BMR Calculator: Mifflin–St Jeor Resting Energy Expenditure Estimator

Why This Page Is Labeled “BMR Calculator” (and Why the Equation Actually Estimates Resting Energy Expenditure)

In everyday fitness and nutrition searches, most readers look for a “BMR calculator” (Basal Metabolic Rate) when they want to know how many calories their body burns at rest. However, in clinical nutrition and human calorimetry, there is an important technical distinction:

• Basal Metabolic Rate (BMR): Measured in a darkened, thermoneutral room immediately upon waking in the morning after an overnight stay in a metabolic laboratory and a strict 12-to-14-hour fast.
• Resting Energy Expenditure (REE) / Resting Metabolic Rate (RMR): Measured via indirect calorimetry while lying quietly at rest after a shorter fast, without requiring an overnight hospital stay.

The landmark 1990 paper by M. D. Mifflin, S. T. St Jeor, and colleagues—“A new predictive equation for resting energy expenditure in healthy individuals” (published in The American Journal of Clinical Nutrition, 1990 Feb;51(2):241–247; Read the study on PubMed)—specifically measured and modeled Resting Energy Expenditure (REE) in 498 healthy adult men and women. We use “BMR Calculator” as the familiar page label so readers can easily find this tool, while clearly explaining that the output is your estimated Mifflin–St Jeor Resting Energy Expenditure.

The Mifflin–St Jeor Equation & Variable Definitions

Mifflin et al. (1990) derived simplified multiple-regression equations using four routine variables:

Variable Definitions

• REE: Estimated Resting Energy Expenditure in kilocalories per 24-hour day (kcal/day), commonly referred to as BMR.
• W: Total body weight in kilograms (kg). If you enter pounds (lb), the calculator converts weight using the exact international avoirdupois definition W = lb × 0.45359237 before applying the equation.
• H: Standing height (stature) in centimetres (cm). If you enter feet and inches, the calculator converts height using the exact definition H = ((feet × 12) + inches) × 2.54 before applying the equation.
• A: Age in years. Each additional year of adult age subtracts 5 kcal/day in the linear model, reflecting the gradual age-associated decline in resting organ and skeletal muscle metabolic rate.
• s: Sex intercept constant (+5 kcal/day for males; −161 kcal/day for females). This 166 kcal/day offset accounts for average population differences in fat-free mass relative to total body mass between adult men and women of identical height, weight, and age.

Worked Example (Matches the Default Calculator Output Above)

Let us verify the hand calculation for the default adult profile pre-loaded in the calculator above—a 32-year-old adult male in Lahore weighing 72 kg and standing 173 cm tall:

1. Weight term: 10 × 72 kg = 720.00
2. Height term: 6.25 × 173 cm = 1,081.25
3. Age term: 5 × 32 years = 160.00
4. Sex constant (male): +5.00
5. Combine all terms: 720.00 + 1,081.25 − 160.00 + 5.00 = 1,646.25 kcal/day
6. Display rounding (nearest whole kilocalorie): 1,646 kcal/day — identical to the output shown in the calculator above!

Comparison with Female Parameter: For a 32-year-old adult female of the same weight (72 kg) and height (173 cm), replacing +5 with −161 gives 720.00 + 1,081.25 − 160.00 − 161.00 = 1,480.25 kcal/day, which rounds to 1,480 kcal/day.

Who This BMR Calculator Is Intended For & Important Boundaries

• Intended Audience (Adults 20+): Although the original Mifflin–St Jeor cohort included adults aged 19 to 78, our calculators standardize on non-pregnant, non-breastfeeding adults aged 20 to 120 for consistency across our site-wide adult metabolic and BMI tools.
• Pregnancy & Breastfeeding Limits: Resting energy expenditure rises significantly during pregnancy (especially in the second and third trimesters) and breastfeeding requires additional maternal energy for milk synthesis. Do not rely on standard adult BMR formulas during pregnancy or lactation; consult your healthcare provider.
• Do Not Eat Below Your Resting Energy Without Medical Supervision: Your BMR/REE represents only the energy needed to keep vital organs functioning while lying still. It does not include walking, working at a desk, cooking, or exercising. Eating aggressively below your resting metabolic rate is neither necessary nor recommended for healthy, sustainable weight management.