By MacroCalculators Editorial Team
What Is BMR?
BMR, or basal metabolic rate, is an estimate of the energy your body requires to maintain essential physiological functions under basal resting conditions. Those functions include the ongoing work required to keep the body alive, such as breathing, circulation, temperature regulation, and other basic processes.
The word basal matters. BMR describes a very specific resting state rather than the energy you use while walking, exercising, working, digesting a meal, or carrying out your normal daily routine.
When people ask, “What is my BMR?”, they are usually looking for a practical estimate of their resting energy requirement. A calculator can provide that estimate from a small set of personal measurements, but the result should be interpreted as a prediction rather than a laboratory measurement.
BMR gives you a useful starting point for understanding your individual resting energy requirements. It is an estimate, not a direct measurement, and it should not be treated as a complete picture of everything your body uses throughout the day.
How Is BMR Calculated?
A BMR calculator usually estimates resting energy expenditure with a predictive equation rather than measuring metabolism directly. Common equations use variables such as body weight, height, age, and sex. Some also use lean body mass.
Because different equations were developed from different groups of people and use different assumptions, two legitimate formulas can produce different BMR estimates for the same person. That does not automatically mean one calculation is wrong.
Mifflin-St Jeor Equation
The Mifflin-St Jeor equation was published in 1990 from data on 498 healthy adults whose resting energy expenditure was measured by indirect calorimetry. The simplified equations use weight, height, age, and a sex-specific constant. (1)
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
The variables are simple: weight is entered in kilograms, height in centimeters, and age in years. For the calculator, Imperial inputs should be converted before the equation is applied.
Revised Harris-Benedict Equation
The Harris-Benedict equations originated from early twentieth-century research on human basal metabolism. A revised version published by Roza and Shizgal in 1984 updated the predictive relationship using resting energy expenditure data. (2)
Men: BMR = 88.362 + (13.397 × weight) + (4.799 × height) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × weight) + (3.098 × height) − (4.330 × age)
These equations also use age, sex, height, and weight. They remain a useful reference point when comparing different approaches to estimating basal or resting energy expenditure.
Katch-McArdle Formula
The Katch-McArdle approach is different because it uses lean body mass rather than relying only on age, height, sex, and total body weight. The underlying research highlighted the relationship between basal energy requirements and lean body mass. (3)
BMR = 370 + (21.6 × lean body mass in kg)
This can be useful when a reasonably reliable estimate of lean body mass is available. The lean body mass calculator can provide an estimate for this input. The trade-off is that any error in the body-composition estimate is carried into the BMR calculation. If you need a body-fat estimate to help assess body composition, use the body fat calculator.
Cunningham Equation
The Cunningham equation is another lean-body-mass-based prediction. Cunningham’s work emphasized fat-free mass as a major predictor of resting energy expenditure. (4)
BMR = 500 + (22 × fat-free mass in kg)
Because this equation depends on fat-free mass, it is most meaningful when the body-composition input is reasonably well estimated. The formula may be useful for people who have a body-composition measurement available, but it should not be treated as automatically superior for every person.
BMR Formula Comparison
There is no single equation that will produce a perfect estimate for everyone. The choice of formula should be understood as a choice of prediction method, not a claim that one number is guaranteed to match an individual’s measured metabolism.
| Formula | Main inputs | Uses lean body mass? | Practical note |
|---|---|---|---|
| Mifflin-St Jeor | Age, sex, weight, height | No | Straightforward when only basic measurements are available. |
| Revised Harris-Benedict | Age, sex, weight, height | No | A long-established predictive approach. |
| Katch-McArdle | Lean body mass | Yes | Requires a body-composition estimate. |
| Cunningham | Fat-free mass | Yes | Puts greater emphasis on lean mass. |
Predictive equations can differ in performance across groups and measurement settings. Research comparing predicted and measured resting energy expenditure therefore supports treating the output as an estimate and considering the context in which the equation is being used. (5)
What Affects Your BMR?
BMR varies from person to person because the body itself varies. Age, body size, sex, and body composition are among the factors reflected in common predictive equations, while the underlying physiology is more complex than any single formula can capture.
| Factor | How it relates to BMR |
|---|---|
| Age | Age is included in several predictive equations and is associated with changes in resting energy requirements across the lifespan. |
| Body weight | Weight is a core variable in the Mifflin-St Jeor and Harris-Benedict equations. |
| Height | Height is included in several common predictive equations. |
| Sex | Common equations use sex-specific constants or coefficients. |
| Lean body mass | Fat-free mass is strongly associated with resting energy expenditure and is the central input in body-composition-based equations. |
| Body composition | People with the same body weight can have different amounts of lean and fat mass, which can contribute to different resting energy requirements. |
The practical takeaway is simple: BMR is personal. Comparing your number with someone else’s is less useful than understanding how the inputs and formula produced your own estimate.
BMR at Rest
BMR is often described as the calories your body needs “at rest,” but the term refers to more than simply lying on the sofa. Basal conditions are intentionally restrictive. The goal is to estimate the minimum level of energy use needed to support basic physiological functions under standardized resting conditions.
