Mifflin-St Jeor BMR Calculator

TL;DR: To calculate your Basal Metabolic Rate using the Mifflin-St Jeor equation, plug your weight, height, age, and sex into the formula: for men, BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5; for women, BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161, giving you the calories your body burns at complete rest.

What Is the Mifflin-St Jeor BMR Calculator?

The Mifflin-St Jeor BMR Calculator is a tool designed to estimate your Basal Metabolic Rate (BMR)—the number of calories your body requires to perform essential life-sustaining functions like breathing, circulating blood, regulating body temperature, and supporting cellular activity while at complete rest. Unlike exercise or daily activity, which burns additional calories, BMR represents the baseline energy expenditure you cannot avoid. This calculator uses the Mifflin-St Jeor equation, widely considered one of the most accurate predictive formulas for BMR in both normal-weight and obese individuals, outperforming older equations like the Harris-Benedict formula.

Who needs this calculator? If you are tracking your calorie intake for weight loss, weight gain, or athletic performance, knowing your BMR is the critical first step. It is also essential for medical professionals creating nutrition plans for patients, fitness coaches designing metabolic conditioning programs, and individuals with metabolic disorders who need precise energy targets. For example, a nutritionist uses BMR to calculate a client’s Total Daily Energy Expenditure (TDEE) by multiplying BMR by an activity factor, allowing them to prescribe a calorie deficit or surplus that is neither dangerously low nor counterproductively high.

The calculator operates on a straightforward logic: you input four variables (weight, height, age, and biological sex), and the algorithm applies the corresponding formula. The output is a single number—your resting calorie burn. However, the real-world context matters: a 30-year-old male weighing 80 kg and standing 180 cm tall will have a BMR of approximately 1,728 calories, but his total daily needs may range from 2,300 to 3,000 calories depending on whether he is sedentary, active, or an athlete.

How to Use the Calculator

Using the Mifflin-St Jeor BMR Calculator is a simple process that requires you to gather basic body metrics before starting. Follow the steps below to obtain an accurate result quickly:

1. Enter your weight: Locate the weight input field. Input your current weight in kilograms (kg). If you prefer pounds (lbs), convert first by dividing your weight in pounds by 2.2046. Ensure you use one unit system consistently; mixing kilograms and pounds will produce a wildly inaccurate result.
2. Enter your height: Find the height field. Input your height in centimeters (cm). If you know your height in feet and inches, multiply the total height in inches by 2.54 to get centimeters. For example, 5 feet 10 inches equals 70 inches, which is 177.8 cm.
3. Enter your age: Type your current age in years. The formula assumes you have passed your most recent birthday. For older adults, this matters significantly because the equation subtracts 5 × age for men and women respectively, meaning BMR naturally declines with age.
4. Select your biological sex: Choose the appropriate sex option (male or female). This choice alters the final arithmetic of the formula: men add 5, while women subtract 161. If you are unsure which to use, consult your biological assignment at birth, as the equation was validated using this metric.
5. Click calculate: After all fields are filled, press the calculate button. The calculator will display your BMR as a whole number of calories per day. Write this number down for later steps like TDEE calculation.

For example, a 35-year-old woman who weighs 65 kg and is 165 cm tall would enter: weight = 65, height = 165, age = 35, sex = female. The calculator then processes these values through the female variant of the formula to yield her BMR—a process that takes under one second.

