The calories you burn doing nothing at all
The calories you burn doing nothing at all
Basal metabolic rate is the energy your body uses at complete rest to stay alive — breathing, circulating blood, maintaining temperature, repairing cells, running your brain. It is what you would burn lying still in a room all day without moving or digesting.
For most people BMR accounts for 60–75% of total daily energy expenditure. That is the striking part: the large majority of the calories you burn have nothing to do with exercise. Your brain alone uses roughly 20% of your resting energy.
This calculator uses Mifflin-St Jeor, which research has generally found more accurate for modern populations than the older Harris-Benedict formula:
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Worked example: a 32-year-old man, 180 lb (81.6 kg), 70 inches (177.8 cm). BMR = (10 × 81.6) + (6.25 × 177.8) − (5 × 32) + 5 = 816 + 1,111 − 160 + 5 = about 1,772 calories per day.
Weight has the largest effect. More body mass means more tissue to maintain, which is why a heavier person burns more at rest than a lighter one.
Height contributes through surface area — taller bodies lose more heat and use more energy maintaining temperature.
Age reduces BMR at roughly 5 calories per year in this equation. Much of that decline reflects gradual loss of muscle rather than an inevitable metabolic slowdown, which is why resistance training tends to blunt it.
Sex appears as a constant because men on average carry more lean mass at a given height and weight. It is a population average, not a statement about any individual.
Mifflin-St Jeor cannot see body composition, and that is its main weakness. Muscle is metabolically more active than fat, so two people of identical height, weight, age, and sex can have genuinely different resting rates if one is lean and muscular and the other is not.
Estimates typically land within about 10% of measured values for most people, which is useful but not precise. For a very muscular or very lean individual, a formula using lean body mass — such as Katch-McArdle — will fit better, though it requires a body fat estimate as an input.
Treat any BMR figure as a starting hypothesis. The real test is what happens over two or three weeks of consistent intake: if weight is stable, your maintenance estimate was close; if it moves, adjust the number rather than doubting the scale.
BMR is not your calorie target. To get total daily energy expenditure, multiply by an activity factor:
Most people overestimate their activity level. Unless you are genuinely training most days, the lower multipliers are usually closer to the truth.
It is tempting to treat BMR as a floor to undercut for faster results. Sustained intake below your resting requirement tends to produce loss of lean tissue alongside fat, poor energy, disrupted sleep, and difficulty maintaining the diet at all — which is usually what ends it.
A moderate deficit taken from your total daily expenditure, rather than from BMR, is both more sustainable and better at preserving muscle. If you are considering a significant or prolonged reduction in intake, that is a conversation worth having with a doctor or registered dietitian rather than a calculator.
BMR is what you burn at complete rest. TDEE is your total daily burn including movement, exercise, and digestion. TDEE is BMR multiplied by an activity factor, and it is the figure to use when setting calorie targets.
It is generally considered the most accurate of the common prediction equations, typically landing within about 10% of measured values. It cannot account for body composition, so very lean or very muscular people may see larger deviations.
Building muscle raises it modestly, since lean tissue uses more energy at rest than fat. The effect is real but smaller than often claimed - a few extra pounds of muscle adds tens of calories per day, not hundreds.
Generally not. Sustained intake below your resting requirement tends to cost lean tissue, energy, and adherence. Take any deficit from your total daily expenditure instead, and speak to a professional before making large or prolonged reductions.