The BMR Calculator estimates basal metabolic rate: the calories the body uses at complete rest to keep organs, breathing and temperature running. Enter weight, height, age and sex (and lean mass if using Katch-McArdle), and the tool returns a resting figure you can multiply by activity to reach daily needs.
Resting burn is the largest single slice of most people's daily energy use, yet it is still an estimate. Formulas disagree by a modest margin, and real resting metabolism shifts with muscle, hormones and health status. This page shows the arithmetic, compares the common equations, and keeps the result framed as general information rather than a clinical reading.
Calculate your basal metabolic rate
Basal metabolic rate is the energy spent lying still, fasting, in a neutral room: the cost of being alive before walking, training or digesting a meal. Indirect calorimetry can measure it in a lab. Online tools estimate it from body size, age and sex using published regression equations.
Enter metric or US measurements; the calculator converts as needed. Sex is required for Mifflin-St Jeor and Harris-Benedict because those equations were fitted separately for male and female adults. Katch-McArdle skips sex and uses lean body mass instead when a body-fat estimate is available.
The number that appears is an estimate for general information. It does not diagnose a slow or fast metabolism, and it is not medical advice. Unexpected fatigue, unexplained weight change or a known endocrine condition belongs with a doctor, not with a website result alone.
Apply the Mifflin-St Jeor formula
Mifflin-St Jeor (1990) is the default here because it tends to track measured resting energy better than older equations for the general adult population. The inputs are ordinary clinic measurements: weight in kilograms, height in centimetres, age in years, and sex, with no need for a body-fat reading on this path.
Male: `` BMR = 10×kg + 6.25×cm - 5×age + 5 ``
Female: `` BMR = 10×kg + 6.25×cm - 5×age - 161 ``
Source: Mifflin MD, St Jeor ST, et al., 1990.
Weight and height pull the estimate up; age pulls it down because resting expenditure tends to fall as lean mass and hormonal profile change over decades. The sex offset (plus 5 versus minus 161) reflects average differences in the samples those authors fitted, not a verdict on any one person.
Use actual measured weight and height. Guessing either by a few kilograms or centimetres moves the result more than switching between Mifflin and Harris-Benedict often does.
Compare BMR formulas
Three published equations sit behind the formula picker on this page. None is perfect for every body, and small disagreements between them are expected. Each option is useful in a different situation, which is why the calculator exposes all three rather than hiding the alternatives.
| Formula | What it uses | When it helps |
|---|---|---|
| Mifflin-St Jeor | Weight, height, age, sex | Default for most adults |
| Harris-Benedict (revised) | Weight, height, age, sex | Second estimate; Roza and Shizgal 1984 revision |
| Katch-McArdle | Lean body mass only | Lean or muscular adults with a body-fat figure |
Harris-Benedict revised: `` Male: 13.397×kg + 4.799×cm - 5.677×age + 88.362 Female: 9.247×kg + 3.098×cm - 4.330×age + 447.593 ``
Katch-McArdle: `` BMR = 370 + 21.6 × lean body mass (kg) ``
Katch-McArdle shines when total weight is a poor proxy for metabolically active tissue: high muscle or, conversely, higher body fat at the same scale weight. It is only as good as the lean-mass input. If body fat is unknown, stay with Mifflin-St Jeor rather than inventing a percentage.
Expect the three methods to land in a band a few percent wide for the same person. That spread is a reminder that the true resting value is nearby, not that one app is lying.
Calculate BMR for an 80 kg male at 180 cm
A fixed adult profile shows every term in Mifflin-St Jeor with units attached, then a brief cross-check against the revised Harris-Benedict equation on the same inputs. Take a 30-year-old man at 80 kg and 180 cm and apply the default formula step by step.
1. Weight term. 10 × 80 = 800
2. Height term. 6.25 × 180 = 1,125
3. Age term. 5 × 30 = 150 (this is subtracted)
4. Sex offset. + 5 for male
5. Sum. `` BMR = 800 + 1,125 - 150 + 5 = 1,780 kcal/day ``
So about 1,780 calories per day is the resting estimate for this profile. For comparison, the revised Harris-Benedict equation on the same inputs yields:
13.397×80 + 4.799×180 - 5.677×30 + 88.362
= 1,071.76 + 863.82 - 170.31 + 88.362
≈ 1,854 kcal/dayRoughly 70 calories apart: close enough that either can seed a daily plan, far enough to show why treating BMR as an exact lab reading is a mistake. If this person knew lean mass was 70 kg, Katch-McArdle would give 370 + 21.6 × 70 = 1,882 kcal/day.
The BMR page stops at the resting number. Turning it into full-day calories is the next section and is owned in more detail by the Calorie Calculator.
