What Is BMR and Why Does It Matter?
Basal metabolic rate is what your body spends keeping itself running: heart, brain, breathing, maintaining temperature, repairing tissue. Nothing voluntary. The BMR Calculator estimates it from height, weight, age and sex.
It is most of your day
The surprise for most people is the proportion. For someone who is not an athlete, BMR is around sixty to seventy-five per cent of everything they burn in a day. Deliberate exercise is usually five to fifteen per cent, and digestion takes about ten.
Which reframes a lot of fitness thinking. An hour's brisk walk might be 250 calories against a BMR of 1,600 - real, worth doing, and small next to the baseline. It also explains why exercise alone is a slow lever on weight and why a sedentary day still burns a great deal.
What actually drives it
Body size, and lean mass in particular. Bigger bodies burn more, and muscle burns noticeably more than fat at rest. This is the main reason men's BMR runs higher than women's at the same weight - more of that weight is muscle on average.
Age. It declines slowly through adult life, and the decline is partly muscle loss rather than metabolism itself slowing, which is the argument for resistance training as people get older.
Genetics. Two people matched on every measurable variable can differ by a couple of hundred calories, and nobody can tell you which you are from a formula.
What barely moves it
A long list of things sold as metabolism boosters. Green tea, chilli, caffeine and cold exposure all have measurable effects that are small and short-lived - tens of calories, not hundreds. Eating frequently does not raise it either; the thermic cost of digestion depends on how much you eat, not how many sittings it arrives in.
The one thing that does move it meaningfully is lean mass, and that takes months of training to change. Which is less exciting than a supplement and considerably more reliable.
Adaptive thermogenesis, briefly
Eat substantially less for a long period and BMR does fall somewhat - partly because there is less of you to maintain, partly a genuine downregulation. The effect is real and routinely exaggerated. It does not stop weight loss, and the phrase "starvation mode" oversells a modest adjustment. If this is something you are weighing up in practice, it is worth discussing with a dietitian rather than a calculator.
Common questions
Is BMR the same as TDEE?
No. BMR is at rest; TDEE is BMR plus everything you do. The TDEE Calculator handles the second.
Which equation is best?
Mifflin-St Jeor is the usual default and is a little more accurate than the older Harris-Benedict for most people. Katch-McArdle is better if you know your body fat percentage.
Can I raise my BMR?
Meaningfully, only by adding lean mass. Everything else is noise.
How accurate is the estimate?
Within about ten per cent for most people, and less accurate the further you are from the population average it was fitted to.
Curious what yours comes out at?