Fitness trackers add steps, but their calorie counts are rough guesses

Key takeaways

• A 2022 review of 39 earlier reviews, covering about 164,000 people, found that activity trackers add roughly 1,800 steps a day.
• In a two-year University of Pittsburgh trial, young adults who added an armband tracker to a weight-loss program lost 7.7 pounds, compared with 13 pounds for those without one.
• In lab tests, wrist trackers measure heart rate fairly well, but a 2020 review found no brand that met accepted accuracy limits for calories burned.
• Watch estimates of VO2 max, a key fitness marker, can be off by several points for one person, so follow the trend, not the single number.
• Set goals in weekly active minutes, plan rest days, and step back if tracking starts to bring guilt instead of motivation.

In 2016, researchers at the University of Pittsburgh published a trial that wearable makers probably wish had gone differently. They had put 470 adults ages 18 to 35 with overweight or obesity on a lower-calorie diet, more exercise and group counseling².

Six months in, half of them also got an armband that tracked their movement and estimated the calories they burned. Two years later, the people with the device had lost an average of 7.7 pounds. The people without it had lost 13 pounds².

Now hold that next to a second finding. In 2022, Australian researchers pooled 39 reviews of activity-tracker studies, covering about 164,000 people. Trackers added roughly 1,800 steps a day¹.

Both results are real. Trackers are good at nudging you to move, but they are much worse at telling you what that movement is worth in food.

About one in five U.S. adults regularly wore a smartwatch or fitness tracker in a 2019 Pew Research Center survey⁴. This guide shows where the device earns its place on your wrist, where it misleads you, and how to set it up so it works for you.

Trackers do get people moving, though the boost fades

The 2022 review, led by Ty Ferguson and Carol Maher at the University of South Australia, appeared in The Lancet Digital Health¹. It gathered systematic reviews, which are studies that pool the results of many earlier studies.

Trackers raised activity in children, adults and older adults, and in healthy people as well as people with chronic disease¹. Beyond steps, people added about 6 minutes a day of moderate-to-vigorous activity, the kind that makes you breathe harder. On average, they lost about 1 kilogram, or 2.2 pounds¹.

The fine print is where it gets useful:

• The 1,800-step figure blends simple pedometers with modern trackers. For consumer trackers alone, one pooled analysis found about 950 extra steps a day¹.
• The effect shrinks with time. One review found about 1,100 extra steps a day at four to six months, but fewer than 500 at up to four years¹.
• In one review, programs that paired trackers with coaching or other support had effects about 50% larger than trackers alone¹.
• Using a standard quality checklist, the authors rated 34 of the 39 reviews as “critically low” in confidence¹.

So the device works best as one part of a plan. The authors themselves recommended trackers as an add-on to physical activity programs¹.

In a two-year weight-loss trial, the tracker group lost less

The Pittsburgh trial, called IDEA and led by John Jakicic, was published in JAMA in September 2016³. Everyone got the same diet, activity goals, group and phone counseling, text reminders and online tools.

At six months, 237 people also got a wearable device and 233 did not. After 24 months, the device group had lost 3.5 kilograms on average, compared with 5.9 kilograms for the standard group³. Both groups improved their fitness, body makeup, activity and diet by similar amounts².

Jakicic offered some possible explanations, which he did not present as proven³. Focusing on activity may have pulled attention away from eating. People may have felt they had “earned” a treat. And some may have lost heart when they fell far short of a daily goal.

He did not tell people to throw out devices that help them. He said trackers could work well for some people, especially tech-savvy people who aren’t sure they’re getting enough activity³.

One detail adds weight to the result. Jakicic disclosed that he led a grant from a tracker company, Jawbone, awarded to his university to study whether activity monitors measure accurately³. A researcher with an industry tie still published a finding that cut against the product. That is how research should work when money is involved, and it is worth noticing when it does.

Heart rate is decent, and calories are a guess with a wide margin

In 2017, a Stanford team led by Euan Ashley tested seven popular wrist devices on 60 volunteers. People sat, walked, ran and cycled while lab equipment measured their true heart rate and calorie burn⁵.

Six of the seven devices had a median heart-rate error under 5% during cycling⁵. Calories were another story, and no device got below 20% error. Across tasks, typical errors ran from about 27% for the best device to 93% for the worst⁵⁶.

Errors were larger for men, for people with darker skin and for people with a higher body mass index, and larger during walking⁵. Ashley’s advice was simple: you can mostly trust the heart rate, but don’t use the calorie number to decide how much to eat⁶.

Those were 2017 models, so a broader check matters. In 2020, Daniel Fuller and colleagues reviewed 158 publications covering 45 devices from nine brands⁷. In lab tests, step counts landed within 3% of the true count about 45% of the time. In daily life, they tended to run a little high, by a median of about 6%⁷.

Calorie estimates did far worse. They landed within 3% of the true burn only about 9% of the time in the lab, and no brand met accepted accuracy limits for energy use⁷.

Here is what that means in practice: picture a walker whose watch reports 400 calories burned. If the estimate is off by 27%, the true burn could be anywhere from about 290 to 510 calories. Eat back the full 400, and you could erase a day’s planned deficit without knowing it.

