Your body burns calories through three main channels: your resting metabolism, the energy used to digest food, and physical activity. Adding these together gives you your total daily energy expenditure, or TDEE. For most people, that number falls somewhere between 1,600 and 3,000 calories per day, depending on body size, age, sex, and how active you are. Calculating yours takes a simple two-step process: estimate your resting metabolic rate, then multiply it by an activity factor.
The Three Components of Daily Calorie Burn
Your resting metabolic rate (often called BMR) is the energy your body uses just to stay alive: breathing, circulating blood, maintaining body temperature, and keeping your organs running. In a mostly sedentary person, this accounts for roughly 60% of total daily calories burned. It’s the largest slice by far, which is why getting this number right matters most.
The thermic effect of food, meaning the calories your body spends digesting and absorbing what you eat, adds another 10 to 15%. Not all foods cost the same amount of energy to process. Protein is the most metabolically expensive, burning 20 to 30% of its own calories during digestion. Carbohydrates use 5 to 10%, and fat uses just 0 to 3%. This is one reason high-protein diets slightly increase total calorie burn.
Physical activity makes up the remaining 15 to 30%. This includes both structured exercise and all the smaller movements throughout your day: walking to the kitchen, fidgeting, standing, carrying groceries. That non-exercise movement (sometimes called NEAT) varies enormously between people and is the most adjustable part of your daily burn.
Step 1: Estimate Your Resting Metabolic Rate
The Mifflin-St Jeor equation is the formula recommended by the American Dietetic Association for most people. You’ll need your weight in kilograms, height in centimeters, and age in years. To convert: divide your weight in pounds by 2.2 for kilograms, and multiply your height in inches by 2.54 for centimeters.
For men: (9.99 × weight) + (6.25 × height) − (4.92 × age) + 5
For women: (9.99 × weight) + (6.25 × height) − (4.92 × age) − 161
As an example, a 35-year-old woman who weighs 70 kg (154 lbs) and stands 165 cm (5’5″) would calculate: (9.99 × 70) + (6.25 × 165) − (4.92 × 35) − 161 = roughly 1,400 calories per day at rest.
If You Know Your Body Fat Percentage
Standard formulas use total body weight, but muscle tissue burns significantly more energy than fat tissue. If you’re unusually muscular or you know your body fat percentage from a DEXA scan or similar measurement, the Katch-McArdle formula gives a more accurate result. First calculate your lean body mass (total weight minus fat weight), then plug it in:
BMR = 370 + (21.6 × lean body mass in kg)
This single equation works for both men and women because it’s based on metabolically active tissue rather than sex-based averages. A 90 kg man at 15% body fat has 76.5 kg of lean mass, giving him a BMR of about 2,022 calories. The same man at 30% body fat would have 63 kg of lean mass and a BMR of roughly 1,731, a difference of nearly 300 calories per day from the same scale weight.
Step 2: Multiply by Your Activity Level
Once you have your resting metabolic rate, multiply it by a physical activity level (PAL) factor that reflects your typical day. These multipliers are based on population-level research and account for both exercise and everyday movement.
- Sedentary (PAL 1.2): Desk job, little to no exercise
- Lightly active (PAL 1.4 to 1.5): Light exercise 1 to 3 days per week, or a job with some walking
- Moderately active (PAL 1.6 to 1.7): Exercise 3 to 5 days per week
- Very active (PAL 1.9+): Hard daily exercise or a physically demanding job
Using the example above, our 35-year-old woman with a BMR of 1,400 who exercises three times a week would multiply by about 1.6, giving a TDEE of roughly 2,240 calories per day. That’s her estimated total burn, including rest, digestion, and movement combined.
The trickiest part is choosing the right multiplier. Most people overestimate their activity level. If you work at a desk and hit the gym for an hour three times a week but are otherwise sitting, you’re closer to “lightly active” than “moderately active.” Be honest with yourself here, because the difference between a 1.4 and a 1.7 multiplier on a 1,500-calorie BMR is over 400 calories per day.
How Age and Body Composition Shift the Numbers
Your calorie burn isn’t static across your lifetime, but the timeline may surprise you. A major study published in Science found that metabolism stays remarkably stable from about age 20 through 60. The common belief that metabolism tanks in your 30s or 40s isn’t supported by the data. What typically happens in those decades is a gradual loss of muscle mass and a decrease in daily movement, both of which reduce calorie burn but aren’t inevitable.
After 60, resting metabolism does genuinely decline at about 0.7% per year, even after accounting for changes in body size. By age 90 and beyond, total energy expenditure is roughly 26% lower than in middle-aged adults. Children and teenagers, on the other hand, have metabolic rates about 30% higher than their body size alone would predict, fueling growth and development.
Muscle mass is the variable you have the most control over. Because lean tissue is metabolically active around the clock, gaining muscle raises your resting calorie burn permanently (as long as you maintain it). This is why two people of the same height, weight, and age can have meaningfully different metabolic rates.
How Accurate Are Fitness Trackers?
Wearable devices like Apple Watch and Fitbit estimate calorie burn using heart rate data, movement sensors, and algorithms. They’re convenient, but their accuracy is limited. A Stanford University study testing seven popular devices found calorie estimates ranged from 27% to 93% error compared to laboratory measurements. The most accurate device in the study, the Fitbit Surge, still overestimated by 27%. For walking and running, average error was around 31%. Cycling was worse at 52%.
This doesn’t mean trackers are useless. They’re reasonably good at tracking relative changes: if your watch says you burned more today than yesterday, that’s probably true. But the absolute number on the screen could easily be off by several hundred calories. If you’re using a tracker to set a calorie target for weight loss, treat its output as a rough estimate and adjust based on what actually happens to your weight over two to three weeks.
Clinical Metabolic Testing
If you want a precise measurement rather than an estimate, indirect calorimetry is the gold standard. You breathe into a mask or canopy hood for 15 to 30 minutes while a machine measures how much oxygen you consume and how much carbon dioxide you produce. Those values are plugged into a formula that calculates your actual resting energy expenditure.
For healthy people, the standard equations (like Mifflin-St Jeor) are reasonably close. Where they fall apart is in people with certain chronic conditions, those recovering from illness, or individuals with unusual body compositions. In critically ill patients, predictive equations agree with indirect calorimetry results only about 55% of the time. Some gyms, nutrition clinics, and sports medicine offices offer metabolic testing for $75 to $250, and it can be worth the investment if formula-based estimates haven’t matched your real-world results.
Putting It All Together
The simplest path to your number: plug your stats into the Mifflin-St Jeor equation, pick an honest activity multiplier, and you’ll have a solid starting estimate. Use it as a baseline, not a fixed target. Track your weight for two to three weeks while eating a consistent amount. If your weight stays stable, your TDEE estimate is close. If you’re gaining, your actual burn is lower. If you’re losing, it’s higher. That real-world feedback loop is ultimately more reliable than any formula, but the formula gives you a sensible place to start.