How calories burned are calculated: METs, formulas and their limits
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
Most calorie-burn figures, from websites to gym machines, start from the same simple formula: an activity's MET value, multiplied by your body weight in kilograms, multiplied by the time in hours. It gives a reasonable average estimate, but real burn varies from person to person by 20 percent or more. The calories burned calculator uses this method; this guide explains how it works, where it goes wrong, and how much weight to put on the result.
What a MET is
A MET, or metabolic equivalent of task, compares the energy cost of an activity with sitting quietly. By convention one MET is 3.5 millilitres of oxygen per kilogram of body weight per minute, which works out at roughly 1 kilocalorie per kilogram per hour. An activity of 5 METs uses about five times the energy of sitting still. Researchers have assigned MET values to hundreds of activities in the Compendium of Physical Activities, first published in 1993 and updated several times since, and the calculator uses approximate values from it: about 3.5 for walking at 3 mph (4.8 km/h), 8 for moderate cycling at 12 to 14 mph, 9.8 for running at 6 mph (9.7 km/h), and 1.3 for sitting at a desk.
The formula and a worked example
The calculation is: calories = MET × body weight in kg × hours. Take a 70 kg (154 lb) person running at 6 mph for 30 minutes: 9.8 × 70 × 0.5 = about 343 kcal. The same person walking at 3 mph for 30 minutes uses 3.5 × 70 × 0.5, about 123 kcal.
The same table shows a useful rule of thumb for running. At 8 km/h the MET is 8.3, at 9.7 km/h it is 9.8 and at 12 km/h it is 11.8; divide each by the speed and running costs close to 1 kcal per kilogram of body weight per kilometre, whatever the pace. Running faster burns more per minute but covers the distance sooner. Walking costs less per kilometre, roughly 0.7 kcal per kg at a 3 mph pace. The steps calculator converts daily steps into distance and calories on the same principle.
Gross versus net calories
The basic formula gives gross calories, which include the energy you would have used anyway by sitting still. For setting exercise against food, the net figure is more honest: subtract one MET for the time spent. In the running example that is 8.8 × 70 × 0.5, about 308 kcal instead of 343. The difference is small for hard exercise but large for gentle activity: half of the gross figure for a slow walk is energy you would have burned sitting on the sofa. The calculator shows both, plus a per-hour figure, and lets you add activities into a running total for the day.
Where exercise fits in your daily total
Exercise is usually the smallest of the three parts of daily energy use. Basal metabolic rate, the energy that keeps you alive at rest, is the largest part for most people, commonly 60 to 70 percent of the total. Digesting food takes roughly another 10 percent. The rest is movement, including both planned exercise and everyday activity such as walking, standing and fidgeting. The calorie calculator estimates the whole daily figure, and How daily calorie needs are calculated — BMR, activity multipliers and what a deficit means explains its formulas.
Why trackers and machines disagree
MET values are averages measured in groups of people, and several things make individual results differ:
- The one-MET assumption. The standard resting value was originally based on one man. Measurements in larger groups have found it overestimates real resting energy use by around 20 to 30 percent on average, mainly because of differences in body composition.
- Fitness and technique. Experienced runners and swimmers are often more economical than beginners at the same speed.
- Terrain, wind and load: hills, soft ground and a backpack all raise the cost.
- Device models. Watches combine heart rate, motion and personal data using their own formulas. Heart rate rises with heat, stress and caffeine as well as effort, and studies of wearables have often found energy estimates to be the least accurate of their measurements.
Gym machines often do not know your weight unless you enter it, and may use gross rather than net figures.
Using the numbers sensibly
Treat any calories-burned figure as a ballpark. If you are managing weight, the most reliable approach is to look at your actual weight trend over several weeks alongside your food intake, and adjust from there rather than eating back every exercise calorie. If you do add exercise calories to a food budget, using the net figure, or half of a watch's estimate, is a reasonable safety margin. The calorie deficit calculator shows how a given daily gap translates into weight change over time. And remember that exercise brings benefits for the heart, mood, sleep and strength that have nothing to do with the calorie count.
Sources and further reading
The claims in this guide rest on these references, which were checked when the guide was last updated. Spotted an error? The contact page says how to report it.