Your watch’s VO2 max is a ballpark, not a lab result

VO2 max is the most oxygen your body can use during an all-out effort. It is a widely used marker of fitness, and many watches now estimate it from your heart rate and pace.

In 2025, a team at University College Dublin checked Apple Watch estimates against a treadmill test with breathing analysis in 28 adults⁸. The watch read low by about 6 points on average (measured in milliliters of oxygen per kilogram per minute), an error of about 13%. For some people, the gap was far wider⁸.

The Dublin team pointed to a 2022 meta-analysis of 14 studies across brands. Average error was close to zero, but single readings could be about 10 points too high or too low⁸. Other research has found watches tend to read high for less fit people and low for very fit people⁸⁹.

A 2025 review of 13 studies, mostly of Garmin watches, found that 7 judged the estimates valid or acceptable⁹. Estimates based on exercise beat estimates made at rest. The review also called for long-term studies to see whether the numbers stay reliable over months⁹.

The lesson: compare your watch to itself, on similar workouts, over several weeks. One new number tells you very little.

Half of new users quit within six months in one study

In a 2017 study, researchers gave a basic Fitbit tracker to people in four French cities and followed 711 of them for 320 days¹⁰. At 100 days, about 74% were still using it. By about 175 days, half had stopped. At 320 days, only 16% were still tracking¹⁰.

The top reason for quitting wasn’t boredom. It was technical trouble: dead batteries, broken devices and lost trackers¹⁰. Older users tended to keep going longer¹⁰.

Who wears trackers matters, too. In the Pew survey, 31% of adults in households earning $75,000 or more used one, compared with 12% in households earning under $30,000⁴.

The practical lesson is that a habit tied to a charger is fragile. The goal is a routine that keeps going after the battery dies.

When counting stops being fun

Researchers have found a quieter cost. In six experiments published in 2016, Duke University’s Jordan Etkin found that measuring an activity led people to do more of it but enjoy it less¹¹. Counting made pleasant activities feel more like work, which lowered people’s interest in continuing¹¹.

For some people, the cost runs deeper. In a 2019 study in JMIR mHealth and uHealth, 41 of 95 young people surveyed reported a bad experience with healthy-eating or fitness apps¹². Obsessive calorie counting or logging was the most common. People described guilt when they missed targets and a sense that the app was in control¹².

The team also interviewed three eating-disorder experts. One, a clinical psychologist, said clinicians actively steer people with eating disorders away from these apps and from wrist monitors¹². Of the top 100 free fitness and healthy-eating apps the team reviewed, 21 let users set a goal weight in the underweight range¹².

The 2022 tracker review also listed obsessive behavior and eating disorders among the concerns raised about these devices¹. The research is early, mostly surveys and short experiments. The warning signs, though, are easy to spot.

Set your tracker to count minutes, not just steps

Federal guidelines are written in minutes. The U.S. Department of Health and Human Services (HHS) recommends 150 to 300 minutes a week of moderate activity, or 75 to 150 minutes of vigorous activity. It also calls for muscle strengthening on two or more days, and balance training for older adults¹³.

The guidelines dropped an old rule that activity had to come in 10-minute blocks to count. Any amount helps, and some benefits start right after a session¹³. For the research on how many daily steps bring the most benefit, see our article on walking clubs.

The Centers for Disease Control and Prevention (CDC) offers an intensity check that needs no gadget¹⁴. During moderate activity, you can talk but not sing. During vigorous activity, you can say only a few words before pausing for breath. On a 0-to-10 effort scale, moderate is about 5 or 6, and vigorous starts around 7 or 8¹⁴.

A 15-minute setup that puts you in charge

1. Wear it for one normal week first. Don’t change anything. This gives you a baseline, your own starting point.
2. Pick one weekly goal in minutes. Choose a number a little above your baseline and build toward 150 minutes of moderate activity. HHS advises inactive people to “start low and go slow”¹³.
3. Hide or ignore the calorie-burn display. If your app allows it, move that number off the main screen, and never use it to set portions.
4. Log strength days yourself. The guidelines call for two a week, and a step count won’t show them¹³. A paper calendar works fine.
5. Plan rest days and turn off streak alerts. A weekly target leaves room for a sick day without “failing.”
6. Treat VO2 max as a monthly trend. Compare similar workouts, not single days⁸⁹.
7. Review once a week, not all day. Five minutes on a Sunday shows whether you met your minutes.
8. Plan for the day it breaks. Keep the walk, the class or the walking partner on your calendar, so the habit doesn’t depend on the device.

