Free BMR Calculator

Calculate your Basal Metabolic Rate and Total Daily Energy Expenditure using the Mifflin-St Jeor equation.

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What Is BMR?

Basal Metabolic Rate (BMR) is the number of calories your body needs to sustain basic physiological functions — breathing, circulation, cell production, protein synthesis, and temperature regulation — while at complete rest. It represents the minimum caloric expenditure required to keep you alive, accounting for roughly 60–75% of total daily energy expenditure for most people. BMR is measured under strict conditions: complete rest (lying still), a fasted state (12–18 hours after eating), and a thermoneutral environment (neither too hot nor too cold). Because those conditions are impractical to replicate at home, BMR is typically estimated using predictive equations based on body weight, height, age, and sex.

Mifflin-St Jeor vs Harris-Benedict Equations

Two equations are commonly used to estimate BMR. The older Harris-Benedict equation, developed in 1919 and revised in 1984, was the clinical standard for decades. The newer Mifflin-St Jeor equation, published in 1990, was developed using a more representative sample and has been shown in multiple studies to be more accurate for most adults. The American Dietetic Association recommends Mifflin-St Jeor as the preferred equation for calculating resting metabolic rate in non-obese adults.

The Mifflin-St Jeor formulas are:

  • Male: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
  • Female: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

This calculator uses the Mifflin-St Jeor equation. Studies show it predicts resting metabolic rate within 10% of measured values for about 82% of individuals, making it the most reliable widely-available estimate without laboratory testing.

TDEE — Total Daily Energy Expenditure

TDEE is your total daily calorie expenditure, calculated by multiplying your BMR by an activity factor that accounts for how much you move throughout the day. The activity multipliers used in this calculator are based on the widely-used Harris-Benedict activity scale:

  • Sedentary (×1.2): Office job, little formal exercise, mostly sitting or standing.
  • Lightly Active (×1.375): Light exercise 1–3 days per week — walking, yoga, casual cycling.
  • Moderately Active (×1.55): Moderate exercise 3–5 days per week — jogging, gym workouts, recreational sports.
  • Very Active (×1.725): Hard training 6–7 days per week or a physically demanding job.
  • Extra Active (×1.9): Very hard daily exercise, twice-daily training, or an extremely physically demanding job (construction, farming, competitive athletics).

TDEE is the number of calories you need to consume to maintain your current weight. Eating below TDEE creates a caloric deficit (weight loss); eating above TDEE creates a caloric surplus (weight gain).

Calorie Deficit for Weight Loss

A caloric deficit of 500 calories per day below TDEE produces approximately 1 lb (0.45 kg) of fat loss per week — based on the often-cited rule that 1 lb of fat contains approximately 3,500 calories. This estimate is somewhat oversimplified (metabolic adaptation, water retention, and lean mass changes all affect actual weight loss), but it provides a practical starting point. Moderate deficits of 500–750 calories per day are generally recommended for sustainable weight loss with minimal muscle loss. Very low calorie diets (1,000+ calorie daily deficit) are associated with muscle loss, metabolic adaptation, nutrient deficiencies, and difficulty maintaining long-term.

Frequently Asked Questions

In clinical studies, Mifflin-St Jeor predicts within 10% of measured RMR for approximately 82% of non-obese adults. Accuracy decreases for severely obese individuals, very muscular athletes, and older adults with significantly reduced lean mass. For a more accurate measurement, a metabolic rate test (indirect calorimetry) performed by a registered dietitian is the gold standard, though this requires clinical equipment.

Key factors include: lean body mass (the most significant predictor — more muscle = higher BMR), age (BMR declines roughly 1–2% per decade after age 20), sex (males have higher BMR due to greater lean mass), thyroid function (hypothyroidism significantly reduces BMR), body surface area, genetics, ambient temperature, and hormonal status. Dietary restriction and significant weight loss also cause metabolic adaptation — a reduction in BMR below what weight alone would predict.

BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) are often used interchangeably but are technically different. BMR is measured under strict fasted, rested, and thermoneutral conditions and represents the absolute minimum caloric need. RMR is measured under less strict conditions (e.g., 3–4 hours after eating, sitting or lying quietly) and is typically 10–20% higher than BMR. Predictive equations like Mifflin-St Jeor estimate RMR rather than true BMR, but the term "BMR" is commonly used for these estimates in everyday contexts.

A common starting point is to subtract 500 calories from your TDEE, which theoretically produces about 1 lb of fat loss per week. However, most experts recommend not going below 1,200 calories per day for women or 1,500 calories for men, as lower intakes risk nutritional deficiencies and muscle loss. Individual calorie needs vary, and sustainable weight loss is best achieved with guidance from a registered dietitian who can account for your specific goals, medical history, and lifestyle.

The idea that eating 5–6 small meals per day "boosts metabolism" is not supported by current research. The total thermic effect of food (the calories burned digesting food) depends on total caloric and macronutrient intake, not meal frequency. Meal frequency is a personal preference issue — what matters for metabolic rate and body composition is total caloric intake relative to TDEE, not how many times per day you eat.

Exercise affects total calorie expenditure directly (calories burned during exercise) and indirectly (elevated post-exercise oxygen consumption, or EPOC). Resistance training builds lean muscle mass, which permanently increases BMR — each pound of muscle burns approximately 6 calories per day at rest, compared to 2 calories per day for fat tissue. Long-term, consistent resistance training is the most effective way to permanently elevate BMR. Aerobic exercise primarily contributes through direct caloric expenditure during the activity itself.

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