BMR Calculator

What is BMR?

Basal metabolic rate (BMR) is the amount of energy your body uses to sustain essential functions at rest under tightly controlled conditions. Those functions include breathing, circulation, cellular activity, temperature regulation and the work of organs such as the brain, heart, liver and kidneys.

BMR is not the same as the calories you normally burn over an entire day. Daily movement, exercise and the energy used to digest food add to basal energy needs. For most people using an online calculator, the result is a prediction based on body measurements rather than a laboratory measurement.

Cleveland Clinic notes that equations can estimate BMR from factors such as sex, body weight, height and age, while an accurate basal measurement requires much more controlled testing conditions. That difference is why this calculator labels its output as an estimate rather than presenting it as an exact metabolic measurement.

How to use this BMR calculator

Choose the equation you want to use, select metric or US units, and enter the required measurements. Mifflin-St Jeor is the default because it is widely used for estimating resting energy expenditure in adults. The revised Harris-Benedict equation is provided for comparison, while Katch-McArdle uses estimated lean body mass and therefore requires body-fat percentage.

• Mifflin-St Jeor: requires weight, height, age and the equation's male/female coefficient.
• Revised Harris-Benedict: also uses weight, height, age and a sex-specific coefficient.
• Katch-McArdle: uses lean body mass, which this calculator derives from weight and body-fat percentage.

You may optionally select an activity level to see a rough total daily energy expenditure (TDEE) estimate. Leaving the activity field at “BMR only” avoids introducing an activity assumption.

Mifflin-St Jeor equation

The Mifflin-St Jeor equation was published in 1990 from data on 498 healthy adults aged 19 to 78, including both normal-weight and obese participants. Resting energy expenditure was measured by indirect calorimetry, then statistical equations were developed from weight, height, age and sex.

Here, W is weight in kilograms, H is height in centimeters and A is age in years. The original paper described the outcome as resting energy expenditure (REE), although online calculators commonly present this equation as a BMR estimate.

Because the study population consisted of adults, this calculator is designed for adults rather than children. It also should not be treated as a specialized equation for pregnancy, breastfeeding, illness, recovery from surgery or other clinical situations in which energy needs can differ substantially.

Worked Mifflin-St Jeor example

Consider a 35-year-old adult using the male coefficient, weighing 80 kg and measuring 180 cm tall:

The predicted resting requirement is therefore about 1,755 kcal/day. In kilojoules, that is roughly 7,343 kJ/day because one dietary kilocalorie is approximately 4.184 kilojoules.

This does not mean that eating exactly 1,755 kcal would maintain body weight. Maintenance needs also include physical activity and other components of total energy expenditure.

Revised Harris-Benedict equation

The original Harris-Benedict equations date to the early twentieth century. In 1984, Roza and Shizgal reevaluated the Harris-Benedict data and published revised coefficients. The revised version remains useful as an alternative prediction method and can show how much equation choice changes an estimate.

Again, weight is in kilograms, height in centimeters and age in years. Different references sometimes round coefficients, so tiny differences between calculators can occur even when they use the same named equation.

Katch-McArdle equation

Katch-McArdle takes a different approach because it uses lean body mass rather than age, height and a sex-specific constant. The commonly used formula is:

If lean mass is not directly known, it can be estimated from body weight and body-fat percentage:

For a person weighing 80 kg at an estimated 20% body fat, lean body mass is 64 kg. Katch-McArdle then estimates BMR at 370 + 21.6 × 64 = 1,752.4 kcal/day.

The major limitation is that body-fat measurements themselves can be inaccurate. A formula that depends on lean mass is only as reliable as the body-composition estimate entered into it.

Which BMR equation should you use?

There is no prediction equation that is perfectly accurate for every person. Equation performance depends on the population being studied, measurement quality and individual physiology. Mifflin-St Jeor is a practical default for many adults because it was derived from measured resting energy expenditure in a relatively large adult sample and remains widely used.

Revised Harris-Benedict provides a useful second estimate using a different set of coefficients. Katch-McArdle can be informative when body-fat percentage is known reasonably well because it responds directly to estimated lean mass.

BMR vs RMR: are they the same?

BMR and resting metabolic rate (RMR), also called resting energy expenditure (REE), are closely related but not perfectly interchangeable in laboratory physiology. True BMR is measured under stricter basal conditions, typically after rest and fasting in a thermoneutral environment. RMR testing is generally somewhat less restrictive.

In everyday fitness websites, the terms are often used loosely and the same predictive equations may be labeled “BMR calculators.” The Mifflin paper itself predicted resting energy expenditure. The practical lesson is to treat equation results as estimates of resting energy needs rather than laboratory-certified BMR.

BMR vs TDEE

Total daily energy expenditure (TDEE) is broader than BMR. It includes basal/resting metabolism plus physical activity and other components of energy expenditure. This is why TDEE is usually higher than BMR.

