Do diet apps and smartwatches help you lose weight?

Short answer

A little. Apps made a weight difference only at 4 to 6 months, and trackers added about 1,800 steps a day and about 1 kg of weight loss. What made the difference was not the device but how many times a day people logged.

Do diet apps and smartwatches help you lose weight?
'Track → notice → lose' — does this formula hold?
You log meals in an app and check steps and calories burned on your watch. The hope is that seeing the numbers makes weight come off. We checked studies that compared the weight of people who used apps and watches with people who did not.
Apps alone: a difference at 4 to 6 months
About 2.3 kg for someone 175 cm tall
An analysis pooled 11 trials of app-only weight programs with 1,717 people. At 4 to 6 months, app users lost more weight and their BMI was 0.76 lower, about 2.3 kg for someone 175 cm (5 ft 9 in) tall. Before 3 months and at 9 to 12 months there was no clear difference.
Past a year, the gap disappeared
No clear difference at 12 or 24 months
A 2024 analysis of 18 trials and 2,703 people found no clear difference in weight change at 12 months between app users and people who got little or nothing. In a trial that followed 245 people for 24 months, the gap was 0.7 kg. Between 5% and 62% of people dropped out, depending on the trial.
Watches and trackers added steps
About 1,800 steps a day, about 1 kg of weight
A review that pooled 39 analyses covering 163,992 people found that wearing an activity tracker added about 1,800 steps and about 40 minutes of walking a day. Weight dropped by about 1 kg. Walking clearly went up, but the weight change was small.
Adding a wearable to a diet program?
People lost 2.4 kg less
471 people followed a diet and exercise program. From month 6, half tracked food and activity with a wearable device and half with a website. After 24 months, the device group had lost 3.5 kg and the website group 5.9 kg. Body composition, fitness and diet improved with no difference between groups.
Calorie-burn numbers were well off
No device got within 20%
When 60 people wore 7 wrist devices while walking, running and cycling, not one device estimated calories burned with less than 20% error. A 2024 review of validation studies found errors from -21% to +15%. With a 20% error, 500 kcal on the watch means anywhere from 400 to 600 kcal.
What mattered was how often people logged
4 times a day vs 1.6
Among 142 people logging food online for 24 weeks, those who lost 5% or more of their weight opened their food log 2.4 times a day; those who did not, 1.6 times. Time spent logging fell from 23.2 minutes a day in month 1 to 14.6 minutes by month 6, and time was not linked to results.
Bottom line: apps and watches work as a log you open often
Just wearing one made little difference
Apps gave about 2.3 kg at 4 to 6 months and watches about 1 kg, and past a year the app effect was not clear. Calorie-burn figures were off by around 20%. So read calorie numbers as rough estimates and use the tools for short, frequent logging through the day.