BMR (Metabolic Rate) Calculator

About BMR (Metabolic Rate) Calculator

The BMR Formulas

Both formulas use age, sex, weight, and height. The Mifflin-St Jeor equation (1990) is the more modern and generally recommended formula. The Harris-Benedict equation (1918, revised by Roza and Shizgal in 1984) is the older standard.

Formula	Men	Women
Mifflin-St Jeor	(10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5	(10 x weight in kg) + (6.25 x height in cm) - (5 x age) - 161
Harris-Benedict (revised)	88.362 + (13.397 x kg) + (4.799 x cm) - (5.677 x age)	447.593 + (9.247 x kg) + (3.098 x cm) - (4.330 x age)

Worked example (Mifflin-St Jeor, male): Age 30, weight 80 kg, height 180 cm

BMR = (10 x 80) + (6.25 x 180) - (5 x 30) + 5 = 800 + 1125 - 150 + 5 = 1,780 calories/day

Same person, Harris-Benedict:

BMR = 88.362 + (13.397 x 80) + (4.799 x 180) - (5.677 x 30) = 88 + 1072 + 864 - 170 = 1,854 calories/day

The two formulas differ by about 74 calories here. Mifflin-St Jeor tends to give slightly lower estimates and has been shown to be more accurate in clinical validation studies.

Which Formula Should I Trust?

Factor	Mifflin-St Jeor	Harris-Benedict (revised)
Year developed	1990	1918 (revised 1984)
Study population	498 healthy adults	239 subjects (original)
Accuracy (within 10% of measured)	~82% of subjects	~69% of subjects
Recommendation	Preferred by ADA and most dietitians	Still widely used, slightly less accurate
Tends to	Slightly underestimate	Slightly overestimate

The American Dietetic Association recommends Mifflin-St Jeor as the best predictive equation for healthy, non-obese and obese adults. Neither formula is perfect - individual variation due to genetics, body composition, and metabolic health can cause actual BMR to differ by 10-15% from any estimate.

What Affects Your BMR?

Factor	Effect on BMR	Why
Lean muscle mass	Higher muscle = higher BMR	Muscle tissue burns roughly 6 calories per pound per day at rest, compared to 2 for fat
Age	BMR decreases ~1-2% per decade after 20	Gradual loss of muscle mass and hormonal changes
Sex	Men typically have higher BMR	Generally more muscle mass and less body fat
Body size	Larger bodies have higher BMR	More tissue to maintain
Thyroid function	Hypothyroidism lowers BMR	Thyroid hormones regulate metabolic rate
Genetics	Varies by up to 200-300 cal/day	Natural variation in metabolic efficiency
Body temperature	Fever increases BMR ~7% per 0.5C	Higher temperature speeds chemical reactions
Extreme dieting	Can lower BMR by 15-20%	Metabolic adaptation / "starvation response"

From BMR to Total Daily Energy Expenditure (TDEE)

BMR is only the calories burned at complete rest. To find your actual daily calorie needs, multiply BMR by an activity factor:

Activity Level	Multiplier	Description	Example TDEE (BMR = 1,780)
Sedentary	1.2	Desk job, little or no exercise	2,136
Lightly active	1.375	Light exercise 1-3 days/week	2,448
Moderately active	1.55	Moderate exercise 3-5 days/week	2,759
Very active	1.725	Hard exercise 6-7 days/week	3,071
Extra active	1.9	Athlete or very physical job	3,382

For a full TDEE calculation with macro breakdowns, use the TDEE Calculator.

Components of Total Daily Calorie Burn

Your body burns calories through three main pathways:

Component	% of Total	What It Covers
BMR (Basal Metabolic Rate)	60-75%	Heart, lungs, brain, liver, kidneys, cell repair - all at rest
TEF (Thermic Effect of Food)	~10%	Energy to digest, absorb, and process food
EAT + NEAT (Activity)	15-30%	Deliberate exercise (EAT) plus all non-exercise movement like walking, fidgeting, standing (NEAT)

Because BMR is the largest component, changes to your BMR (through building muscle, for example) have the biggest impact on your total daily calorie expenditure.

Can I Increase My BMR?

The most effective way to raise your BMR is to increase lean muscle mass through resistance training. Each pound of muscle burns roughly 6 calories per day at rest, compared to about 2 calories per pound of fat. Over time, adding 10 pounds of muscle could raise your resting calorie burn by about 40-50 calories per day.

Other factors that support a healthy BMR: adequate protein intake (the thermic effect of protein is higher than carbs or fat), consistent sleep (sleep deprivation can reduce BMR), staying hydrated, and avoiding prolonged severe calorie restriction, which triggers metabolic adaptation.

The Katch-McArdle Formula

Both Mifflin-St Jeor and Harris-Benedict estimate BMR from total body weight, which means they treat muscle and fat the same. The Katch-McArdle formula takes a different approach: it uses lean body mass (LBM) instead. The formula is:

BMR = 370 + (21.6 x lean body mass in kg)

This formula is the same for men and women, because the sex difference in BMR is almost entirely explained by differences in lean mass. If you know your body fat percentage (from a DEXA scan, skinfold calipers, or the Body Fat Calculator), you can calculate lean mass as: weight x (1 - body fat % / 100).

