The number of calories you burn in a day comes from three sources: your resting metabolism, the energy it takes to digest food, and physical activity. For most people, resting metabolism alone accounts for 60 to 75 percent of total daily burn. The good news is you can estimate your number with a simple formula and then refine it based on your activity level, body composition, and other personal factors.
Start With Your Resting Metabolic Rate
Your resting metabolic rate (RMR) is the calorie cost of simply keeping your body alive: breathing, circulating blood, maintaining body temperature, and running your brain. It’s the largest slice of your daily calorie burn, and it’s determined mostly by your size, age, sex, and how much muscle you carry. One pound of muscle burns about 6 calories per day at rest, while one pound of fat burns only 2. That difference is small per pound, but it adds up across your whole body.
The most reliable formula for estimating RMR is the Mifflin-St Jeor equation. A systematic review in the Journal of the American Dietetic Association found it predicted resting metabolism within 10 percent of the actual measured value more often than any competing formula, with the narrowest error range. Here’s how it works:
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
For example, a 35-year-old woman who weighs 70 kg (154 lbs) and stands 165 cm (5’5″) would calculate: (10 × 70) + (6.25 × 165) − (5 × 35) − 161 = 1,396 calories per day at rest. That number only covers lying still. Everything else you do adds on top of it.
One important caveat: this equation was developed and validated mostly in younger, non-Hispanic white populations. It can be less accurate for older adults and certain ethnic groups, so treat it as a starting estimate rather than a precise measurement.
Add Calories From Digestion
Your body spends energy breaking down and absorbing food, a process called the thermic effect of food. This typically adds 5 to 15 percent on top of your resting metabolism, depending heavily on what you eat. Protein costs the most to digest, burning 15 to 30 percent of its own calories during processing. Carbohydrates cost 5 to 10 percent. Fats are the cheapest to process at just 0 to 3 percent.
This is one reason high-protein diets can nudge total calorie burn upward. If you eat 2,000 calories and 30 percent comes from protein, you’ll burn noticeably more through digestion alone compared to a diet where most calories come from fat. For a rough daily estimate, adding 10 percent to your RMR gets you close enough for most mixed diets.
Estimate Calories From Activity
Physical activity is the most variable part of your daily burn, and it’s where most people have the hardest time estimating. Exercise scientists use a unit called a MET (metabolic equivalent of task) to express how hard an activity is relative to sitting still, which counts as 1 MET. The formula to convert METs into calories is straightforward:
Calories burned = METs × body weight in kg × hours spent doing the activity
Here are MET values for some common activities to give you a sense of scale:
- Slow walking (2 mph): 2.8 METs
- Brisk walking (3.5 mph): 4.3 METs
- Very brisk walking (4.5 mph): 7.0 METs
- Weight training, moderate effort: 3.5 METs
- Weight training, vigorous effort: 6.0 METs
- Swimming laps, moderate pace: 5.8 METs
- Swimming laps, fast pace: 9.8 METs
- Leisurely swimming: 6.0 METs
So a 70 kg person walking briskly for 45 minutes would burn roughly 4.3 × 70 × 0.75 = 226 calories. That same person swimming moderate laps for 30 minutes would burn about 5.8 × 70 × 0.5 = 203 calories. These numbers include your resting burn during that time, so they’re total calories, not “extra” calories on top of your RMR.
Don’t forget non-exercise activity. Fidgeting, standing at your desk, cooking, cleaning, and walking around stores all add up. Some people burn several hundred extra calories a day just through these everyday movements, while others who sit most of the day burn very little beyond their RMR.
How Accurate Are Fitness Trackers?
Wrist-worn devices like the Apple Watch and Fitbit estimate calories by combining your heart rate with movement data and personal details like age and weight. The number on your wrist after a workout feels precise, but research from Harvard’s School of Engineering found that calorie estimates from smartwatches carry error rates of 30 to 80 percent. That means a reading of 400 calories burned could represent anything from roughly 220 to 720 actual calories.
These devices tend to be better at tracking relative effort (did you work harder today than yesterday?) than absolute calorie counts. If you use a tracker, treat the calorie number as a rough guide rather than something to eat back calorie-for-calorie. Watching trends over weeks is more useful than fixating on any single day’s reading.
The Clinical Gold Standard
If you want a truly accurate number, clinical testing exists. The most precise method is indirect calorimetry, where you breathe into a device that measures the oxygen you consume and carbon dioxide you produce. From that gas exchange, technicians can calculate your exact resting metabolic rate. It takes about 15 to 30 minutes, and many sports medicine clinics and university labs offer it for $100 to $250.
For measuring total daily expenditure over days or weeks, including all activity, a technique called doubly labeled water is the research gold standard. You drink water containing harmless tracers and provide urine samples over 1 to 2 weeks. The best equations used in this method produce results within about 1 percent of values measured by laboratory chambers. It’s almost exclusively used in research settings, though, and isn’t something most people need.
Factors That Shift Your Burn
Several things can raise or lower your calorie burn in ways a formula won’t capture. Age is the most predictable: you lose muscle gradually each decade after your 30s, which pulls your resting metabolism down. This is one reason strength training becomes more important as you get older. Building or maintaining muscle mass directly increases the number of calories your body uses at rest.
Thyroid function plays a significant role. An underactive thyroid slows metabolism, sometimes substantially, while an overactive thyroid speeds it up. If your actual calorie burn seems far lower than any formula predicts, and you’re gaining weight despite careful tracking, a thyroid panel is worth considering.
Other less obvious factors include sleep (chronic sleep deprivation disrupts hormones that regulate metabolism), environmental temperature (your body burns extra calories maintaining its core temperature in cold conditions), and hormonal fluctuations related to menstrual cycles or menopause. Even caffeine temporarily raises metabolic rate by a small amount.
Putting It All Together
The most practical approach for most people is layering these estimates. Start with the Mifflin-St Jeor equation for your resting metabolism. Add 10 percent for digestion. Then estimate your activity calories using MET values for your typical workouts and daily movement. The sum gives you a reasonable daily total.
If that feels like too much math, a simpler shortcut is to multiply your RMR by an activity factor: 1.2 if you’re mostly sedentary, 1.375 for light exercise a few days a week, 1.55 for moderate exercise most days, and 1.725 for hard daily training. This bundles digestion and activity into one multiplier.
Then validate against real-world results. If you eat at your estimated maintenance calories for two to three weeks and your weight stays stable, your estimate is close. If you’re gaining or losing, adjust by 200 to 300 calories and reassess. No formula, tracker, or lab test is perfect for predicting what happens in your individual body over time. The numbers get you in the right neighborhood, and your scale and energy levels tell you the rest.