BMR and TDEE, without the guesswork

BMR, RMR and TDEE

Three abbreviations, one quantity measured under different conditions.

• BMR — basal metabolic rate. What your body costs to stay alive at complete rest: breathing, circulation, brain activity, maintaining tissue. Measured after an overnight fast, lying still, in a temperature-controlled room. For most adults it is 60% to 70% of everything they burn in a day.
• RMR — resting metabolic rate. The same measurement taken under less strict conditions, which puts it around 10% higher than BMR. The two are used interchangeably in most calculators, including this one. The difference is well inside the error margin of the equation producing either.
• TDEE — total daily energy expenditure. Everything: BMR plus the cost of digesting food, plus incidental movement, plus deliberate exercise. This is the number a deficit is measured against.

Only TDEE matters for planning. BMR matters because it is the part that can be estimated from four things you already know, and because it is the floor below which intake stops being a diet and starts being a different kind of problem.

The Mifflin-St Jeor equation

Published in 1990 and still the default choice, because it validates better against measured expenditure in modern populations than the older Harris-Benedict equation it replaced. It takes one form with one term that changes:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + s

Where s is +5 for the male coefficient and−161 for the female one. That term is a parameter of this particular regression — it absorbs average differences in lean mass between the groups the equation was fitted on. It is not a statement about identity, which is why this tool labels the field biological sex and uses it for nothing else.

Each coefficient is worth reading directly, because they explain a lot:

• 10 per kilogram. Every kilogram you carry costs about 10 kcal a day to maintain. This is the term that makes weight loss self-limiting: lose 10 kg and your BMR falls by roughly 100 kcal a day whether you like it or not.
• 6.25 per centimetre. Taller people burn more at the same weight, because more of that weight is metabolically active tissue rather than stored fat.
• −5 per year of age. A 50-year-old burns about 100 kcal a day less than their 30-year-old self at identical height and weight. Most of that is lean mass lost over two decades rather than metabolism mysteriously slowing.

A worked example

Take a 35-year-old male profile, 178 cm, 85 kg:

A 32-year-old female profile at 165 cm and 72 kg comes out at 1,430 kcal: 720 + 1,031 − 160 − 161. The 362 kcal gap between the two is mostly the sex constant and the weight term, and it is why intake advice pitched as a single number for everybody is close to useless.

Choosing an activity factor

BMR becomes TDEE by multiplication. The factors this calculator uses describenon-exercise movement only — your job, your commute, how much you are on your feet:

The rungs are anchored on step count and job type deliberately. Every ladder that asks you to rate your own activity as "moderate" is asking for a judgement people get wrong in a predictable direction, and one rung is worth 150 to 270 kcal a day.

The double-counting trap

The familiar ladder you will find elsewhere runs 1.2 / 1.375 / 1.55 / 1.725 / 1.9, and its rungs are labelled in terms of training — "moderate exercise 3-5 days a week", up to "twice-daily training". Those multipliers already contain your workouts.

So if you pick 1.55 because you lift three times a week, and then add the calories from those sessions on top, you have billed the same training twice. For the 85 kg profile above, the gap between 1.55 and this tool's 1.45 is about 180 kcal a day, and picking 1.725 instead would put it near 490. Add a genuine session estimate on top of that and maintenance is overstated by several hundred calories.

An overstated maintenance figure makes every projection built on it read fast, which is the specific error this whole tool exists to correct. The fix is to keep the two accounts separate: a compressed multiplier for incidental movement, and training counted once, from METs. The exercise burn guide covers the second half.

How accurate any of this is

Mifflin-St Jeor predicts measured BMR within 10% for roughly 80% of people. That is good for a four-input equation and still means a plausible 200 kcal error before you have chosen an activity factor. The multiplier adds its own uncertainty, and it multiplies the first error rather than averaging it out.

Two cases where the estimate is worse than usual: at the extremes of body composition, since the equation cannot see the difference between 85 kg of lean mass and 85 kg carrying substantial fat; and in anyone with a thyroid condition or on medication affecting metabolism, which no population formula accounts for.

The practical answer is to treat the calculated figure as a starting hypothesis and let three weeks of weight data correct it. If you are holding a projected 500 kcal deficit and nothing is moving, your real maintenance is lower than the estimate — or your intake is higher than you logged. Both are common, and reading the trend properly is how you tell them apart.

Sources

The research this guide draws on, and the claim each one supports. Figures that come from the calculator itself — the NEAT penalty, the danger thresholds, the break model — are documented with their own sources on the methodology page.

1. The Mifflin-St Jeor equation and its coefficientsMifflin MD, St Jeor ST, et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 51(2):241–247.
2. Mifflin-St Jeor as the most accurate of the common equations, within 10% for most adultsFrankenfield D, Roth-Yousey L, Compher C (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association 105(5):775–789.
3. Physical activity level multipliers, which the activity ladder here adapts to exclude exerciseFAO/WHO/UNU (2004). Human energy requirements: report of a joint FAO/WHO/UNU expert consultation. FAO Food and Nutrition Technical Report Series 1.
4. MET values for counting training separately from the multiplierHerrmann SD, Willis EA, et al. (2024). 2024 Adult Compendium of Physical Activities: a third update of the energy costs of human activities. Journal of Sport and Health Science 13(1):6–12.