Basal metabolic rate is the energy your body spends at complete rest to keep the lights on: breathing, circulating blood, maintaining temperature, running the sodium-potassium pumps in every cell membrane. Measured properly it requires a darkened room, an overnight fast and a metabolic cart, conditions no web page can reproduce.
What a calculator offers instead is a prediction from a regression equation fitted to measured data. The distinction matters, because the number that comes out is the centre of a distribution, not a personal measurement. It is still the most useful starting point available for anyone planning calorie intake, because the alternative — guessing — is worse.
Mifflin-St Jeor, published in 1990, is the equation most dietetics bodies now prefer for healthy adults. Everything is computed in metric after conversion: imperial height is treated as inches and multiplied by 2.54, imperial weight is multiplied by 0.45359237.
male: BMR = 10w + 6.25h − 5a + 5
female: BMR = 10w + 6.25h − 5a − 161
with w in kilograms, h in centimetres and a in years. Note that the imperial height field here is in inches, not the decimal feet used by the BMI page — do not carry a number between the two pages without converting.
The tool reports four figures. The first is the raw rate. The next three multiply it by 1.375, the “light activity, one to three days a week” factor, to give a maintenance estimate, and then subtract or add 500 kcal for roughly half a kilogram of loss or gain per week. The full ladder of factors is printed beneath the results: 1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 active, 1.9 very active.
A 30-year-old male, 172 cm, 68 kg:
Change the sex selector only, and the female form gives 680 + 1075 − 150 − 161 = 1444 kcal/day, about 166 kcal lower. That gap is the equation's average offset, not a statement about any individual.
The five hundred calorie adjustment is worth checking against arithmetic: 500 kcal a day is 3,500 a week, and a kilogram of body fat stores roughly 7,700 kcal. So the true figure is nearer 0.45 kg, which is why the label says about. It also assumes the loss is entirely fat and that metabolism does not adapt, and it does adapt.
Nearly, and not exactly. Resting metabolic rate is measured under looser conditions and typically lands five to ten per cent above the basal figure. The equations used here are calibrated close to resting values, so treating the output as RMR rather than strict BMR is the more honest reading.
Eating exactly at BMR leaves no energy for movement at all — it is what a body burns lying still. Maintenance is the figure to plan around; BMR is the floor beneath which a plan becomes difficult to sustain.
The activity multiplier already accounts for your usual training. Adding logged workout calories on top double-counts them, which is one of the most common ways a deficit quietly stops working.
Because age, sex and above all lean mass differ. Two people can share a height and weight and differ by several hundred kilocalories a day. The equation gives your best estimate, not a league table.
BMI answers how your weight sits for your height, and the Body Fat page estimates how much of that weight is lean tissue. All three read the same two or three inputs and answer different questions; none of them replaces a measurement.