Calories burned are calculated by combining three components: the energy your body uses at rest, the energy spent digesting food, and the energy used during physical activity. Every calorie calculator, fitness tracker, and nutrition label works from this same basic framework, though the specific methods range from simple math formulas to sensor-packed wristbands. Here’s how each piece works.
Your Body Burns Most Calories at Rest
The biggest chunk of your daily calorie burn, roughly 60 to 70%, comes from simply keeping your body alive. Breathing, circulating blood, maintaining body temperature, repairing cells: all of this costs energy. This baseline is called your resting metabolic rate (RMR), and it’s the starting point for virtually every calorie calculation.
The most widely recommended formula for estimating RMR is the Mifflin-St Jeor equation. The Academy of Nutrition and Dietetics rates it as the most accurate option when you can’t measure metabolic rate directly in a lab. It uses four inputs: weight in kilograms, height in centimeters, age in years, and sex.
- For men: (10 × weight) + (6.25 × height) − (5 × age) + 5
- For women: (10 × weight) + (6.25 × height) − (5 × age) − 161
So a 35-year-old woman who weighs 70 kg (about 154 lbs) and stands 165 cm (5’5″) would calculate: (10 × 70) + (6.25 × 165) − (5 × 35) − 161 = 1,370 calories per day at rest. That’s before any movement or digestion is factored in.
This formula lands within 10% of lab-measured values for about 70% of people. It tends to underestimate more often than it overestimates, meaning your actual resting burn could be slightly higher than the number you get. The gap comes from things the equation can’t see, especially body composition. A kilogram of muscle burns roughly 10 to 15 calories per day at rest, while the same weight in fat burns far less. Two people with identical height, weight, age, and sex can have meaningfully different metabolic rates if one carries more muscle.
How Activity Calories Are Calculated
Physical activity is the most variable part of your daily burn and the part most people are trying to track. The standard method uses a unit called a MET, or metabolic equivalent. One MET equals the energy you use sitting quietly, about 1 calorie per kilogram of body weight per hour. Every activity has a published MET value: walking at a moderate pace is around 3.5 METs, running at 6 mph is about 10 METs, and cycling at a leisurely pace is roughly 4 METs.
The formula to turn METs into calories is straightforward:
Calories per minute = 0.0175 × MET value × body weight in kilograms
To convert pounds to kilograms, divide by 2.2. If you weigh 80 kg (176 lbs) and run at 10 METs for 30 minutes, the math is: 0.0175 × 10 × 80 = 14 calories per minute, multiplied by 30 minutes, giving you 420 calories for that run.
This calculation is what most online calorie calculators use behind the scenes. You select an activity, enter your weight and duration, and the tool plugs in the corresponding MET value. The limitation is that MET values are population averages. They don’t account for your fitness level, how hard you’re actually working, or environmental factors like heat or altitude. A well-trained runner and a beginner running the same pace for the same time will get the same MET-based estimate, even though the beginner is working harder and likely burning more.
What Digestion Adds to the Total
Your body spends energy breaking down and absorbing food, a cost known as the thermic effect of food. This typically accounts for about 10% of your total daily calorie burn, but the exact amount depends heavily on what you eat.
Protein costs the most to digest, increasing your metabolic rate by 15 to 30% of the calories consumed. If you eat 200 calories of protein, your body uses 30 to 60 of those calories just processing it. Carbohydrates raise your metabolic rate by 5 to 10%, and fats by just 0 to 3%. This is one reason high-protein diets can slightly increase total daily calorie burn compared to diets higher in fat, even when total calorie intake is the same.
Most calorie calculators don’t ask you to enter what you ate. They simply assume the thermic effect adds roughly 10% on top of your resting and activity calories. It’s a reasonable average, but it means the total can shift by a few percentage points depending on your diet’s macronutrient balance.
How Fitness Trackers Estimate Your Burn
Wearable devices like the Apple Watch, Fitbit, and Garmin use a combination of sensors and algorithms to estimate calories throughout the day. The two key sensors are an accelerometer, which measures movement in three dimensions, and a photoplethysmography (PPG) sensor, the green light on the back of the watch that reads your heart rate through the skin.
Movement data tells the device how active you are, how many steps you’ve taken, and the intensity of your motion. Heart rate adds a layer of metabolic information because your heart beats faster as your body demands more oxygen to fuel activity. The device’s algorithm filters out motion artifacts from the heart rate signal (your arm swinging while running can distort the reading) by subtracting the accelerometer’s movement pattern from the raw PPG data. What’s left is a cleaner heart rate estimate that the software can pair with your profile information, including age, weight, height, and sex, to estimate calorie expenditure in near real time.
The accuracy of these estimates, however, has consistent limitations. A 2025 study from the University of Mississippi found that wearable devices had a mean error of about 28% for energy expenditure, compared to just 4.4% for heart rate and 8.2% for step counts. In practical terms, if your watch says you burned 500 calories during a workout, the actual number could reasonably be anywhere from 360 to 640. The error tends to be larger during activities that don’t involve rhythmic arm movement, like cycling or weight training, where the accelerometer has less useful data to work with.
Putting It All Together
When you see a “total daily energy expenditure” number from an app or calculator, it’s combining all three components. It starts with your resting metabolic rate (estimated from a formula using your age, weight, height, and sex), adds the thermic effect of food (usually estimated at 10%), and layers on activity calories (from METs, heart rate data, or a general activity multiplier). Many simpler calculators skip the thermic effect and just multiply your RMR by an activity factor: 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, and so on.
No method available outside a laboratory is perfectly accurate. The Mifflin-St Jeor equation misses by more than 10% for roughly 3 in 10 people. MET-based calculations ignore fitness level and body composition. Wearable devices add real-time data but introduce their own sensor errors. The most reliable approach is to use any of these methods as a starting point, then adjust based on what actually happens to your weight and energy levels over two to three weeks. If you’re losing weight faster than expected, you’re eating below your true burn. If nothing is changing, the estimate is probably too high. The formulas give you a solid first draft. Your body gives you the final answer.