What Is a BMR Calculator
A BMR calculator uses a population equation to predict how much energy an adult body would expend over a day at rest. This page uses the commonly searched term basal metabolic rate, but its main Mifflin-St Jeor result is technically a prediction of resting energy expenditure, or REE, expressed in kilocalories per day.
The estimate represents energy associated with basic physiological work such as circulation, breathing, temperature regulation, and cellular activity. It is not a measurement of metabolism, an activity-adjusted maintenance estimate, a minimum food intake, or a diagnosis of a fast or slow metabolism.
BMR, RMR, REE, and TDEE Are Related but Not Interchangeable
Strict basal metabolic rate is measured under tightly controlled conditions after sleep, fasting, and complete physical rest. Resting metabolic rate and resting energy expenditure are measured under less restrictive resting conditions and are often slightly higher. Online pages frequently use BMR and RMR as practical synonyms, which can hide this methodological difference.
Total daily energy expenditure, or TDEE, adds physical activity, non-exercise movement, and the thermic effect of food to resting energy. This calculator deliberately does not multiply the BMR estimate by an activity label. Use the TDEE Calculator when the question is total daily expenditure rather than resting energy alone.
| Term | Question answered | Includes activity? | How to obtain it |
|---|---|---|---|
| BMR | Energy under strict basal conditions | No | Controlled laboratory measurement |
| RMR or REE | Energy while resting | No | Indirect calorimetry or a prediction equation |
| TDEE | Energy across the whole day | Yes | Measurement or a model that adds activity and food thermogenesis |
How to Use the BMR Calculator
Choose the coefficient set and measurement system that match the equation inputs you intend to model. Switching units converts values already in the form, so the same physical measurements remain comparable. Enter whole years of age and recent measurements rather than an aspirational weight or rounded height.
- Choose the male or female coefficient published by the equations.
- Enter age in completed whole years; this page is limited to adults age 18 and older.
- Select metric for kilograms and centimetres or U.S. customary for pounds and total inches.
- Enter body weight and standing height, then calculate resting energy.
- Read the Mifflin-St Jeor component ledger, equation comparison, units, and limitation note before using the estimate in another calculation.
What the Mifflin-St Jeor Equation Predicts
Mifflin and colleagues published the equation in 1990 after measuring resting energy expenditure by indirect calorimetry in 498 healthy adults, including 247 women and 251 men. Participants were 19 to 78 years old and included people described in the paper as normal weight and obese. Weight, height, age, and the study's binary sex variable explained part, not all, of individual variation.
The study reports an R-squared value of 0.71 for its regression model. That describes how the model fitted its development data; it does not mean an online result is 71% accurate for a particular person. A regression estimate can be above or below a measured value even when every input is correct.
BMR Calculator Formula Guide
Both equations require weight in kilograms, height in centimetres, and age in years. Pounds are divided by 2.2046226218 and inches are multiplied by 2.54 before either formula is evaluated. The result is kilocalories per day; one dietary Calorie is one kilocalorie.
- W is measured body weight in kilograms.
- H is standing height in centimetres.
- A is age in completed years.
- The sex coefficient changes the regression equation; it is not a judgment about identity or health.
Mifflin-St Jeor, male coefficient: REE = 10W + 6.25H - 5A + 5.Mifflin-St Jeor, female coefficient: REE = 10W + 6.25H - 5A - 161.Revised Harris-Benedict, male coefficient: REE = 88.362 + 13.397W + 4.799H - 5.677A.Revised Harris-Benedict, female coefficient: REE = 447.593 + 9.247W + 3.098H - 4.330A.Kilojoules per day = kilocalories per day x 4.184.
Worked BMR Example With a Formula Ledger
For the male coefficient set at age 30, 75 kg, and 178 cm, the Mifflin weight component is 750, the height component is 1,112.5, the age subtraction is 150, and the sex constant is 5. The calculation is 750 + 1,112.5 - 150 + 5 = 1,717.5 kcal/day.
The 1984 revised Harris-Benedict equation produces about 1,777.0 kcal/day from the same measurements. The roughly 59.5 kcal/day difference is an equation comparison, not an uncertainty range and not evidence that one number is the person's measured metabolism.
| Component | Operation | Value | Running total |
|---|---|---|---|
| Weight | 10 x 75 | 750.0 | 750.0 |
| Height | 6.25 x 178 | 1,112.5 | 1,862.5 |
| Age | -5 x 30 | -150.0 | 1,712.5 |
| Sex constant | +5 | 5.0 | 1,717.5 kcal/day |
Why Compare Revised Harris-Benedict
Roza and Shizgal reevaluated the older Harris-Benedict work in 1984 and published revised coefficients. Showing that result beside Mifflin-St Jeor makes equation choice visible. It is more honest than presenting one regression output with false precision or silently switching methods between pages.
A comparison is not a vote. The closer equation for one population or person may not be closer for another. Validation studies find meaningful individual error and differences by body size, age, athletic status, health status, and study population. Indirect calorimetry remains the reference when a measured resting value is clinically important.
