same device, not from any single number.

How accurate are heart rate, activity minutes and calories on a wearable?

Wrist heart rate is the most dependable of the three, though individual readings can be off. Calorie estimates are the weakest, and activity minutes are best treated as rough too.

Heart rate. Wrist-worn heart rate typically shows an error of about ±3%. How close it gets depends on the person, how hard they're working and the device model¹. Readings stay close to the reference during sleep, at rest and on a treadmill from walking to running, differing by roughly half a beat a minute or less on average. The gap is larger during resistance training (7.26 beats a minute lower on average) and cycling (4.55 lower)⁶. Even when the average error is small, single readings can be far off⁷. Movement, moisture and poor skin contact can disturb the signal⁸. Wrist heart rate may also be misestimated more in people with heart disease, so people with a heart condition shouldn't rely on a watch reading⁸.

Calories. No tested device proved accurate. The error was above 30% for all brands⁹, and fewer than 10% of estimates fell within acceptable limits in controlled settings¹⁰. Wearables tend to underestimate, by about 3 kcal per minute on average¹. Over a whole day, all devices underestimated total energy use, by 475 to 898 kcal, which makes them of little use for balancing what you eat with what you use¹¹.

What to do with this:

• Don't use a watch's calorie figure to decide how much to eat or to judge a session. Treat it as a very rough guess.
• Check how hard the activity feels as well as the number on the watch.
• Expect heart rate to read less well during weights or cycling, or if the watch is loose or the skin is wet.
• Look at your own pattern over weeks rather than any single reading.

When to check with a health professional

If you're starting exercise after a long break, it's always a good idea to see your doctor first. For an injury or a health condition, see a health professional, such as your doctor or a physio.

Most accuracy studies were in healthy adults, so tracker numbers aren't medical information^1,7,8. Go by how your body feels rather than the watch. Blood oxygen readings have limited accuracy when oxygen levels are low, and a normal-looking reading doesn't mean symptoms can be ignored.

Call your local emergency number for chest pain or pressure (or pain spreading to the arm, neck, jaw or back, especially with sweating or feeling sick), severe breathlessness, fainting or collapse, signs of a stroke, a serious injury, or someone who isn't safe.

For a sore muscle or joint, ease off and get advice if it persists.