Your body burns calories through three main channels: your resting metabolism, the energy needed to digest food, and physical activity. Resting metabolism alone accounts for 60 to 70% of your total daily burn. To estimate your personal number, you need a formula for that baseline metabolism, then a multiplier that factors in how active you are.
The Three Parts of Your Daily Calorie Burn
Your total daily energy expenditure (TDEE) isn’t just about exercise. It breaks down into three components that work together:
- Resting metabolic rate (RMR): The calories your body uses just to keep you alive, powering your brain, heart, lungs, and cells. This is the biggest slice, making up roughly 60 to 70% of your total burn.
- Thermic effect of food: Digesting and absorbing nutrients costs energy, about 10% of your daily total. Protein is the most expensive to process (15 to 30% of the calories in protein go toward digestion), followed by carbohydrates (5 to 10%) and fats (0 to 3%).
- Physical activity: Everything from formal exercise to walking around your kitchen. This is the most variable piece and the one you have the most control over.
A surprising amount of your activity calories come not from workouts but from everyday movement: fidgeting, standing, walking to the mailbox, carrying groceries. Researchers call this non-exercise activity thermogenesis, or NEAT. According to Mayo Clinic researcher Dr. James Levine, NEAT can differ by up to 2,000 calories per day between two people of similar size. That gap explains why someone with an active job and a habit of pacing during phone calls burns far more than someone who sits most of the day, even if neither person sets foot in a gym.
Step 1: Estimate Your Resting Metabolism
The most widely recommended formula is the Mifflin-St Jeor equation. A systematic review in the Journal of the American Dietetic Association found it was the most reliable option, predicting resting metabolic rate within 10% of the measured value in more people than any other common equation.
Here’s how it works:
- For men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- For women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
You’ll need to convert your measurements if you think in pounds and inches. Divide your weight in pounds by 2.2 to get kilograms, and multiply your height in inches by 2.54 to get centimeters.
As a quick example: a 35-year-old woman who weighs 150 pounds (68 kg) and stands 5’6″ (167.6 cm) would calculate (10 × 68) + (6.25 × 167.6) − (5 × 35) − 161, which comes out to about 1,392 calories per day at rest.
If You Know Your Body Fat Percentage
The Mifflin-St Jeor equation uses total body weight, which means it doesn’t distinguish between muscle and fat. If you’re particularly muscular or carry more body fat than average, the Katch-McArdle formula can give you a better estimate because it uses lean body mass instead. Lean tissue is more metabolically active than fat, so this approach adjusts for that difference.
The formula: 370 + (21.6 × lean body mass in kg)
To find your lean mass, you need your body fat percentage. If you weigh 180 pounds and carry 20% body fat, your fat mass is 36 pounds and your lean mass is 144 pounds, or about 65.5 kg. Plugging that in gives you 370 + (21.6 × 65.5) = roughly 1,785 calories at rest. You can get your body fat percentage from a DEXA scan, a body composition scale (less accurate but convenient), or skinfold calipers.
Step 2: Factor In Your Activity Level
Once you have your resting metabolic rate, multiply it by an activity factor to get your total daily burn:
- Sedentary (desk job, little to no exercise): multiply by 1.2
- Lightly active (light exercise 1 to 3 days per week): multiply by 1.375
- Moderately active (moderate exercise 3 to 5 days per week): multiply by 1.55
- Very active (heavy exercise 6 to 7 days per week): multiply by 1.725
- Extremely active (intense training twice daily or a hard labor job): multiply by 1.9
Using the example above, if that 35-year-old woman exercises moderately three to five days a week, her estimated TDEE would be 1,392 × 1.55 = about 2,158 calories per day. That’s the number she’d need to eat to roughly maintain her current weight.
The hardest part of this step is being honest about your activity level. Most people overestimate. If you work out three times a week but spend the rest of your day sitting, “lightly active” is probably more accurate than “moderately active.” The labels describe your overall lifestyle, not just your gym sessions.
How Accurate Are These Estimates?
Even the best formula is still an estimate. The Mifflin-St Jeor equation predicts within 10% of measured values for most people, but it can be less accurate for older adults, certain ethnic groups, and people at the extremes of body weight. A 10% error on a 2,000-calorie estimate means your actual number could be anywhere from 1,800 to 2,200. That’s a meaningful range if you’re trying to lose or gain weight.
Activity multipliers add another layer of imprecision. They’re broad categories, and the difference between a 1.2 and a 1.375 multiplier on a 1,500-calorie RMR is over 250 calories per day. Small differences in how much you move outside of exercise, how physical your job is, or even how much you fidget can shift your real number within or between categories.
The most precise way to measure your resting metabolic rate is a clinical test called indirect calorimetry, where you breathe into a device that analyzes your oxygen consumption. It’s available at some hospitals, sports medicine clinics, and nutrition practices. It’s not cheap, and the standard predictive equations are accurate enough for most people’s purposes.
What About Fitness Trackers?
Wrist-worn trackers estimate calorie burn using heart rate data and motion sensors, but their accuracy is limited. Research has found calorie estimates from popular devices range from 27% to 93% error. Even the most accurate tracker tested still overestimated by 27%. Error rates vary by activity type too: walking and running estimates averaged about 31% off, while cycling estimates were off by roughly 52%.
That doesn’t make trackers useless. They’re helpful for tracking relative trends. If your tracker says you burned more on Tuesday than Monday, that’s probably directionally correct even if the exact numbers are wrong. Just don’t treat the calorie display as precise enough to plan your meals around.
Putting Your Number to Work
Once you have a TDEE estimate, what you do with it depends on your goal. To maintain your current weight, eat roughly that many calories. To lose weight, eat below it. A deficit of about 500 calories per day translates to roughly one pound of weight loss per week. To gain weight, eat above it by a similar margin.
Because these formulas are estimates, treat your calculated TDEE as a starting point rather than a final answer. Track your weight over two to three weeks while eating at your target calorie level. If your weight stays stable when you expected it to change, adjust your intake by 100 to 200 calories and observe again. Your body is giving you more accurate data than any equation can.
Keep in mind that your TDEE isn’t static. It shifts as your weight changes, as you age, as your activity habits evolve, and even seasonally. Recalculating every few months, or whenever your routine changes significantly, keeps your estimate relevant.