This is why BMR should not be confused with the total energy you use during an ordinary day. Physical activity, movement, digestion, and other processes add to energy use beyond basal requirements.
Keep in mind: Think of BMR as the body’s resting baseline, not as a complete daily calorie recommendation.
BMR vs RMR
BMR and RMR, or resting metabolic rate, are closely related terms. Both describe energy use while the body is at rest, but the measurement conditions are not necessarily identical. BMR uses more strictly defined basal conditions, while RMR is commonly measured under somewhat less restrictive resting conditions. (6)
| BMR | RMR | |
|---|---|---|
| Full term | Basal metabolic rate | Resting metabolic rate |
| Main idea | Energy use under basal conditions | Energy use while resting |
| Measurement conditions | More strictly defined | Usually less restrictive |
| Calculator use | Commonly predicted with equations | Also commonly predicted or measured |
For everyday calculator use, the distinction is mainly about terminology and measurement conditions. A BMR calculator is still providing an estimate of resting energy needs, not a direct metabolic measurement.
BMR for Men and Women
Most common BMR equations use sex-specific constants or coefficients. That means the same age, height, and weight can produce different estimates depending on which sex-specific equation is used.
BMR for Men
In the Mifflin-St Jeor equation, the male version ends with a +5 constant. Other equations use different coefficients. The difference is built into the equation itself rather than being an additional adjustment made after the result is calculated.
BMR for Women
In the Mifflin-St Jeor equation, the female version ends with a −161 constant. As with the male equation, this is part of the predictive model. A calculator should apply the appropriate equation based on the user’s selected sex.
Important: These formulas describe statistical relationships in the populations used to develop or validate them. They are not a statement that every individual within a sex category has the same metabolism.
BMR Examples
Seeing the calculation with real numbers makes the formula easier to understand. The examples below use the Mifflin-St Jeor equation and are for illustration rather than personal recommendations.
| Example | Age | Sex | Weight | Height | Estimated BMR |
|---|---|---|---|---|---|
| A | 25 | Female | 60 kg | 165 cm | 1,345 kcal/day |
| B | 35 | Male | 80 kg | 180 cm | 1,755 kcal/day |
| C | 50 | Female | 70 kg | 160 cm | 1,289 kcal/day |
Worked Example
Consider a 30-year-old man who weighs 70 kg and is 175 cm tall.
| Input | Value |
|---|---|
| Age | 30 years |
| Sex | Male |
| Weight | 70 kg |
| Height | 175 cm |
Worked example — Mifflin-St Jeor
BMR = 10 × 70 + 6.25 × 175 − 5 × 30 + 5
BMR = 700 + 1,093.75 − 150 + 5
BMR = 1,648.75 kcal/day
Rounded result: 1,649 kcal/day
The calculated result is an estimate of this person’s basal energy expenditure. It does not mean that 1,649 calories is automatically their full daily requirement, because normal daily life involves much more than basal activity.
How to Read Your BMR Result
The number produced by a BMR calculator is easiest to understand when you treat it as a baseline estimate. It tells you approximately how much energy a predictive equation expects your body to use under basal resting conditions based on the information you entered.
A larger or smaller BMR is not, by itself, a score of health or fitness. It reflects the relationship between your personal measurements and the equation being used.
Your BMR is an estimate, not a calorie prescription. For a practical daily calorie estimate, use the calorie calculator. For broader daily energy planning, a separate TDEE calculation accounts for activity and other components beyond BMR. See the TDEE calculator.
Once you have a calorie target and want to see suggested protein, carbohydrate, and fat targets alongside it, use the macro calculator.
Can BMR Change Over Time?
Yes. Your estimated BMR can change as your body changes. Age, body weight, and body composition can all change over time, and those changes can alter the inputs used by predictive equations.
That does not mean your metabolism changes dramatically every day. Rather, the estimate can shift as your underlying characteristics change. This is one reason an older calculation may no longer describe your current resting energy requirements as well as a calculation using your current measurements.
Common BMR Calculation Mistakes
Most BMR calculator errors are not complicated. They come from entering the wrong information, choosing the wrong method, or asking the result to tell you more than it actually can.
Using the Wrong Units
Mifflin-St Jeor and many other equations expect weight in kilograms and height in centimeters. Entering pounds or inches directly into a metric equation produces a meaningless result. The calculator should handle unit conversion automatically when Imperial units are selected.
Entering the Wrong Height or Weight
Check the number before calculating. A misplaced digit or an outdated measurement can change the result even when the formula itself is working correctly.
Confusing BMR With a Complete Daily Requirement
BMR represents resting energy requirements. It does not account for everything involved in a normal day. Treating the BMR number as a complete daily calorie requirement is a common misunderstanding.
Treating an Estimate as a Measurement
Predictive equations estimate resting energy expenditure. They do not directly measure your oxygen consumption or carbon dioxide production. Direct measurement requires a suitable metabolic testing method, such as indirect calorimetry.
Using an Unreliable Body-Composition Estimate
Equations based on lean or fat-free mass are only as reliable as the body-composition value entered. If the lean-mass estimate is inaccurate, the resulting BMR estimate will be affected too.
