Can you trust your smartwatch? A deep dive into calorie burn estimates

For many of us, our smartwatch is almost always on our wrist, monitoring our sleep, steps, heart rate, workouts, and even our calorie burn. We use it to provide insights into our health, fitness, and recovery. In fact, they often influence how hard we train, when we rest, and how much we eat. But how accurate are our fitness trackers, and can we trust them to determine our eating habits?

How Fitness Trackers Estimate Calories

To understand how accurate fitness trackers are, we first need to look at how they estimate caloric burn. These devices use a combination of sensors, algorithms, and user data to calculate an individual’s total daily energy expenditure (TDEE).

To start, these devices approximate your Basal Metabolic Rate (BMR) — a measure of the number of calories your body burns while at rest, maintaining only essential functions like breathing and blood circulation. Most fitness trackers estimate BMR by using an algorithm called the Mifflin-St. Jeor equation that accounts for user inputs like age, sex, weight, and height. Some trackers may also use other, more advanced measures of fitness like VO₂ max to calculate a more accurate BMR.

Once the BMR calculation is complete, it becomes the basis for estimating Total Daily Energy Expenditure (TDEE), which reflects the total number of calories you burn in a day. TDEE is made up of four components:

Basal Metabolic Rate (BMR): energy burned at rest

Non-Exercise Activity Thermogenesis (NEAT): energy burned through daily activities like walking, talking, cleaning, and fidgeting

Thermic Effect of Food (TEF): energy used during digestion, absorption, and processing of nutrients

Exercise Activity Thermogenesis (EAT): energy expended during planned and intentional exercise or sport

To estimate the non-BMR components of the TDEE equation, fitness trackers use a combination of heart rate technology, motion sensors, GPS, and other more specific features. Many devices have specific algorithms that use these data points to present an estimate of “active calories” — a measurement of calories burned during physical activity. The device adds its estimates for active calories and BMR to produce an approximate total daily caloric burn.

Can you really trust the calorie estimates on your smartwatch? Find out as we take a deep dive into all things calorie tracking and wearable technology. #saslife
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The Accuracy Problem

Although it may seem straightforward, there is a lot of nuance in caloric expenditure estimation. The research consistently shows that calorie burn estimates are highly variable and often inaccurate. Because of these discrepancies, wearables cannot reliably reflect your true energy output and should be used cautiously—especially when informing nutrition decisions like how much to eat.

So, how inaccurate are these devices when it comes to measuring calorie burn?

A large review looked at many studies on wearable fitness trackers and found that calorie estimates were often inaccurate across the 22 brands and 36 devices. On average, the devices over- or underestimated energy burn by more than 30%. Although some devices were found to be more accurate in controlled environments, with error readings as low as 3%, none of the devices were considered reliably accurate in real-world conditions. In fact, the error measurements exceeded the 10% standard for all devices tested in day-to-day scenarios.

Researchers have also looked at how accurate fitness trackers are during different activities, and the results are mixed. One review found that trackers were more accurate during consistent activities like walking and running, especially in controlled lab settings. Irregular movements like cycling and household chores provided less reliable estimates. Still, overall, these estimates were inconsistent and generally unreliable.

Another study tested three devices at different running paces and estimates were again inconsistent. One device overestimated calorie burn at every pace. The second underestimated at lower speeds and overestimated at higher speeds, and the third showed the exact opposite pattern. Even during simple activities with steady movement, devices showed different results.

Pitfalls of Relying on Wearables

Let’s talk about some common ways that people use data from their wearables to guide their eating habits. Since our smartwatches often overestimate our calorie burn, relying on this number to inform our food choices and determine our caloric intake can lead to consistent overeating, which may not align with our personal goals and could lead to undesired weight gain.

Alternatively, some watches underestimate calories burned. This could lead to undereating and unintentional weight loss, leaving us under-fueled and fatigued, giving us inadequate energy for our daily activities and workouts. Either way, depending too heavily on these numbers can disrupt our body’s natural energy balance.

