By Peter Conroy, Founder & CEO, The Difference App · October 17, 2026
If you’ve ever finished a workout and gotten two wildly different “calories burned” numbers from different devices (apps, wearables, or equipment like a treadmill), you may already question their accuracy. You’re right to. It’s a guess. But these devices aren’t trying to be malicious. They just don’t know you well enough. Instead, they use averages, population-wide assumptions, and generic formulas.
People want an app that gives them an accurate activity calorie burn, not generic formulas. They want a tool that considers more of what makes them unique but can still do automatic, real-time calculations. They want a calorie estimate grounded in real exercise physiology, built on science and honest about its own limits of personalization.
That’s why we built Activity Burn, a Premium feature of The Difference App. It uses your weight, height, gender, and age to personalize the intensity level of whatever you’re doing, so the calories you see reflect you, not a hypothetical average person. Here’s why most estimates miss, and how we approached the problem.
What’s Wrong With Just Guessing at Activity Calorie Burn?
Technology can measure activity calorie burn in many ways, including indirect calorimetry, heart rate, steps, and intensity level. However, all of these methods can have significant error. Better estimates incorporate an individual’s unique qualities.
The measurement gap matters to anyone trying to manage their weight. A 20% to 30% error in estimating activity calorie burn can meaningfully distort total daily energy expenditure (TDEE), especially when it’s compounded day after day. For example, someone trying to lose weight could overeat by 100 to 150 calories a day if they overestimate their activity calorie burn by 20% to 30%.
Indirect Calorimetry: The Gold Standard, but Impractical
Indirect calorimetry is the gold standard for measuring calories burned during activity and exercise. It uses a medical-grade metabolic cart to measure the oxygen consumed and carbon dioxide produced during exercise in a closed test environment. The test is called a cardiopulmonary exercise test (CPET), also known as a metabolic stress test or VO₂ max test.
However, the equipment is only available in a laboratory, and the test itself can cost as much as $300. While this method is accurate, it’s impractical for estimating calorie burn in everyday life. No app will ever come with a mask, tubes, a treadmill, and a medical cart to pull.
Heart Rate: Easy to Measure, Hard to Turn Into Calories
Some apps, through wearables, measure heart rate extremely well. Accuracy falls short when formulas translate heart rate into calories burned. A widely cited Stanford study benchmarked several popular wrist-worn devices against a medical-grade metabolic cart and found that estimating energy expenditure could be off by as much as 93%, depending on the device and the activity (1).
Simply put, while heart rate can be relatively easy to accurately measure, it isn’t the same thing as energy expenditure. Nevertheless, some consumer technology quietly treats them as interchangeable.
Steps: Great for Counting, Not for Calories
Apps do a great job counting steps. Some stop at counting steps, and maybe they should, because estimating calorie burn from steps is difficult. Our review of research on how apps estimate energy expenditure found that step count alone is inadequate for calculating activity calories. Similar step counts can reflect very different energy costs, such as walking on level ground versus climbing stairs or hills.
One study found that combining phone motion data with elevation readings and personal details may improve calorie estimates compared with steps alone. The takeaway: apps should present calorie results as estimates, and stronger models draw on several signals rather than steps alone.
Intensity Level (METs): How Most Apps Estimate Calorie Burn
Ever wondered how most apps estimate activity calorie burn? Most base it on intensity level. Large studies have assigned intensity levels to a wide variety of activities and exercises, in units called metabolic equivalents (METs).
Treadmills and stationary bikes often detect intensity automatically from speed, incline and resistance. Apps can do the same with data from wearables and mobile devices. Most apps go further and let you select the exact activity you did, from household chores and occupations to daily living, exercise and sports. But how does an intensity level tell you how many calories you burned? Let’s look closer.
How Are METs Used to Measure Activity Calorie Burn?
What Is a MET?
A MET (Metabolic Equivalent of Task) is a ratio of how much energy an activity burns compared with sitting completely still. One MET is roughly 1 kilocalorie per kilogram of body weight per hour. An activity rated at 6 METs burns energy at six times that resting rate.