Worked example: Male, 80 kg, 18% body fat

Lean mass = 80 x (1 - 0.18) = 65.6 kg

BMR = 370 + (21.6 x 65.6) = 370 + 1,417 = 1,787 cal/day

For this person, Katch-McArdle gives 1,787, Mifflin-St Jeor gives around 1,780, and Harris-Benedict gives about 1,854. The formulas converge when body composition is average. But for someone very lean (like a bodybuilder at 8% body fat) or someone with a high body fat percentage, Katch-McArdle will be more accurate because it accounts for the actual amount of metabolically active tissue.

Measuring BMR in a Clinical Setting

The gold standard for measuring actual BMR (rather than estimating it) is indirect calorimetry. The subject lies down in a quiet, temperature-controlled room after fasting for 10-12 hours. A transparent hood or mask is placed over their face, and the device measures the volume of oxygen consumed and carbon dioxide produced over 15-30 minutes. Since the body uses oxygen to burn fuel and produces CO2 as a byproduct, the ratio of these gases (called the respiratory quotient) reveals both total calorie burn and what fuel mix the body is using (more carbs vs. more fat).

Indirect calorimetry machines are found in hospitals, sports science labs, and some high-end gyms. A single test typically costs between $100-$250 / GBP75-GBP200. It is worth considering if you have been tracking calories consistently but not getting expected results, as it can reveal whether your actual BMR is significantly different from what the formulas predict. About 18% of people will have a measured BMR that is more than 10% off from the Mifflin-St Jeor estimate.

How BMR Changes with Age

BMR declines as you age, primarily because of gradual loss of lean muscle mass and changes in hormonal activity. The drop is not dramatic in any single year, but it compounds over decades. This table shows approximate BMR decline by decade for an average-height person, based on cross-sectional data from multiple metabolic studies:

Age Range	Estimated BMR (Men, 80 kg)	Estimated BMR (Women, 65 kg)	Change from Age 20-29
20-29	1,800	1,400	Baseline
30-39	1,750	1,370	-2 to 3%
40-49	1,700	1,340	-4 to 6%
50-59	1,640	1,300	-7 to 9%
60-69	1,580	1,260	-10 to 12%
70+	1,500	1,200	-14 to 17%

The Pontzer et al. (2021) study in Science, using doubly labeled water data from over 6,400 people, found that the sharpest decline in metabolic rate actually does not begin until after age 60, later than previously thought. Before 60, the decline is mostly explained by changes in body composition (less muscle, more fat) rather than any change in how efficiently cells burn energy.

BMR During Pregnancy and Lactation

Pregnancy increases BMR significantly. The Institute of Medicine (IOM) and the American College of Obstetricians and Gynecologists (ACOG) estimate the additional energy cost of pregnancy at roughly 340 extra calories per day in the second trimester and 450 extra calories per day in the third trimester, compared to pre-pregnancy BMR. This extra demand supports foetal growth, increased blood volume, and the metabolic activity of the placenta and uterine tissue.

During lactation, the energy cost is even higher. Producing breast milk requires approximately 500 additional calories per day (WHO estimate), though some of this is offset by mobilisation of fat stores accumulated during pregnancy. The NHS recommends that breastfeeding mothers eat to appetite rather than trying to count calories, as individual variation in milk production and activity levels makes fixed numbers unreliable.

How Muscle Mass Affects BMR

Muscle tissue is more metabolically active than fat tissue at rest, but the difference per kilogram is often overstated in fitness media. Careful measurements show that each kilogram of skeletal muscle burns about 13 calories per day at rest, while each kilogram of fat tissue burns about 4.5 calories per day (Elia, 1992, from a chapter in Energy Metabolism: Tissue Determinants and Cellular Corollaries, Raven Press, 1992). That is a real difference, but it means adding 5 kg of muscle (a substantial amount, representing months or years of training) would increase your resting calorie burn by about 43 calories per day.

So why does strength training matter so much for metabolism? Because the calorie burn from muscle is not just at rest. Resistance exercise creates an "afterburn" effect (excess post-exercise oxygen consumption, or EPOC) that elevates calorie expenditure for 24-48 hours after a session. The training itself burns calories. And maintaining a higher training volume keeps NEAT elevated. The total metabolic impact of carrying more muscle goes well beyond the 13 cal/kg/day resting figure.

BMR Reference Ranges

Group	Typical BMR Range (cal/day)
Women, sedentary, 18-30	1,200 - 1,500
Women, active, 18-30	1,400 - 1,700
Men, sedentary, 18-30	1,600 - 1,900
Men, active, 18-30	1,800 - 2,200
Women, 50+	1,100 - 1,400
Men, 50+	1,400 - 1,700

These are rough ranges. Your actual BMR depends on your specific height, weight, body composition, and genetics.

Once you know your BMR, combine it with the Calorie Calculator to set daily intake targets for weight loss, maintenance, or gain. For a detailed macronutrient split, the Macro Calculator divides your calories into protein, carbs, and fat. To see how your BMR translates to total daily calorie burn with activity factored in, the TDEE Calculator applies the appropriate multiplier.

All calculations run entirely in your browser. No personal data is sent to any server.

Sources

• Mifflin MD et al. (1990) - A new predictive equation for resting energy expenditure in healthy individuals, American Journal of Clinical Nutrition
• Roza AM, Shizgal HM (1984) - The Harris Benedict equation reevaluated, American Journal of Clinical Nutrition
• Pontzer H et al. (2021) - Daily energy expenditure through the human life course, Science
• USDA - Dietary Reference Intakes Calculator
• NHS - Eating a balanced diet and energy needs
• WHO - Energy requirements and growth standards