BASELINE
Your resting burn — the calories your body spends before you do anything. Needs four numbers you'll only type once: sex, age, height, weight. Everything else in The Workup builds on this.
What a BMR actually is
Your basal metabolic rate is the tab your body runs before you contribute anything to the situation. Breathing, pumping blood, holding 37 degrees, rebuilding tissue, keeping your brain online — all of it costs calories, and all of it happens while you are asleep.
Do not eat it. BMR is a floor, not a target — it is what you would burn in a coma, and nobody reading this is in a coma. Its job is to be the base that BURN multiplies by your actual life.
The one thing to take from it directly: this is roughly 60 to 70 percent of everything you burn in a day. Which is why the fastest way to move the number is not cardio — it is carrying more muscle.
Muscle is metabolically expensive and fat is cheap, so two men at the same weight can sit hundreds of calories apart. Age drags it down a little each decade. Aggressive dieting drags it down faster, and it does not fully bounce back the week you stop.
Mifflin–St Jeor is accurate to within about ten percent for most people — which on a 1,800 kcal baseline is a 180 calorie swing, and 180 calories is not nothing. It was validated on general populations, so it systematically underestimates visibly muscular bodies and overestimates very heavy ones.
Run TAPE first and this station switches to Katch–McArdle, which is meaningfully sharper on a trained body because it stops guessing at your composition and starts using it.
Estimates, not diagnoses. These figures describe averages and your body is not one. Nothing here treats, cures or prevents anything — and if you are managing a health condition, taking medication, or working through a relationship with food, talk to a professional before acting on a number a website gave you.