Researchers measured METs through indirect calorimetry. They measured people’s actual oxygen consumption and carbon dioxide output while they performed an activity in a lab, then averaged the results across many participants and studies. This research was published in the Compendium of Physical Activities, first released in 1993 (2).
The MET Scale: From Sitting Still to Olympic Running Pace
MET values in the Compendium range from about 1 (sitting quietly) to roughly 23 (running at 14 miles per hour, a 4:20-per-mile pace). Almost everything you’d recognize as daily life or exercise falls somewhere in between (3):
| Activity | METs |
|---|---|
| Sitting quietly | 1.0 |
| Laundry | 2.3 |
| Yoga (Hatha) | 2.3 |
| Walking, 2.5 mph | 3.0 |
| Bicycling, < 10 mph | 4.0 |
| Walking, 3.5 mph | 4.8 |
| Swimming Laps, Recreational | 5.8 |
| Tennis (General) | 6.8 |
| Jogging (General) | 7.5 |
| Basketball (Full Game) | 8.0 |
| Elliptical Trainer, Vigorous Effort | 9.0 |
| Bicycling, 14 to 15.9 mph | 10.0 |
| Running, 7 mph (8:30/mile) | 11.0 |
| Running, 14 mph (4:20/mile) | 23.0 |
The Compendium Just Got Its Biggest Update in Over a Decade
The Compendium of Physical Activities was substantially revised after nearly a decade and a half (3). The update introduced hundreds of new official activities that weren’t measured before. Just as significantly, hundreds of existing activities had their intensity ratings revised based on newer research.
Some moved up, like competitive trampoline jumping, from 4.5 to 10.3 METs. Others moved down, like an “easy pace” bike ride, from 7.5 to 4.3 METs. These aren’t small rounding corrections. They can meaningfully shift calorie burn estimates for activities millions of people log every day. The Difference has updated its activity records with these new activities and MET values.
Calculating Calories From METs: The Simple Math
Have you ever peeked under the hood of your fitness app? You might be surprised how antiquated the technology is: decades out of date. Here’s the original formula, which many apps still use as a rough approximation:
𝐂𝐚𝐥𝐨𝐫𝐢𝐞𝐬 𝐩𝐞𝐫 𝐌𝐢𝐧𝐮𝐭𝐞= 𝐌𝐄𝐓𝐬 × 𝐁𝐨𝐝𝐲 𝐖𝐞𝐢𝐠𝐡𝐭 𝐢𝐧 𝐤𝐠 ÷ 𝟔𝟎.
Later, since 1 MET equals 3.5 milliliters of oxygen per kilogram of body weight per minute, and the body burns about 5 kilocalories per liter of oxygen, the formula was rewritten by multiplying by 3.5 milliliters and then by 5 kilocalories per 1,000 milliliters:
𝐂𝐚𝐥𝐨𝐫𝐢𝐞𝐬 𝐩𝐞𝐫 𝐌𝐢𝐧𝐮𝐭𝐞= 𝐌𝐄𝐓𝐬 × 𝟑.𝟓 × 𝐁𝐨𝐝𝐲 𝐖𝐞𝐢𝐠𝐡𝐭 𝐢𝐧 𝐤𝐠 ÷ 𝟐𝟎𝟎.
So a 154-pound (70 kg) person walking at 3.5 mph is doing an activity rated at 4.8 METs. They burn roughly (4.8 × 3.5 × 70) ÷ 200 = 5.9 calories per minute. Walking for 30 minutes, they’d burn about 176 calories by this simple formula.
These formulas are handy because you can do the math quickly. Simple, right? Deceptively so. If you think other factors affect how many calories you burn, including major ones like height, age, and gender, you’re 100% correct. Nevertheless, many technologies stop there.
The 3.5 Constant: The Major Culprit in the Equation
That “3.5” in the formula is a fixed constant. It represents one assumption about resting oxygen consumption, applied identically to every person who has ever used the formula, regardless of age, sex, body composition, or fitness level.
