Every calorie calculator ends in one number: how many calories you burn in a day. It looks like a measurement. It is an estimate built from an equation, and the honest answer to "how many do I actually burn?" is "somewhere near that, and nobody knows exactly." This article covers where the number comes from, how big each piece is, and how to test it against your own life.
The short version
- Daily burn has three parts: resting burn (about two thirds), digesting food (5 to 15%) and movement (the rest).
- Equations predict resting burn from your age, sex, height and weight. They are right on average and wrong for many individuals.
- Treat any calculator's number as a starting point, then correct it with two or three weeks of your own weight and food data.
The three parts
Resting burn. Your heart, brain, liver, kidneys, breathing and body heat. It makes up about two thirds of the energy an adult uses in a day (Hall and colleagues, 2012). It follows how much of you there is. In a large database of people measured with doubly labeled water, a method that tracks specially labeled water through the body over a week or two, total daily expenditure rose with fat-free mass. Once that was accounted for, expenditure stayed flat in adults from about 20 to 60, even during pregnancy (Pontzer and colleagues, 2021).
Digesting food. Breaking down and storing what you eat costs energy. For a mixed diet eaten at energy balance it is 5 to 15% of daily expenditure. It is higher with more protein or alcohol and lower with more fat (Westerterp, 2004).
Movement. Everything else: workouts, walking, standing, fidgeting, carrying groceries. It is the most variable part, which is why calculator activity factors (1.2 for mostly sitting, up to 1.725 for hard training six or seven days a week) are a convention, not a measurement.
Where a calculator's number comes from
A calculator takes your age, sex, height and weight, puts them in an equation for resting burn, and multiplies by an activity factor. The most used equation is Mifflin-St Jeor, fitted to 498 healthy adults whose resting burn was measured in a lab. In a systematic review of four common equations it was the most likely to land within 10% of a measured value. The same review warned of notable errors when any of them is applied to one person (Frankenfield and colleagues, 2005).
A worked example
A 30-year-old man, 180 cm and 80 kg:
- Resting burn: 10 x 80 + 6.25 x 180 - 5 x 30 + 5 = 1,780 calories.
- With moderate activity (x 1.55): 2,759, so a daily burn of about 2,760.
- If his true resting burn is 250 calories higher or lower, about the size of the person-to-person variation researchers cannot explain, his daily burn lands between roughly 2,370 and 3,150 once the activity factor is applied.
How to check your number
You are the experiment. Log what you eat and weigh yourself regularly for two or three weeks while keeping your habits steady. If your weight trend is flat, you ate about what you burned, so your average intake is your maintenance. If it fell or rose, the energy in that change tells you how far off the estimate was. The usual rule of thumb is about 3,500 calories per pound of body weight. This beats any equation, as long as the logging is honest.
Vital does the arithmetic for you. Its estimated maintenance card works out the calories that would hold your weight from your food log and weight trend. It shows a range with a confidence label, and nothing at all when the data is too thin.
What we are less sure about
- How much of the person-to-person gap is real. About 250 calories a day of variation in resting burn is not explained by body size and composition (Hall and colleagues, 2012). Some of it is real differences in organ and tissue size, and some is measurement noise. Doubly labeled water, the best field method, has a precision of about 5% itself, which is more than 100 calories a day.
- Who the equations fit. In 620 healthy Chilean women, only about a third to two fifths landed within 10% of measured resting burn with Mifflin-St Jeor or Katch-McArdle (Molina-Luque and colleagues, 2021). The systematic review above also found older adults and ethnic minorities underrepresented in the studies behind the equations.
- Activity factors. They are round numbers agreed by convention. How active you really are depends on far more than "three to five workouts a week".
- The 3,500-calorie rule. It is only about right for people with a lot of body fat, and it overstates the energy in a pound lost by leaner people (Hall, 2008). That affects the check above, so read the answer as a range.
What to do
Start with a calculator, eat to it for two or three weeks, and adjust by what your weight trend says rather than what one morning says. Single days mostly show water, not fat, which the scale article explains. The BMR calculator shows the resting part on its own, with both equations.
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51(2):241-247. doi:10.1093/ajcn/51.2.241
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc 2005;105(5):775-789. doi:10.1016/j.jada.2005.02.005
- Hall KD, Heymsfield SB, Kemnitz JW, Klein S, Schoeller DA, Speakman JR. Energy balance and its components: implications for body weight regulation. Am J Clin Nutr 2012;95(4):989-994. doi:10.3945/ajcn.112.036350
- Westerterp KR. Diet induced thermogenesis. Nutr Metab (Lond) 2004;1:5. doi:10.1186/1743-7075-1-5
- Pontzer H, Yamada Y, Sagayama H, et al. Daily energy expenditure through the human life course. Science 2021;373(6556):808-812. doi:10.1126/science.abe5017
- Molina-Luque R, Carrasco-Marín F, Márquez-Urrizola C, et al. Accuracy of the resting energy expenditure estimation equations for healthy women. Nutrients 2021;13(2):345. doi:10.3390/nu13020345
- Hall KD. What is the required energy deficit per unit weight loss? Int J Obes 2008;32(3):573-576. doi:10.1038/sj.ijo.0803720
Written November 5, 2026. This is general information, not medical advice. Talk to a doctor or dietitian before big changes to how you eat or train, especially with a medical condition or a history of disordered eating.