How Accurate Is a BMR Calculator?
A BMR calculator can be useful, but its result should be understood as an estimate. Predictive equations are built from measured data and statistical relationships, and their accuracy varies across individuals, populations, and measurement conditions.
The Mifflin-St Jeor study, for example, developed its equation from a specific sample of healthy adults. Other equations were developed from different groups and periods. That history matters because a prediction equation is not the same thing as a universal biological law. (1, 2)
Calculated BMR vs Measured Resting Energy Expenditure
Indirect calorimetry is a method used to measure resting energy expenditure by analyzing respiratory gases. Research has shown that measured resting expenditure can vary with testing conditions, which is another reason a calculated BMR should not be presented as an exact measurement. (6)
| Approach | What it provides | Main limitation |
|---|---|---|
| BMR calculator | A predicted estimate based on an equation | Individual error can be meaningful. |
| Indirect calorimetry | A measurement of resting energy expenditure under controlled conditions | Requires appropriate equipment and standardized testing. |
Bottom line: A calculator is valuable because it is convenient and uses information most people already know. The trade-off is that convenience comes with uncertainty around the individual prediction.
Frequently Asked Questions
These questions cover common BMR searches and practical points without creating a separate page for every keyword variation.
What is BMR?
BMR stands for basal metabolic rate. It is an estimate of the energy your body requires to maintain basic physiological functions under basal resting conditions.
What does BMR stand for?
BMR stands for basal metabolic rate.
What is my BMR?
Your BMR depends on the personal information entered into the calculator, including age, sex, height, and weight, as well as the equation used to make the estimate.
How do I calculate my BMR?
Enter your age, sex, height, and weight into a BMR calculator and apply the selected predictive equation. If using the formula by hand, make sure the units match the equation.
How is BMR calculated?
Common BMR calculations use predictive equations based on factors such as age, sex, height, weight, and sometimes lean body mass. Different equations can give different estimates.
What is a normal BMR?
There is no single normal BMR that applies to everyone. Resting energy requirements vary with body size, age, sex, body composition, and other individual factors.
Is BMR the same as resting metabolic rate?
They are closely related, but the terms can refer to measurements taken under different resting conditions. BMR uses more strictly defined basal conditions, while RMR usually refers to resting conditions that are somewhat less restrictive.
What is BMR at rest?
BMR describes the body’s energy use under basal resting conditions. It is meant to represent the energy required for essential physiological functions rather than activity or exercise.
Can BMR change?
Yes. Your estimated BMR can change when factors such as age, body weight, and body composition change.
What affects BMR?
Common predictors include age, sex, height, weight, and in some equations lean body mass. Individual physiology is more complex than any single equation.
Can I use BMR to lose weight?
BMR can help you understand resting energy requirements, but it is not by itself a complete daily calorie target. A broader calculation is needed to account for activity and other daily energy use.
Is a BMR calculator accurate?
A BMR calculator provides an estimate. Predictive equations can be useful, but the result may differ from an individual measurement of resting energy expenditure.
What is the difference between BMR and TDEE?
BMR estimates energy use under basal resting conditions. TDEE is a broader estimate that includes energy used beyond the basal level, such as daily activity. See the TDEE Calculator for the broader calculation.
What is the difference between BMR and RMR?
BMR and RMR are closely related measures of resting energy use, but BMR is associated with more strictly defined basal conditions. The terms are sometimes used loosely in everyday nutrition discussions.
Can I calculate BMR without a calculator?
Yes. You can calculate BMR manually with a published equation such as Mifflin-St Jeor, but a calculator reduces arithmetic and unit-conversion errors.
Why do different BMR calculators give different results?
They may use different equations, updated coefficients, rounding rules, or different assumptions about body composition. A different result does not automatically mean one calculator is malfunctioning.
Why does my BMR seem low or high?
BMR varies across people, so a number should be interpreted in the context of the inputs and equation used. A calculator result alone is not a diagnosis or a measure of overall health.
References
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A New Predictive Equation for Resting Energy Expenditure in Healthy Individuals. American Journal of Clinical Nutrition. 1990;51(2):241–247. PMID 2305711. DOI: 10.1093/ajcn/51.2.241. PubMed
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition. 1984;40(1):168–182. PMID 6741850. DOI: 10.1093/ajcn/40.1.168. PubMed
- McArdle-related BMR work: Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. American Journal of Clinical Nutrition. 1980;33(11):2372–2374. PMID 7435418. PubMed
- Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. American Journal of Clinical Nutrition. 1991;54(6):963–969. PMID 1957828. PubMed
- Thompson J, Manore MM. Predicted and measured resting metabolic rate of male and female endurance athletes. Journal of the American Dietetic Association. 1996;96(1):30–34. PMID 8537566. PubMed
- Compher C, Frankenfield D, Keim N, Roth-Yousey L. Best practice methods to apply to measurement of resting metabolic rate in adults: a systematic review. Journal of the American Dietetic Association. 2006;106(6):881–903. PMID 16720129. DOI: 10.1016/j.jada.2006.02.009. PubMed