If you’ve ever checked your smartwatch and wondered if you actually burned the number of calories you see on the screen, you’re not alone.

A lot of people use those numbers to guide their daily nutrition and fitness habits, but the problem is that most wearables aren’t very good at estimating calorie burn. A team at Stanford tested the accuracy of 7 of the most popular fitness trackers and smartwatches, and the results were eye-opening.

This doesn’t mean wearables are useless, but the helpful parts are different from what you might assume. 

How Stanford Researchers Put Your Smartwatches to the Test 

Researchers brought 60 adults into the lab for a structured experiment. The group was intentionally diverse in age, size, fitness level, skin tone, and gender. Participants completed a series of activities:

• Sitting

• Walking

• Running

• Cycling

During each trial, they wore up to seven consumer devices, including the Apple Watch, Fitbit Surge, Samsung Gear S2, Microsoft Band, and others. At the same time, the researchers measured their heart rate and energy expenditure using clinical-grade tools. These tools acted as the baseline for comparison.

The devices were tested on two things:

• Heart rate

• Energy expenditure (calories burned)

The study then compared how close the wearables came to the measurements taken using clinical-grade instruments.

What the Study Found

Using the clinical tool measurements as a baseline, all the smartwatches tested were consistently better at tracking heart rate compared to calories burned. Error margins varied a bit based on how much arm movement was involved. 

In this context, “margin of error” simply refers to how far off a device’s reading is from the real, lab-measured value. A small margin of error means the number is close to accurate, while a large margin means it could be significantly higher or lower than the truth.

Heart Rate: Mostly Accurate

Takeaway: Smartwatch heart rate reliability is high across nearly all activities. Running is the only place where accuracy dips slightly, but not enough to make the data unusable. 

Looking at the graph below, it’s easy to see that all the smartwatches tested were mostly accurate in measuring heart rate, with the greatest margins of error occurring during the walking and running tests. 

Calories Burned: Consistently Inaccurate

Takeaway: Calories burned aren’t trustworthy in any activity. Even the best-case scenarios fall short of being accurate.

What This Means for Your Workouts

Instead of relying on calorie burn numbers to guide decisions, it’s better to focus on the data your wearable handles well. There is still plenty of value in the information these devices provide. You just have to know what to use and what to ignore.

Trust your smartwatch for:

• Heart rate zones

• Workout intensity

• Step counts

• Activity streaks

• General movement patterns

Heart rate gives you a reliable sense of how hard you are working. Steps and streaks help track your consistency. These metrics can support a strong foundation for long-term habits.

Be cautious with:

• “Active calories”

• “Total daily calories burned”

• Calories “earned back” from exercise

Smarter Ways to Use Your Smartwatch

Here are a few practical strategies based on what the research shows.

1. Use heart rate to guide your workouts.
   Pay attention to intensity zones rather than calorie totals. This helps keep training effective and purposeful.

2. Track behaviors, not burn.
   Steps. Minutes of movement. Daily consistency. These patterns matter far more than estimated calories.

3. Build habits around patterns you can trust.
   If your device shows you move less on weekends or sit for long periods in the afternoon, use that information to shift your routine.

4. Pay attention to trends, not precise numbers.
   These trends help you understand whether your routine is working, and you can adjust your nutrition based on real progress rather than estimated burn numbers.

The Bottom Line

Smartwatches are useful tools, as long as you understand what they measure well. Heart rate data is trustworthy enough to help guide the intensity and structure of your workouts. Calories burned, on the other hand, aren’t accurate enough to rely on.

This is good news. You don’t need perfect calorie numbers to build strong habits. You need consistency, awareness, and a few reliable metrics that help you show up each day.

The biggest takeaway from this study is that your smartwatch is really good at giving you data to map the scope of your long-term progress, even if the exact accuracy of some metrics isn’t reliable. 

Use your smartwatch for the things it does well, ignore the misleading numbers, and focus on the behaviors that actually move you toward your goals.