Energy balance in human physiology is governed by thermodynamic conservation: energy in versus energy out. To design nutritional strategies for athletic performance or weight management, practitioners distinguish between Basal Metabolic Rate (BMR) — what your body burns at complete rest — and Total Daily Energy Expenditure (TDEE), the full 24-hour cost of living.
What BMR Actually Measures
BMR is the caloric cost of keeping vital cellular, neurological, and circulatory functions operational at complete physical and digestive rest in a thermoneutral environment. It covers breathing, heart function, brain activity, temperature regulation, and cellular repair — and it accounts for the largest single share of what most people burn in a day. Contrary to popular belief, deliberate exercise is usually the smallest share.
BMR declines with age (roughly 1-2% per decade after 20, independent of weight change) and varies with lean body mass far more than with total weight. This is why two people of identical scale weight can have 300+ kcal/day differences in BMR.
The Mifflin-St Jeor Equation
The modern clinical gold standard is the Mifflin-St Jeor equation, validated across diverse demographic cohorts with the lowest prediction variance:
Women: BMR = (10 × weight[kg]) + (6.25 × height[cm]) - (5 × age[yr]) - 161
The older Harris-Benedict equation (1919, revised 1984) overestimates BMR for most modern populations, and the Katch-McArdle variant — which adds lean mass — is preferable only when you know your body-fat percentage: BMR = 370 + (21.6 × lean mass in kg).
The Four Components of TDEE
TDEE totals the metabolic throughput across four physiological categories:
- BMR (~60-70%) — basal maintenance at rest.
- NEAT (~15%) — Non-Exercise Activity Thermogenesis: walking, typing, fidgeting, standing posture. This is the most variable component between individuals.
- TEF (~10%) — Thermic Effect of Food: energy spent digesting and processing macronutrients. Protein has the highest TEF (20-30% of its calories), carbs 5-10%, fat 0-3%.
- EAT (~5-15%) — deliberate exercise and training.
TDEE is computed by applying a Physical Activity Level (PAL) multiplier to BMR: TDEE = BMR × PAL.
Choosing an Activity Multiplier
1.55 moderate (3-5 days/week) • 1.725 hard (6-7 days/week)
1.9 athlete (physical job + daily training)
The most common error is over-selecting. Most people who "work out 3-5 days a week" still have mostly sedentary days and belong at 1.55 only if those sessions are genuinely strenuous. If the number produces faster-than-expected gain or loss after 3 weeks of tracked intake, adjust by 5-10% rather than restarting from scratch.
Applying the Numbers
For fat loss, a sustainable deficit is 10-20% below TDEE (never below BMR). For muscle gain, a surplus of 5-10% above TDEE limits fat gain while supporting growth. Re-estimate BMR whenever body weight shifts more than 5%, since the equation inputs have changed. Both our BMR and TDEE calculators run Mifflin-St Jeor with your unit system and return the full four-component breakdown.
Calculate Your BMR & TDEE
Get your Mifflin-St Jeor BMR, activity-multiplied TDEE, and macro split in seconds.