Simple calculators often use broad multipliers such as 1.2 for sedentary lifestyles and larger values for higher activity. These categories are convenient, but they are not direct measurements of your actual movement, exercise intensity, occupation or adaptive changes in energy expenditure.

The NIH Body Weight Planner uses a more sophisticated dynamic model and treats physical activity level as an adjustable input rather than assuming that a single multiplier perfectly predicts everyone. For that reason, the TDEE feature here is clearly presented as an optional rough estimate.

Activity multiplier table

Do not interpret these labels too literally. Two people who both exercise four days per week can have very different daily energy expenditure if one has a sedentary job and the other performs heavy physical labor.

What affects basal metabolic rate?

BMR varies between people because the body tissues and processes that consume energy are not identical. Important influences include body size, fat-free mass, age, sex-related physiology, genetics, hormones, health status and environmental conditions.

Lean tissue is especially important because organs and muscle are metabolically active. The Mifflin study found fat-free mass to be the strongest single predictor among the variables it examined, although the practical equation uses weight, height, age and sex because those measurements are easier to obtain.

Changes in body weight can therefore change predicted BMR, but equal changes in weight do not necessarily have identical metabolic effects when body composition differs.

Why age changes the estimate

Both Mifflin-St Jeor and revised Harris-Benedict include age as a negative term. That means the predicted value decreases as age rises when the other inputs are held constant. This statistical relationship reflects the populations used to derive the formulas; it should not be interpreted as a precise biological rule that every birthday lowers an individual's actual metabolism by exactly the formula coefficient.

Changes in body composition, organ mass, activity and health across adulthood also matter. Prediction equations summarize group patterns rather than directly measuring those mechanisms in one person.

Why body composition matters

Two adults can have the same body weight but different proportions of fat mass and fat-free mass. Because fat-free tissues generally require more energy than adipose tissue, their true resting expenditure may differ even when a weight-based equation gives similar estimates.

Katch-McArdle attempts to capture this by using lean body mass. But body-fat percentage can be estimated by many methods — scales, skinfolds, circumference equations, imaging and others — and those methods have different error ranges. A precise-looking BMR output does not eliminate uncertainty in the input.

Why BMR calculators are estimates, not measurements

Predictive equations are regression models built from groups of people. Even a good equation will overestimate some individuals and underestimate others. The revised Harris-Benedict reevaluation reported substantial individual prediction uncertainty, and later validation research has repeatedly shown that no single equation perfectly predicts resting expenditure across all populations.

For someone who needs a more direct assessment — for example because of a medical condition, unusual body composition or unexplained discrepancies between expected and observed energy needs — indirect calorimetry performed under appropriate conditions is a more direct measurement approach.

What is indirect calorimetry?

Indirect calorimetry estimates energy expenditure from respiratory gas exchange, typically by measuring oxygen consumption and carbon dioxide production. It is widely used in research and clinical settings to measure resting energy expenditure.

Testing conditions matter. Food intake, recent exercise, stimulants, room temperature, stress and insufficient rest can influence a resting measurement. That is why a laboratory BMR/RMR assessment is not equivalent to simply lying on a couch while wearing a consumer fitness tracker.

Calories vs kilojoules

Food energy may be reported in kilocalories (kcal) or kilojoules (kJ). The “Calories” shown on food labels in some countries are kilocalories.

If estimated BMR is 1,700 kcal/day, that corresponds to about 7,113 kJ/day. Changing the display unit in this calculator does not change the underlying metabolic estimate; it only converts the energy unit.

Metric and US unit conversion

The equations are calculated internally with kilograms and centimeters. When you choose US units, the calculator converts pounds to kilograms and feet/inches to centimeters before applying the selected formula.

This prevents a common calculation error: inserting pounds or inches directly into formulas whose coefficients were derived for kilograms and centimeters.

Worked comparison of two equations

Take a 40-year-old adult using the female coefficient, weighing 65 kg and measuring 165 cm:

Mifflin-St Jeor: 10(65) + 6.25(165) − 5(40) − 161 = about 1,320 kcal/day.

Revised Harris-Benedict: 9.247(65) + 3.098(165) − 4.330(40) + 447.593 = about 1,386 kcal/day.

The difference is about 66 kcal/day. That does not mean one calculation must be “wrong.” It shows the uncertainty created by using different population-derived prediction models.

Can BMR be used as a calorie target?

BMR by itself is generally not a maintenance-calorie target because it excludes normal daily activity. If your purpose is estimating how much energy you expend in a day, TDEE or a more comprehensive energy-balance model is more relevant.

Likewise, simply subtracting a fixed number of calories from BMR is not a robust weight-management method. Energy needs change with body weight, activity and time. The NIH Body Weight Planner uses a dynamic mathematical model specifically because static rules can misrepresent how body weight responds to energy changes.

