BMR Calculator (Mifflin-St Jeor)

Article — BMR Calculator (Mifflin-St Jeor)

BMR calculator: Mifflin-St Jeor basal metabolic rate

Basal metabolic rate is the number of kilocalories your body burns at complete rest to keep heart, lungs, brain, kidneys, and other organs running. The Mifflin-St Jeor equation, published in 1990, predicts BMR for adults more accurately than the older Harris-Benedict formula, with an average error of about 10% against measured values from indirect calorimetry. For most adults BMR falls between 1200 and 2000 kilocalories per day.

This calculator runs Mifflin-St Jeor as the primary formula and reports Harris-Benedict alongside as a cross-check. Below the result you get TDEE (total daily energy expenditure) for five activity levels, so you can see how a desk job differs from an active week without typing different numbers.

What is BMR?

BMR is the minimum metabolic activity your body needs to stay alive. It is measured under strict conditions: subject awake but lying still, room at thermal neutrality (about 22 degrees Celsius), no food intake for 12 hours, and no physical activity in the previous 24 hours. Under those conditions, oxygen consumption directly reflects basal energy use, which converts to kilocalories at 4.825 kcal per litre of oxygen consumed.

BMR makes up 60 to 75% of total daily energy expenditure in a sedentary adult. The next biggest chunk is the thermic effect of food (8-10% of intake, used to digest and absorb meals), followed by activity-related energy expenditure that ranges from 15% in a desk worker to 40%+ in an athlete. Exercise-induced burn is usually a smaller share of TDEE than NEAT, the non-exercise activity thermogenesis from fidgeting, posture maintenance, and casual movement.

The Mifflin-St Jeor BMR formula

Mark Mifflin and Sachiko St Jeor published their equation in the American Journal of Clinical Nutrition in 1990. They fit it to indirect calorimetry on 498 healthy adults (264 women and 234 men) aged 19 to 78. The result is a linear formula with the same four inputs as Harris-Benedict (weight, height, age, sex) but smaller weight and height coefficients that handle modern body compositions better.

The variables are W in kilograms, H in centimetres, and A in years. The sex constant is +5 for men and -161 for women, a 166-kcal gap that mostly captures the difference in average lean body mass. Plug in a 30-year-old male at 80 kg and 180 cm and the formula gives 1780 kcal/day. A 30-year-old female at 65 kg and 165 cm comes to 1370 kcal/day.

BMR Mifflin vs Harris-Benedict

The Academy of Nutrition and Dietetics ran a systematic review in 2005 and found that Mifflin-St Jeor predicted measured resting metabolic rate within 10% in 82% of non-obese adults. Harris-Benedict managed only 64%. The gap widens with body mass: for obese subjects, Harris-Benedict over-estimates BMR by 5-15% because its 1918 cohort had an average BMI around 21, far leaner than modern populations.

Harris-Benedict still appears in some textbooks and online calculators, which is why this widget reports both numbers. If the two values differ by more than 10%, the inputs likely sit at the edge of one equation's validation range. Mifflin-St Jeor is the safer default for most users today.

What affects your BMR

Five factors drive BMR variation in healthy adults:

• Lean body mass — 60-70% of BMR variance, the single biggest factor
• Body size — bigger people burn more at rest in absolute kcal
• Age — BMR drops 2-3% per decade after 30, mostly through muscle loss
• Sex — men average 5-10% higher BMR than women of matched weight
• Thyroid status — hyperthyroidism raises BMR up to 50%, hypothyroidism drops it 25%
• Genetics — explains 20-30% of inter-individual variation
• Environment — cold exposure raises BMR through brown adipose thermogenesis

BMR to TDEE activity multipliers

TDEE is BMR multiplied by an activity factor between 1.2 and 1.9. The multipliers were popularised by the American College of Sports Medicine in the 1990s and have stuck around because they are simple and roughly accurate for population averages. The widget shows all five levels in the result table.

Using BMR for weight loss

For weight loss, the standard advice is a 300-500 kcal/day deficit below TDEE. That deficit, sustained for a week, yields roughly 0.45 to 0.9 kg (1 to 2 lb) of fat loss, since one pound of fat stores about 3500 kcal. Eating below BMR is generally not recommended because it triggers metabolic adaptation: the body lowers thyroid output and NEAT, which makes further loss harder.

For weight gain, the same logic runs in reverse. A 300-500 kcal/day surplus drives 0.25-0.5 kg/week of mass gain. With resistance training and adequate protein (1.6-2.2 g/kg/day), most of that gain can be lean tissue rather than fat. Without training, the lean-to-fat split is closer to 30:70.

BMR accuracy and equation limits

Mifflin-St Jeor has roughly 10% standard error against indirect calorimetry, which is the gold standard. That means a calculator output of 1700 kcal/day represents a measured value most likely between 1530 and 1870. Real-world TDEE error is bigger because the activity multiplier is self-reported and the thermic effect of food varies with diet composition.

How to raise your BMR

Lean body mass is the lever you can actually move. Each kilogram of skeletal muscle burns roughly 13 kcal/day at rest, against 4.5 kcal/day for the same mass of fat. Adding 2-3 kg of muscle through structured resistance training (3 sessions per week, progressive overload) raises BMR by 25-40 kcal/day. The effect is modest but compounds with the higher TDEE from the training sessions themselves.

Adequate sleep (7-9 hours), thyroid health, protein intake of 1.6-2.2 g/kg/day, and consistent movement throughout the day all support BMR maintenance. Crash dieting does the opposite: weeks of intake below BMR can reduce metabolic rate by 10-25% beyond what the weight loss alone explains, and recovery takes months of normalised eating.