What it covers
BMR is the energy needed to keep the body running with no movement and no digestion: the heart pumping, lungs working, brain active, body temperature held steady. In most adults it is the largest single part of daily energy use, often cited as roughly 60 to 70 percent of the total, with the rest coming from movement and from digesting food.
What changes it
Lean tissue such as muscle and organs burns more than fat, so people with more lean mass tend to have a higher BMR. Body size matters, since a bigger body needs more energy. Age lowers it gradually, men on average have a higher BMR than women of the same weight, and conditions such as thyroid disease can raise or lower it.
How it is estimated
Measuring BMR properly needs a lab, so calculators use an equation. The most common is Mifflin-St Jeor (1990): 10 × weight in kg + 6.25 × height in cm − 5 × age in years, plus 5 for men or minus 161 for women. A 30-year-old woman who is 165 cm and 60 kg comes out at about 1,320 kcal a day. Try your own numbers in the BMR and TDEE Calculator.
Common misunderstanding
BMR is not how much you should eat. It excludes all movement, so it is a floor and not a target. To plan intake, start from TDEE, and see BMR vs TDEE for the comparison.
Source: Mifflin MD, St Jeor ST, et al. Am J Clin Nutr 1990;51(2):241–247, doi:10.1093/ajcn/51.2.241. General information only, not medical advice.