Energy and weight
Estimate BMR and TDEE, then understand the estimate
What are BMR and TDEE, and how are they estimated?
BMR is the energy your body uses at complete rest. TDEE adds the energy of digesting food and all of your movement. Calculators estimate BMR from height, weight, age and sex with an equation such as Mifflin-St Jeor, then multiply it by an activity factor to estimate TDEE.
Both steps carry error. The most reliable common equation predicts resting rate within 10% of measured in more people than the others, and the activity factor is a broad assumption about a group. Treat the result as a starting estimate to compare with your own weight trend.
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BMR and TDEE calculator
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What is the difference between BMR and TDEE?
BMR is the energy used at complete rest; TDEE is everything you spend in a day. BMR, basal metabolic rate, is the energy used at complete rest to keep the body running. The FAO/WHO/UNU expert consultation on human energy requirements puts it at about 45 to 70% of total energy expenditure in adults, determined mainly by sex, body size, body composition and age.
TDEE, total daily energy expenditure, is everything: resting expenditure, the energy of digesting food, and physical activity. A consensus panel lists those three as the main expenditure terms in The American Journal of Clinical Nutrition (2012). The incidental movement of an ordinary day sits inside activity, and it varies between people more than they tend to expect.
How does a calculator estimate BMR?
A BMR calculator applies an equation fitted to measurements of other people; nobody measures your metabolism. Mifflin and colleagues derived theirs, published in American Journal of Clinical Nutrition (1990), from 498 healthy adults aged 19 to 78, 247 women and 251 men. Each person’s resting expenditure was measured by indirect calorimetry.
Show the Mifflin-St Jeor equation
With weight in kilograms, height in centimeters and age in years:
Men: 10 × weight + 6.25 × height − 5 × age + 5Women: 10 × weight + 6.25 × height − 5 × age − 161The paper first fitted one equation with a sex term, 166 × sex (1 for men, 0 for women), and reports that separating it into the two forms above did not change how well it predicted.
Why the calculator asks about sex
The equation was fitted to two groups of people recorded as women and men, and it carries a different constant for each. Mifflin-St Jeor cannot return a number without one of them. The constant is a property of how the equation was derived, not a judgment about anyone’s identity. The equation also cannot see the things that differ inside either group, such as body composition.
How well it does
A systematic review in Journal of the American Dietetic Association (2005) compared the four equations most used in practice. Mifflin-St Jeor predicted resting rate within 10% of the measured value in more people than any other. The same review noted that “noteworthy errors and limitations exist when it is applied to individuals.” Older adults and US ethnic minorities were underrepresented in the studies that developed and validated these equations.
Where do activity multipliers come from?
Activity multipliers come from population averages, not from measurements of you. The FAO/WHO/UNU consultation expresses daily energy needs as a multiple of BMR, called the physical activity level. It uses a population’s average BMR and activity level to estimate energy needs for that population, and describes lifestyles in broad groups, from sedentary or light activity to vigorously active.
A calculator borrows that idea and applies one band to one person. The four bands here, 1.2, 1.375, 1.55 and 1.725, are conventions shared by many calculators, not values measured for you. Choosing between them is usually the larger source of error.
Measuring activity with a device does not remove the uncertainty. Testing seven wrist-worn devices in 60 volunteers, Journal of Personalized Medicine (2017) found that no device estimated energy expenditure with an error below 20%.
Why does body composition change the estimate?
Lean tissue uses more energy at rest than fat. Validating long-used organ and tissue figures in The American Journal of Clinical Nutrition (2010), Wang and colleagues found about 13 kcal per kilogram per day for skeletal muscle and 4.5 for adipose tissue. Those values held across adulthood, with a small age adjustment above 50. Two people who match on height, weight, age and sex can differ in resting expenditure. An equation built on those inputs cannot see the difference.
Equations built on fat-free mass exist. Reviewing earlier studies in The American Journal of Clinical Nutrition (1991), Cunningham proposed one that explained 65 to 90% of the variation in resting expenditure. In 50 athletes around 20 years old, Journal of Strength and Conditioning Research (2018) found all five equations tested underestimated measured resting rate. Cunningham’s was the closest on average. Fat-free mass equations need a reliable body-composition measurement, which most people do not have.
Why does TDEE change as your weight changes?
TDEE is not fixed; it falls as weight falls. In The New England Journal of Medicine (1995), people holding their weight 10% or more below their usual weight spent 6 to 8 kcal less per kilogram of fat-free mass per day. Resting and non-resting expenditure each fell by 3 to 4. Six years after a televised weight-loss competition, Obesity (2016) found resting expenditure in 14 former contestants about 500 kcal a day below what their body composition and age predicted.
An estimate that fit at the start therefore fits less well as weight changes. The drift is part of why a straight-line projection overstates progress.
How should you use a TDEE estimate?
Use a TDEE estimate as a starting point, then compare it with what your weight trend does over several weeks. Shorter periods are dominated by water and other short-term changes: in Physiological Reports (2017), 84% of the change in body weight over two weeks was fat-free mass rather than fat.
