Health & Medicine · Dietetics & Metabolism
Basal Metabolic Rate Calculator
Calculates your Basal Metabolic Rate (BMR) — the number of calories your body burns at rest — using the Mifflin-St Jeor equation.
Calculator
Formula
W = body weight in kilograms, H = height in centimetres, A = age in years. The result is expressed in kilocalories per day. The male and female equations differ by a constant offset (5 vs. −161) reflecting differences in baseline metabolic activity.
Source: Mifflin MD, St Jeor ST, et al. 'A new predictive equation for resting energy expenditure in healthy individuals.' American Journal of Clinical Nutrition, 1990;51(2):241–247.
How it works
Your body expends energy continuously, even while sleeping. This baseline energy expenditure, the BMR, accounts for roughly 60–75% of total daily calorie burn for sedentary individuals. It is influenced by body size, lean muscle mass, age, and biological sex. Larger bodies with more muscle tissue have higher BMRs because muscle is metabolically more active than fat tissue. As we age, BMR tends to decline, partly due to loss of muscle mass — a process called sarcopenia.
This calculator uses the Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate predictive formula for resting energy expenditure in healthy, non-obese adults. For males: BMR = 10W + 6.25H − 5A + 5, and for females: BMR = 10W + 6.25H − 5A − 161, where W is weight in kg, H is height in cm, and A is age in years. The constant offset differs by sex to reflect hormonal and physiological differences in baseline metabolism. Studies comparing predictive equations have consistently shown Mifflin-St Jeor to be more accurate than the older Harris-Benedict equation, with mean prediction errors typically under 10%.
The calculator also computes your Total Daily Energy Expenditure (TDEE) by multiplying BMR by an activity factor (PAL — Physical Activity Level). TDEE represents your true daily caloric need including exercise and non-exercise activity thermogenesis. This figure is what dietitians use to set a caloric surplus for muscle gain, a deficit for fat loss (typically 300–500 kcal below TDEE), or maintenance targets. TDEE is essential for personalised meal planning, sports nutrition, and clinical dietetic assessment.
Worked example
Consider a 35-year-old male who weighs 80 kg and stands 180 cm tall, with a moderately active lifestyle (activity factor 1.55).
Step 1 — Apply the Mifflin-St Jeor male equation:
BMR = 10 × 80 + 6.25 × 180 − 5 × 35 + 5
BMR = 800 + 1125 − 175 + 5
BMR = 1,755 kcal/day
Step 2 — Calculate TDEE using the activity multiplier:
TDEE = 1,755 × 1.55
TDEE = 2,720 kcal/day
This means the individual needs approximately 2,720 kcal per day to maintain his current weight given his activity level. To lose approximately 0.5 kg per week (a 500 kcal daily deficit), he would target around 2,220 kcal/day. To gain lean mass, a surplus of 250–300 kcal above TDEE (~2,970–3,020 kcal/day) would be appropriate.
Limitations & notes
The Mifflin-St Jeor equation was validated primarily in healthy, non-obese adults aged 19–78. Its accuracy may be reduced in the following groups: individuals with a BMI above 40 (morbid obesity), highly trained athletes with exceptionally high lean muscle mass, elderly populations over 80, individuals with thyroid disorders or other metabolic conditions, and pregnant or breastfeeding women. In clinical settings, indirect calorimetry — which measures actual oxygen consumption — remains the gold standard for measuring resting energy expenditure and may be preferred over predictive equations for critically ill patients or those with complex metabolic conditions. Activity multipliers are inherently subjective and represent a major source of error; TDEE estimates can be off by 20% or more depending on how accurately activity level is self-reported. This calculator should not replace individualised advice from a registered dietitian or physician.
Frequently asked questions
What is the difference between BMR and TDEE?
BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest to sustain basic physiological functions. TDEE (Total Daily Energy Expenditure) adds the calories burned through physical activity and daily movement on top of BMR. For most people, TDEE is 20–90% higher than BMR depending on activity level.
How accurate is the Mifflin-St Jeor equation?
In the original 1990 validation study, the Mifflin-St Jeor equation predicted measured resting energy expenditure within ±10% for approximately 82% of participants. A 2005 comparative study in the Journal of the American Dietetic Association found it outperformed the Harris-Benedict, Owen, and WHO equations. However, it remains a population-level average; individual predictions can vary by up to 15–20%.
How does age affect basal metabolic rate?
BMR generally declines with age at a rate of roughly 1–2% per decade after the age of 20, largely due to decreasing muscle mass (sarcopenia) and hormonal changes. In the Mifflin-St Jeor equation, this is captured by the −5A term. A 60-year-old with identical weight and height to a 30-year-old will have a BMR approximately 150 kcal/day lower.
Can I use this calculator if I am trying to lose weight?
Yes. Start by calculating your TDEE, which represents the calories needed to maintain your current weight. To lose approximately 0.5 kg per week, subtract 500 kcal from your TDEE. A deficit greater than 1,000 kcal/day is generally not recommended without medical supervision, as it risks muscle loss and nutritional deficiencies.
Why does the formula differ for males and females?
Biological sex influences basal metabolic rate primarily through differences in body composition and hormones. On average, males have a higher proportion of lean muscle mass and lower fat mass relative to body weight than females of the same size, and muscle tissue has a higher metabolic rate than adipose tissue. The Mifflin-St Jeor equation accounts for this with a constant offset: +5 for males and −161 for females, reflecting the average difference in resting energy expenditure observed in the validation dataset.
Last updated: 2025-01-15 · Formula verified against primary sources.