How Each Input Moves the Equation
Mifflin-St Jeor adds 10 kcal/day for each additional kilogram entered, adds 6.25 kcal/day for each additional centimetre, and subtracts 5 kcal/day for each additional year, with the other inputs held fixed. Those are regression coefficients, not promises about how an individual's metabolism will change after gaining weight, growing, aging, or changing body composition.
Fat-free mass is an important determinant of resting energy, but this calculator does not measure body composition. Illness, medication, hormones, menstrual status, pregnancy, lactation, growth, environmental temperature, recent food, sleep, and prior activity can also affect measured energy expenditure without appearing in the equation.
Predicted Resting Energy Is Not Measured Metabolism
Prediction equations estimate a population average relationship from easy-to-obtain inputs. A correct calculation can still differ materially from indirect calorimetry for one person. Systematic reviews and validation studies commonly judge an estimate against a band such as within 10% of measured energy, and they still report people outside that band.
Do not label yourself as having a damaged, slow, or fast metabolism because the estimate differs from a wearable, food log, or scale trend. Those sources measure or infer different quantities and carry their own errors. Unexplained weight change, fatigue, temperature intolerance, appetite change, or other symptoms deserve qualified medical assessment rather than formula troubleshooting.
When Indirect Calorimetry May Matter
Indirect calorimetry estimates energy expenditure from oxygen consumption and carbon-dioxide production under a defined protocol. It requires equipment, preparation, and trained interpretation, so it is not necessary for every casual planning question. It can be valuable when clinical nutrition support, significant illness, unusual body composition, or repeated prediction failure makes an equation inadequate.
A measured value is also protocol-dependent. Fasting, recent exercise, rest before testing, room conditions, equipment calibration, and steady-state criteria can change the result. Ask the clinician or laboratory what was measured, under which conditions, and whether the output is BMR, RMR, or REE.
Benefits of a Transparent BMR Calculator
A transparent calculator makes the unit conversions, selected coefficient, arithmetic components, comparison equation, and exclusions visible. That helps a user reproduce the estimate, identify a mistyped measurement, and understand why two sites disagree before carrying the number into a TDEE or nutrition-planning model.
The benefit is auditability, not personalization. A displayed daily total is easier to inspect than a hidden formula, but it remains a resting-energy hypothesis based on a research equation.
BMR Calculator Features
- Metric and U.S. customary measurements with in-form conversion.
- Strict adult, unit, coefficient, weight, and height validation.
- Mifflin-St Jeor result calculated before display rounding.
- Weight, height, age, and sex-constant component ledger.
- Revised Harris-Benedict comparison using the same inputs.
- Absolute and percentage difference between equations.
- Daily kcal, daily kJ, hourly, weekly, and 30-day arithmetic views.
- Explicit separation of resting energy from TDEE and food intake.
- Formula steps, research references, copy, and PDF export.
Common BMR Calculator Use Cases
Use the page to reproduce a Mifflin-St Jeor calculation, convert a pounds-and-inches estimate into its metric basis, compare two published equations, or supply a clearly labeled resting estimate to a separate TDEE scenario. Recalculate when age or measured body size changes rather than carrying an old result forward indefinitely.
Do not use the result alone to prescribe calories, diagnose a metabolic condition, plan nutrition support, determine whether someone is eating enough, or judge another person's body. Those questions require more evidence than age, height, weight, and one coefficient set.
- Audit a BMR value shown by another app or worksheet.
- Explain why Mifflin-St Jeor and revised Harris-Benedict differ.
- Convert resting energy between kcal/day and kJ/day.
- Prepare a labeled starting assumption for a TDEE calculation.
- Document the exact measurements and formula used in a personal planning note.
Who Needs Individual Clinical Guidance
This general adult calculator is not designed for children or teens, pregnancy or breastfeeding, growth, frailty, an eating disorder or disordered-eating history, severe undernutrition, major recent weight change, hospitalization, critical illness, nutrition support, or conditions and medications that affect metabolism. Athletes with unusual body composition may also need a population-specific method or measured value.
NIDDK's Body Weight Planner likewise limits its model to adults and excludes pregnancy and breastfeeding. A registered dietitian or qualified healthcare professional can decide whether a prediction equation, a different equation, indirect calorimetry, or another assessment is appropriate.
Accuracy and Trust Notes for the BMR Calculator
The arithmetic is deterministic for valid entries and uses full precision internally. Unit conversions use 1 lb = 0.45359237 kg, 1 in = 2.54 cm, and 1 kcal = 4.184 kJ. Display rounding happens after each full equation is evaluated, and the comparison difference uses unrounded values.
Accuracy of the equation is a separate question from accuracy of the code. The result inherits limitations from the development sample, measurements, regression model, coefficient categories, and the user's inputs. It does not detect fluid retention, measurement error, endocrine disease, medication effects, or metabolic adaptation.
Helpful BMR and Resting Energy References
These research and government sources support the equations, comparison method, energy terminology, measurement context, and limitations used on this page.