Turn BMR into daily calorie needs
Daily maintenance, often labelled TDEE, is BMR multiplied by an activity factor that accounts for training and everyday movement. The same factor table is shared with the Calorie Calculator so the two pages stay consistent. The factors used across this health set are:
| Activity level | Factor |
|---|---|
| Sedentary - little or no exercise | 1.2 |
| Light - exercise 1 to 3 times/week | 1.375 |
| Moderate - exercise 4 to 5 times/week | 1.465 |
| Active - daily exercise or intense 3 to 4 times/week | 1.55 |
| Very active - intense exercise 6 to 7 times/week | 1.725 |
| Extra active - intense daily or physical job | 1.9 |
For the 1,780 kcal example at moderate activity:
TDEE = 1,780 × 1.465 ≈ 2,608 kcal/dayThat maintenance figure is the natural starting point for planning intake. Moderate deficit or surplus adjustments, calorie floors, and the full TDEE workflow live on the Calorie Calculator, which keeps those guardrails in one place. This page owns the resting estimate and the formula comparison.
Understand what affects your metabolic rate
Body size is the obvious driver: more mass generally means more resting burn. Composition matters too. Muscle tissue uses more energy at rest than fat tissue, which is why two people at the same weight can differ and why Katch-McArdle keys off lean mass when that number is known.
Age tends to lower BMR estimates because the equations were fitted on populations whose lean mass and hormones change over time. Sex offsets in Mifflin and Harris-Benedict reflect average sample differences. Individual thyroid status, fever, recent diet, medications and genetics sit outside every simple equation.
Short-term "starvation mode" myths overstate what happens in a moderate deficit. Severe under-eating can reduce expenditure, but that is a clinical situation, not a reason to fear a modest, supervised change. For everyday planning, treat BMR as a stable-enough baseline and adjust daily calories from real weekly feedback rather than from anxiety about a hidden metabolic collapse.
Read the estimate as a planning baseline
A resting calorie figure is most useful when treated as a baseline for weekly experiments rather than a verdict on metabolism. Track body weight, training quality and hunger for two or three weeks at a maintenance intake near BMR times the chosen activity factor, then nudge calories in 100 to 200 kcal steps if the trend disagrees with the estimate.
Lab calorimetry, wearable burn totals and equation results will not match exactly. Prefer the method that can be repeated under similar conditions. The BMR Calculator remains an educational estimate for general information; unexplained weight change, endocrine symptoms or a need for supervised intake planning still belong with a clinician or dietitian who can see the full history.
Frequently asked questions
What is BMR?
BMR is basal metabolic rate: estimated calories used at complete rest. It covers the energy cost of organs, breathing and temperature control before added movement. Most online tools, including this one, estimate it from equations rather than measuring it directly.
Is BMR the same as RMR?
They are close cousins. Resting metabolic rate (RMR) is measured under less strict conditions than true basal rate. Consumer calculators usually estimate something nearer to RMR while still labelling it BMR. For planning purposes the difference is small next to activity and intake error.
Which formula should I use?
Mifflin-St Jeor is the default for most adults. Use Katch-McArdle when a reliable lean-mass figure is available, especially for muscular builds. Harris-Benedict revised is a useful cross-check when comparing two estimates.
Why do I need sex for some formulas?
Mifflin-St Jeor and Harris-Benedict were validated with separate male and female equations. The calculator needs sex to pick the matching coefficients. Katch-McArdle does not use sex; it uses lean body mass instead.
How do I get from BMR to daily calories?
Multiply BMR by an activity factor from the table above. The product is an estimated maintenance intake (TDEE). The Calorie Calculator applies those multipliers with maintenance shown first and with deficit and floor guardrails enforced.
Does building muscle raise BMR?
More lean mass usually raises resting expenditure, which is why Katch-McArdle increases when lean mass increases. The change per kilogram is real but modest; most of the calorie difference people notice after training comes from the activity itself.
Can medication or illness change BMR?
Yes. Thyroid disease, infection, some medications and recovery from injury can shift resting burn. Equation results will not capture that. Anyone with a suspected metabolic or endocrine issue needs clinical assessment.
Is a higher BMR always better?
Not as a moral score. A higher estimate often tracks larger body size or more lean mass. Health decisions rest on overall habits, labs and how a person feels, not on maximising a resting calorie number from a website.
Summary
The BMR Calculator estimates resting calorie burn from Mifflin-St Jeor by default, with Harris-Benedict revised and Katch-McArdle as alternatives. A worked adult male profile at 80 kg, 180 cm and age 30 returns about 1,780 kcal/day on Mifflin-St Jeor.
Multiplying by an activity factor yields daily maintenance; fuller TDEE planning, including the 500-calorie adjustment cap and intake floors, sits on the Calorie Calculator.
The result is an estimate for general information, shaped by size, age, sex or lean mass, and open to individual variation no equation fully captures.