More importantly, inaccurate calorie estimations may also cause us to dismiss important cues like hunger, fullness, and fatigue. These signals exist to help us respond to what our bodies actually need — something that our watches cannot fully replicate. When we listen more to our watches than ourselves, we risk missing out on valuable feedback from the body that supports energy, recovery, and overall well-being.

Using Calorie Data Mindfully

It’s clear that wearables aren’t giving us precise calorie burn estimates—and relying on them too heavily can disconnect us from our body’s natural cues. But that doesn’t mean the data is useless. So, how can we use it more mindfully when it comes to nutrition?

The great news is that our wearables are collecting data over time as we wear them. This gives us a great way to look at trends in our overall activity rather than daily fluctuations. For example, if your estimated calorie burn increases over several weeks, that likely indicates an increase in physical activity.  We can use this information, along with our body’s natural signals, to inform our eating habits.

Consistency is key! Instead of using daily energy expenditure to determine your energy intake, aim for a regular eating schedule and adapt as needed throughout the week based on your body’s cues. Maintaining a steady average intake over a long period is much easier for your body to handle than an unpredictable eating schedule based on your watch data. This can help to promote healthy digestion and reduce stress by regulating appetite, blood sugar, and circadian rhythm, and reducing metabolic strain.

Although not for everyone, activities like food journaling and reflection can help us to increase awareness. Tracking how we feel before and after meals, around our workouts, and when we wake up can reveal relationships between our food, mood, energy, and overall health. Over time, by combining tracking with increased body awareness, we can use our calorie trend data to help guide more balanced food choices without getting caught up in the imperfect numbers.

You’re Smarter Than Your Smartwatch

Wearable technology is a great tool that provides us with daily insight into our health, but not all data is created equal. Calorie estimates from our smartwatches are often inaccurate, making them unreliable when it comes to understanding our energy needs. We’re much better off when we listen to our bodies, pay attention to long-term trends in our smartwatch data, and use other, more dependable data sources to inform our food choices. At the end of the day, your body is the most reliable data source you have, and it’s always worth listening to.

About the Author

Sarah Frost, Dietetic Intern

Sarah holds a BS in Applied Nutrition from NC State University and an MPH in Nutrition and Dietetics from UNC-Chapel Hill. Before her internship at SAS, she was a Research Assistant at UNC-Chapel Hill, interned at St. Francis Medical Center in Richmond and worked as a Community Health Education Programs Coordinator at the Inter-Faith Food Shuttle.

Pre-Workout 5-Ingredient Granola Bars

Recipe Adapted From: Minimalist Baker
Makes 10 bars
PRINT RECIPE

Ingredients
1 heaping cup packed dates, pitted (deglet noor or medjool*)
¼ cup maple syrup, agave nectar or honey
¼ cup creamy natural peanut butter or almond butter
1 cup roasted unsalted almonds, loosely chopped
1 ½ cups rolled oats (gluten-free for GF eaters)
Optional additions: nuts, seeds, dried fruit, vanilla, etc.

Instructions

Process dates in a food processor until small bits remain (about 1 minute). It should form a "dough" like consistency.

Optional step:Toast your oats (and almonds, if raw) in a 350˚F (176˚C) oven for 10-15 minutes or until slightly golden brown. Otherwise, leave them raw.

Place oats, almonds and dates in a large mixing bowl – set aside.

Warm maple syrup (or agave or honey) and peanut butter in a small saucepan over low heat. Stir and pour over oat mixture and then mix, breaking up the dates to disperse throughout.

Once thoroughly mixed, transfer to an 8×8-inch baking dish or other small pan lined with parchment paper so they lift out easily.

Press down firmly until uniformly flattened – use something flat, like a drinking glass, to press down and really pack the bars, which helps them hold together better.

Cover with parchment paper and let firm up in fridge or freezer for 15-20 minutes.

Remove bars from pan and chop into 10 even bars. Store in an airtight container for up to 3-4 days or store them in the freezer for up to 1 month to keep them extra fresh.

* Although not ideal, if your dates don’t feel sticky and moist, you can soak them in water for 10 minutes then drain before processing. This will help them blend better and hold the bars together better. But ideally, you can find fresh, sticky moist dates.