What the evidence supports, claim by claim

Claim	Evidence level	What the research shows
Wearing a tracker raises daily activity	Strong evidence	Consistent gains across 39 reviews and all age groups, though most reviews were low in quality1
The extra activity lasts for years	Promising but early	Gains shrink to under 500 steps a day at up to four years1
Trackers lead to modest weight loss	Promising but early	About 1 kilogram on average across reviews, and the reviews were mostly low in quality1
Adding a tracker to a strong weight-loss program boosts results	Evidence says no, or unsafe	In a two-year trial of 470 adults, the tracker group lost less weight23
Wrist heart rate is accurate during steady exercise	Strong evidence	Most devices within about 5% in lab tests, with more error for some skin tones and body sizes57
Step counts are accurate	Strong evidence	Close in the lab, though they tend to run a bit high in daily life7
Calorie-burn estimates are good enough to plan meals around	Evidence says no, or unsafe	Errors of 27% to 93% in one test; no brand met accuracy limits in a review of 158 publications57
Watch VO2 max gives a useful ballpark for healthy adults	Promising but early	About half of studies call it acceptable; single readings can miss by about 10 points89
Measuring an activity can make it less enjoyable	Promising but early	Six experiments from one research team11
Fitness tracking causes eating disorders	Not enough evidence	Surveys link app use to compulsive habits in some people, but they can’t show cause12

The tracker your body already came with

Long before wristbands, people judged effort by their breath, their sweat and how they felt the next morning. The CDC’s talk test is a modern name for that old skill¹⁴.

A tracker can sharpen that sense for a season. It can show how little you move on workdays, or how a daily walk adds up over a month. But the research suggests the benefit comes from paying attention, and that attention can outlast the gadget.

Keep in mind that the device has its own goals. In the 2019 app review, 60 of the top 100 free fitness and eating apps rewarded continuous use¹². A company that profits when you check your wrist all day is not the same as a coach who wants you healthy.

That fits a core belief in natural health: your body gives you feedback all day long. A good tracker can teach you to read it. A bad relationship with one teaches you to ignore it.

Questions readers ask

The bottom line

Trackers are good at one job: making movement visible. For most people, that visibility leads to more movement, though the boost is modest and tends to fade.

Treat the other numbers with care. Heart rate is fairly reliable, steps are close, VO2 max is a ballpark, and calories are a rough guess. Set goals in minutes, give yourself rest days, and build a habit that keeps going when the battery dies.

Related reading

• Walking clubs work, and you need fewer steps than you were told
• Your wearable’s best number is your own normal, not the average
• “FDA registered” isn’t “FDA approved”: a buyer’s map to wellness devices
• The wellness subscription you meant to cancel

Sources

1. Ferguson T, Olds T, Curtis R, et al. “Effectiveness of wearable activity trackers to increase physical activity and improve health: a systematic review of systematic reviews and meta-analyses.” The Lancet Digital Health, 2022 (full text via Newswise). newswise.com
2. University of Pittsburgh Epidemiology Data Center. “Activity Trackers Are Ineffective at Sustaining Weight Loss.” University of Pittsburgh, September 22, 2016. edc.pitt.edu
3. TCTMD. “A Step in the Wrong Direction: Weight-Loss Efforts Could Be Hampered by Addition of Wearable Device” (on Jakicic JM et al., JAMA, 2016). TCTMD, September 21, 2016. tctmd.com
4. Pew Research Center. “About one-in-five Americans use a smart watch or fitness tracker.” Pew Research Center, January 9, 2020. pewresearch.org
5. Shcherbina A, Mattsson CM, Waggott D, et al. “Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort.” Journal of Personalized Medicine, 2017. pubmed.ncbi.nlm.nih.gov
6. Stanford Medicine News Center (via Stanford Bio-X). “Fitness trackers accurately measure heart rate but not calories burned.” Stanford University, May 24, 2017. biox.stanford.edu
7. Fuller D, Colwell E, Low J, et al. “Reliability and Validity of Commercially Available Wearable Devices for Measuring Steps, Energy Expenditure, and Heart Rate: Systematic Review.” JMIR mHealth and uHealth, 2020. pubmed.ncbi.nlm.nih.gov
8. Lambe R, O’Grady B, Baldwin M, Doherty C. “Investigating the accuracy of Apple Watch VO2 max measurements: A validation study.” PLOS ONE, 2025. journals.plos.org
9. Železnik Mežan L. “Accuracy of wearables for determining the maximal oxygen uptake and lactate threshold: a qualitative systematic review.” Frontiers in Sports and Active Living, 2025. frontiersin.org
10. Hermsen S, Moons J, Kerkhof P, Wiekens C, De Groot M. “Determinants for Sustained Use of an Activity Tracker: Observational Study.” JMIR mHealth and uHealth, 2017. pubmed.ncbi.nlm.nih.gov
11. Etkin J. “The Hidden Cost of Personal Quantification.” Journal of Consumer Research, 2016. doi.org
12. Honary M, Bell BT, Clinch S, Wild SE, McNaney R. “Understanding the Role of Healthy Eating and Fitness Mobile Apps in the Formation of Maladaptive Eating and Exercise Behaviors in Young People.” JMIR mHealth and uHealth, 2019. pubmed.ncbi.nlm.nih.gov
13. U.S. Department of Health and Human Services. “Physical Activity Guidelines for Americans, 2nd edition: Executive Summary.” HHS, 2018. odphp.health.gov
14. Centers for Disease Control and Prevention. “How to Measure Physical Activity Intensity.” CDC, accessed October 2026. cdc.gov