BMR and weight loss

When body weight decreases, energy expenditure often changes as well. Part of that change is expected because a smaller body generally requires less energy, while physiological adaptation and changes in activity can also contribute.

For long-term weight planning, recalculating an equation from time to time may provide an updated estimate, but a dynamic tool or professional assessment can account for changes more appropriately than assuming the original BMR remains constant.

BMR and weight gain

Gaining body mass usually raises predicted BMR because weight is a positive term in commonly used equations. However, the amount of change depends on the tissue gained and the equation used. An increase in lean mass and an equal increase in fat mass do not necessarily have the same effect on actual resting expenditure.

This is another reason to avoid thinking of BMR as a fixed “metabolism score.” It is an energy expenditure estimate that changes with body size, composition and physiology.

BMR and exercise

Exercise does not simply get added into the BMR equation. BMR describes resting needs. Exercise contributes to total daily expenditure, and training can also influence body composition over time.

Strength training that increases lean mass may affect resting expenditure indirectly through body-composition changes. Endurance exercise can raise total daily energy use through the activity itself. The size of these effects varies widely between people and training programs.

BMR and sleep, stress, illness and hormones

Metabolism is influenced by physiology that a simple height-weight-age equation cannot fully represent. Thyroid function, fever, illness, injury, medications, sleep patterns, stress responses and other factors can alter energy expenditure or appetite.

If your estimated needs seem dramatically inconsistent with your experience and there is a health concern, use the calculator as a starting point rather than attempting to diagnose the cause from the number.

Pregnancy and breastfeeding

This adult calculator is not designed to estimate the additional energy needs of pregnancy or breastfeeding. Those life stages involve nutritional requirements that are not represented by ordinary BMR equations or a generic activity multiplier.

For individualized advice during pregnancy or breastfeeding, use guidance from a qualified healthcare professional who can consider gestational stage, health history, body changes and nutritional needs.

Children and teenagers

The default equations on this page were developed for adult populations, so this calculator restricts age input to adults. Children and adolescents have growth-related energy requirements and age-specific predictive equations.

Using an adult Mifflin or revised Harris-Benedict result for a growing child can create a misleading number. Pediatric nutrition questions should use age-appropriate methods and professional guidance where needed.

Older adults

Prediction accuracy can also vary in older adults. Research comparing equations with measured resting metabolic rate in older populations has found equation-specific over- and underestimation. Changes in lean mass, health and body composition can make a population average less representative of one individual.

For routine planning, an equation may still be useful as a starting estimate. For clinical nutrition decisions, measured or professionally assessed needs can be more appropriate.

Why your fitness tracker may show a different number

Wearable devices often estimate resting calories and active calories using proprietary algorithms. Their “resting” figure may not use the same equation or definition as a BMR calculator. Some devices include small amounts of routine movement in what they display as resting energy.

Differences can therefore arise even when your height, weight and age are entered correctly. Compare trends within one system more cautiously than treating figures from different systems as directly interchangeable.

Common BMR calculator mistakes

• Entering pounds into a kilogram formula. Unit mismatch can produce a huge error.
• Confusing BMR with maintenance calories. BMR excludes normal activity.
• Treating an equation as an exact measurement. Predictive equations have individual error.
• Using Katch-McArdle with an unreliable body-fat estimate. Lean-mass error flows directly into the result.
• Comparing calculators that use different equations. Different coefficients can produce different estimates.
• Assuming activity categories are precise. Weekly exercise frequency alone does not capture total movement.
• Using adult equations for children. Growth changes energy needs and equation choice.
• Using BMR as a rigid minimum intake rule. Nutritional adequacy and clinical needs cannot be determined from BMR alone.
• Ignoring pregnancy, illness or other special conditions. Generic equations may not apply.
• Overreacting to small differences. A 30–80 kcal difference between equations can be smaller than prediction uncertainty.

How to interpret your result realistically

Think of the BMR result as a model-based estimate of resting energy expenditure. It can be useful for comparing formulas, converting between kcal and kJ, and serving as one component of broader energy planning.

Avoid treating the final digit as meaningful precision. A result of 1,742 kcal/day should not be interpreted as proof that your body burns exactly 1,742 calories every day at rest. Real biological expenditure changes and prediction error are larger than one calorie.

If you use the estimate for general planning, monitor real-world outcomes over time and update assumptions when body weight, activity or circumstances change.

Frequently asked questions

Metabolic estimate note

BMR equations are population-based estimates, not direct measurements or personalized medical prescriptions. Use the same equation and unit conventions when comparing results, and treat optional activity-based TDEE as a rough planning estimate rather than a guaranteed maintenance intake. This calculator is intended for adults and is not designed for children, pregnancy or breastfeeding. If illness, medication, unusual body composition or another health factor may affect energy needs, a qualified healthcare professional or measured resting-energy assessment can provide more appropriate guidance.