What is Basal Metabolic Rate (BMR)?

Basal Metabolic Rate (BMR) is the minimum number of calories your body requires at complete rest — fasted, supine, and in a thermoneutral environment — to sustain life-critical functions. These include cardiac output, respiration, thermoregulation, brain activity, and organ maintenance.

BMR typically accounts for 60–70% of Total Daily Energy Expenditure (TDEE). It is the foundation of evidence-based nutrition planning — without an accurate BMR, calorie targets for fat loss or muscle gain are little better than guesses.

Clinically, BMR is measured via indirect calorimetry (oxygen consumption). In practice, it is estimated using validated predictive equations. This calculator implements the three most widely cited: Mifflin-St Jeor (1990), Harris-Benedict Revised (1984), and Katch-McArdle (1996).

The 3 BMR Formulas Explained

Mifflin-St Jeor (1990) — Current Gold Standard

Validated against 498 subjects via indirect calorimetry. Recommended by the Academy of Nutrition and Dietetics (AND) and ACSM as the most accurate general equation for adults. Error: typically ±10% of measured RMR.

Harris-Benedict Revised (Roza & Shizgal 1984) — Clinical Standard

The original 1919 Harris-Benedict formula revised by Roza & Shizgal using 337 subjects. Remains the standard in WHO technical reports and clinical pharmacokinetics. Slightly overestimates BMR in obese individuals.

Katch-McArdle (1996) — Best for Known Body Composition

Uses lean body mass (LBM) directly, eliminating the weight/height/sex proxies. This is the most accurate formula for athletes and individuals with a measured body fat %. It is body-composition neutral — it does not assume a "typical" fat-to-lean ratio. Best paired with DXA-measured body fat %.

6 Calculator Modes Explained

• Mifflin-St Jeor BMR — 4 fields (weight, height, age, sex). The AND/ACSM gold standard for general adults. Outputs BMR, BMI, sedentary TDEE, and moderate TDEE.
• Harris-Benedict BMR — 4 fields (weight, height, age, sex). The WHO/clinical-research standard. Same outputs as Mifflin for direct comparison.
• Katch-McArdle BMR — 3 fields (weight, body fat %, sex). LBM-based formula — most accurate for athletes. Outputs BMR, lean mass, fat mass, sedentary TDEE.
• TDEE Estimator — 5 fields (weight, height, age, sex, activity level). TDEE = BMR × WHO/FAO PAL multiplier. Shows all 5 activity levels simultaneously for comparison.
• Calorie Goal Planner — 6 fields (all TDEE fields + goal type). Sets evidence-based calorie targets for fat loss, maintenance, or lean gain. Outputs target calories, projected weekly change, and 30/40/30 P/C/F macro split.
• Formula Comparison — 5 fields (weight, height, age, sex, body fat %). All 3 formulas computed simultaneously. Consensus average and formula spread shown — spread <50 kcal = high agreement; >100 kcal = atypical body composition.

TDEE & Physical Activity Level (PAL) Multipliers

TDEE = BMR × PAL, where PAL (Physical Activity Level) is defined by WHO/FAO/UNU 2004:

Non-Exercise Activity Thermogenesis (NEAT) — fidgeting, standing, walking — accounts for 200–800 kcal/day variation between individuals. Accurate activity classification is the most important variable after BMR itself.

Calorie Goals & Macros (Evidence-Based)

1 kg of body fat ≈ 7,700 kcal. The macro split used (30% protein / 40% carbs / 30% fat) follows ACSM 2016 evidence-based guidelines. During fat loss, protein should be increased to ≥1.6–2.2 g/kg body weight to minimise lean mass catabolism (Phillips & Van Loon 2011).

Average BMR by Age & Sex (Mifflin-St Jeor)

Based on Mifflin-St Jeor using reference body sizes: 70 kg male (175 cm) / 60 kg female (163 cm). Individual BMR varies with actual body composition — athletes typically have BMR 10–20% above these averages.

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