The newer formula looks more precise because it’s more complicated. But we took a deeper dive into the old versus new formula and found that it simply multiplies METs by a different constant. Dividing 3.5 by 200 yields 0.0175, whereas the original formula uses 1 ÷ 60, or about 0.0167. The two differ and unilaterally increase calories burned by only 5%, and neither one knows anything about you.
That’s the gap our Premium Activity Burn feature is built to close. It uses your actual profile (weight, height, gender, and age) to personalize the intensity level for whatever you’re doing. So the calculation reflects your physiology rather than that of a hypothetical generic person.
Real example: On the same bike ride, a generic estimate showed 787 calories. Activity Burn, a premium feature of The Difference, personalized to the rider’s profile, showed 502 calories. This is nearly a 300-calorie difference from a single workout. Such an inaccuracy can really add up over days, weeks, and months.
Why Getting the Number Right Actually Changes Behavior
This isn’t just scientific tidiness. Research on self-monitoring consistently shows that people who track their behavior more accurately and consistently see better weight management outcomes. Moreover, the quality of data tracked matters as much as the habit of tracking itself (4). An activity calorie burn you can trust isn’t just a nicer feature. It’s a more useful tool for the goal you’re tracking it for.
How The Difference Calculates Accurate Activity Calorie Burn
The Difference’s Premium Activity Burn feature shows you the real impact of your exercise. It combines the latest, largest, and most comprehensive activity database with some of the most advanced formulas available, personalized to your weight, height, gender, and age.
And because no reference library, however up to date, covers everything a real, varied life includes, The Difference also lets you add your own custom activities and workouts to a crowd-sourced database alongside hundreds of official, research-backed entries.
Activity Burn works alongside True Burn, The Difference’s formula that learns your real daily calorie burn, so you can see what you’ll weigh tomorrow based on what you eat and do today.
Frequently Asked Questions
How accurate are fitness tracker calorie burn numbers?
Less accurate than most people think. A Stanford study found that wrist-worn devices measure heart rate well, but their energy expenditure estimates could be off by as much as 93%, depending on the device and activity (1). Most trackers turn heart rate or steps into calories with generic formulas.
Why don’t the calorie counts from my devices match?
Each device uses its own formula and its own assumptions about an “average” person. A treadmill, smartwatch, and app may each estimate differently, and few account for your weight, height, gender, and age together.
How do I know my real calorie burn, not a generic estimate?
The most accurate method, indirect calorimetry, requires a lab. For everyday tracking, use an app that personalizes the intensity of each activity to your own profile and uses current MET values, like The Difference’s Activity Burn.
Which app gives accurate activity calorie burn, not generic formulas?
The Difference App. Its Premium Activity Burn feature personalizes calorie burn using your weight, height, gender, and age, with activity values from the 2024 Compendium of Physical Activities. Premium costs $24.99/year on iOS and Android.
Stop Guessing: Get to Know Your Real Activity Burn
If you’re ready to discover your real activity calorie burn and stop guessing at the impact of your activities and exercise, download The Difference App on iOS or Android and try Activity Burn with your Premium subscription.
References
(1) Shcherbina A, et al. “Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort.” Journal of Personalized Medicine, 2017. https://doi.org/10.3390/jpm7020003.
(2) Ainsworth BE, et al. “Compendium of Physical Activities: Classification of Energy Costs of Human Physical Activities.” Medicine & Science in Sports & Exercise, 1993. https://doi.org/10.1249/00005768-199301000-00011.
(3) Herrmann SD, et al. “2024 Adult Compendium of Physical Activities.” Journal of Sport and Health Science, 2024. https://doi.org/10.1016/j.jshs.2023.10.010.
(4) Burke LE, Wang J, Sevick MA. “Self-Monitoring in Weight Loss: A Systematic Review of the Literature.” Journal of the American Dietetic Association, 2011. https://doi.org/10.1016/j.jada.2010.10.008.
