What Is BMR?
Basal Metabolic Rate is the energy your body spends just staying alive — heart beating, lungs breathing, brain thinking, cells repairing — measured at complete rest in a neutral environment. For most people it's 60–70% of everything they burn in a day, dwarfing workouts.
Where it goes is counterintuitive:
| Organ/tissue | Share of resting burn |
|---|---|
| Liver | ~20% |
| Brain | ~19% |
| Skeletal muscle (at rest) | ~18% |
| Kidneys + heart | ~17% |
| Everything else | ~26% |
Muscle's share is why body composition nudges BMR: each kilogram of muscle adds roughly 10–15 kcal/day at rest — modest per kilo, meaningful over years of training.
The Equation and a Worked Example
This calculator uses Mifflin-St Jeor — the equation research finds most accurate for the general population:
Women: BMR = 10×weight(kg) + 6.25×height(cm) − 5×age − 161
Example — woman, 42, 160 cm, 72 kg
That's her cost of simply existing — before a single step. Formulas carry ±10% individual variation (genetics, thyroid status, muscle mass); lab measurement (indirect calorimetry) is the gold standard when precision matters medically.
BMR Is a Foundation, Not a Food Target
The most common misuse of BMR is eating at it "to lose fast." BMR is the input to your real budget, not the budget:
Our example (1,349 BMR, moderately active ×1.55) actually burns ≈ 2,090/day. A sensible 500-kcal deficit puts her at ~1,590 — comfortably above BMR while losing ~0.45 kg/week. Chronic sub-BMR intakes tend to shed muscle, tank energy, and rebound. Use the TDEE Calculator for the full budget and the Calorie Calculator for goal targets.
What Raises or Lowers BMR
| Factor | Effect | Notes |
|---|---|---|
| Muscle mass | ↑ | ~10–15 kcal/day per kg — the trainable lever |
| Age | ↓ ~1–2%/decade after 20s | Mostly via muscle loss — largely preventable with training |
| Body size | ↑ with size | More tissue = more burn; also why BMR falls as you diet down |
| Sex | Men higher on average | Driven by size and lean-mass differences |
| Thyroid function | ± | Hypo/hyperthyroid shift BMR clinically — see a doctor if suspected |
| Extreme dieting | ↓ 5–15% | Adaptive thermogenesis; refeeds and maintenance breaks mitigate |
Practical upshot: the only lever both meaningful and controllable is lifting and keeping muscle — especially through weight loss and aging.
The Bottom Line
BMR is your metabolic baseline — the number everything else stacks on. Compute it above, multiply through your honest activity level for TDEE, and set food targets from there (never below the baseline for long without medical guidance). Then protect the baseline itself the only way that works: resistance training and adequate protein, decade after decade.