If your trend and the estimate disagree, the estimate was wrong for you. The planning scenario above is arithmetic, not advice about how much to eat. What intake is right for you depends on health and circumstances a calculator cannot see. That is a conversation for a clinician or registered dietitian.
How far the equations can be trusted
What validation studies show
- Mifflin-St Jeor predicted resting rate within 10% of measured in more people than other common equations (Frankenfield et al., 2005).
- BMR is about 45 to 70% of total expenditure in adults, and activity is expressed as a multiple of BMR for population groups (FAO/WHO/UNU).
- Wrist-worn devices estimated energy expenditure with errors of at least 20% in a laboratory comparison (Shcherbina et al., 2017).
- Expenditure falls when weight is held below its usual level (Leibel et al., 1995).
Where the equations fall short
- An equation that is within 10% for most people is off by more for some, and nothing in the calculation tells you which group you are in.
- Mifflin-St Jeor was derived from 498 adults aged 19 to 78, and older adults and US ethnic minorities were underrepresented in the studies Frankenfield reviewed.
- The athlete comparison tested 50 athletes around 20 years old, so its ranking of equations does not transfer to other groups.
- The activity bands are conventions, not measurements, and the calculation cannot see body composition.
Using the number
- Read the output as an estimate with a margin, which is why it is rounded.
- Compare it with several weeks of your own weight trend before trusting it.
- Treat a planning scenario as arithmetic. The right intake for you is a question for a clinician, especially with a medical condition, a medication, pregnancy or nursing.
Product design, not a scientific finding
Where Loopa uses the same equation
Loopa uses the same equation and activity factors to calculate starting calorie targets from what you enter during setup. Those calculated targets are free in Loopa Basic. Loopa keeps a target at or above your estimated BMR, the same floor this calculator applies to a planning scenario.
The weight forecast is Premium. The forecast will not project below an intake floor of 1,500 kcal a day for men and 1,200 for women, and gives no date beyond 52 weeks. It says so when your current plan does not reach your goal.
This is general information, not medical advice, and the review date under the title is an editorial check, not a medical approval. Anything involving a medical condition, a medication, pregnancy or nursing belongs with your doctor, pharmacist or dietitian, who can see your whole situation.
Sources
- 1
Human Energy Requirements: Report of a Joint FAO/WHO/UNU Expert Consultation.
Food and Agriculture Organization, World Health Organization, United Nations University, FAO Food and Nutrition Technical Report Series 1 (2001).
expert consultation reportClaim re-read September 16, 2026
- 2
Energy balance and its components: implications for body weight regulation.
Hall KD, Heymsfield SB, Kemnitz JW, et al., The American Journal of Clinical Nutrition (2012).
consensus statementDOI 10.3945/ajcn.112.036350PubMed 22434603Claim re-read September 16, 2026
- 3
A new predictive equation for resting energy expenditure in healthy individuals.
Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO, American Journal of Clinical Nutrition (1990).
primary researchDOI 10.1093/ajcn/51.2.241PubMed 2305711Claim re-read September 16, 2026
- 4
Frankenfield D, Roth-Yousey L, Compher C, Journal of the American Dietetic Association (2005).
systematic reviewDOI 10.1016/j.jada.2005.02.005PubMed 15883556Claim re-read September 16, 2026
- 5
Shcherbina A, Mattsson CM, Waggott D, et al., Journal of Personalized Medicine (2017).
primary researchDOI 10.3390/jpm7020003PubMed 28538708Claim re-read September 16, 2026
- 6
Wang Z, Ying Z, Bosy-Westphal A, et al., The American Journal of Clinical Nutrition (2010).
primary researchDOI 10.3945/ajcn.2010.29885PubMed 20962155Claim re-read September 16, 2026
- 7
Cunningham JJ, The American Journal of Clinical Nutrition (1991).
reviewDOI 10.1093/ajcn/54.6.963PubMed 1957828Claim re-read September 16, 2026
- 8
Accuracy of Resting Metabolic Rate Prediction Equations in Athletes.
Jagim AR, Camic CL, Kisiolek J, et al., Journal of Strength and Conditioning Research (2018).
primary researchDOI 10.1519/JSC.0000000000002111PubMed 28682934Claim re-read September 16, 2026
- 9
Changes in energy expenditure resulting from altered body weight.
Leibel RL, Rosenbaum M, Hirsch J, The New England Journal of Medicine (1995).
primary researchDOI 10.1056/NEJM199503093321001PubMed 7632212Claim re-read September 16, 2026
- 10
Persistent metabolic adaptation 6 years after "The Biggest Loser" competition.
Fothergill E, Guo J, Howard L, et al., Obesity (2016).
primary researchDOI 10.1002/oby.21538PubMed 27136388Claim re-read September 16, 2026
- 11
Composition of two-week change in body weight under unrestricted free-living conditions.
Bhutani S, Kahn E, Tasali E, et al., Physiological Reports (2017).
primary researchDOI 10.14814/phy2.13336PubMed 28676555Claim re-read September 16, 2026