Formula and Calculation Method

The Mifflin-St Jeor equation is a linear regression model developed in 1990 by MD Mifflin and ST Jeor. It predicts BMR based on body weight, stature, age, and sex. The plain-language explanation: your BMR increases as your weight and height increase, but decreases as you age. Sex modifies the constant term, reflecting typical differences in lean body mass between men and women. The formula is expressed as follows:

For Men: BMR (kcal/day) = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5

For Women: BMR (kcal/day) = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161

Let us walk through a concrete worked example. Consider a 40-year-old man weighing 90 kg (198 lbs) and standing 180 cm (5 feet 11 inches) tall. Using the male formula:

• First, multiply his weight by 10: 90 × 10 = 900
• Second, multiply his height by 6.25: 180 × 6.25 = 1,125
• Third, multiply his age by 5: 40 × 5 = 200
• Next, add the first two results: 900 + 1,125 = 2,025
• Then, subtract the age term: 2,025 – 200 = 1,825
• Finally, add the male constant of 5: 1,825 + 5 = 1,830

His BMR is therefore 1,830 calories per day. This means that if he lay in bed awake all day without moving, his body would burn 1,830 kcal just to keep him alive. His actual daily burn—if he has a sedentary desk job—would be approximately 1,830 × 1.2 = 2,196 calories, meaning he would maintain his weight eating around 2,200 calories daily.

Practical Examples

To illustrate the versatility of the Mifflin-St Jeor calculator, here are three realistic scenarios showing how different inputs change the result and what that means in practice. The table below summarises these examples.

In the first scenario, the young female athlete has a relatively high BMR for her weight because she is young and likely has more lean muscle mass. Her BMR of 1,374 kcal means her maintenance needs, even with moderate training, exceed 2,100 calories. In contrast, the senior woman at 70 years old has a much lower BMR of 1,269 kcal, partly due to her age term (5 × 70 = 350 subtracted) and partly due to lower weight relative to height. She would struggle to lose weight on a 1,200-calorie diet because that is already near her resting burn. The middle-aged man illustrates the midpoint: his BMR of 1,650 kcal means he needs around 1,980 kcal to maintain weight if sedentary, so a common 2,000-calorie meal plan would cause slow weight gain, not maintenance.

Tips for Accurate Results

Achieving a reliable BMR reading from the Mifflin-St Jeor calculator depends on input precision and understanding the formula's limitations. Below are specific tips to avoid common pitfalls and ensure your result reflects your true resting metabolism.

• Always use metric units consistently: The single most common error is entering pounds into a kilogram field or feet into a centimetre field. If you only know your weight in pounds, divide by 2.2046 to convert to kg. For height in inches, multiply by 2.54 to get cm. Never mix units—for instance, entering weight in kg and height in inches yields a result that is off by a factor of 2.54, often making your BMR appear incorrectly low or high.
• Do not round intermediary values too early: When calculating manually, keep at least one decimal place during multiplication steps. Rounding 6.25 × 177.8 to 1,111 instead of 1,111.25 introduces a small error, but when multiplied by 10 for weight, it can shift your BMR by 10–20 calories. For the calculator, enter your exact height in centimetres, not a rounded whole number like 178 if your true height is 177.8.
• Re-calculate after weight changes: BMR is highly sensitive to weight. A 75 kg person has a BMR that is 100 calories higher than a 65 kg person with identical height, age, and sex. If you are dieting or bulking, update your weight weekly to keep your calorie targets accurate.
• Use your biological sex, not gender identity: The formula was validated using sex assigned at birth because it correlates with hormonal differences affecting lean body mass. If you are taking hormone replacement therapy, consult a doctor, as your metabolic rate may shift away from the formula’s prediction.
• Treat the result as an estimate, not a precise lab value: The Mifflin-St Jeor equation is accurate within about ±10% for most people, but it can overestimate BMR by 5% in obese individuals and underestimate by 3% in highly muscular athletes. If your goal is clinical precision, compare your calculated BMR against an indirect calorimetry test if available.

Frequently Asked Questions

Question 1: How does the Mifflin-St Jeor formula compare to the Harris-Benedict equation, and which is better for weight loss?

The Mifflin-St Jeor formula is generally more accurate than the revised Harris-Benedict equation, which was developed in 1919 and later updated in 1984. In a landmark 2005 study published in the Journal of the American Dietetic Association, researchers found that the Mifflin-St Jeor equation predicted resting metabolic rate within ±10% of measured values in 85% of participants, compared to only 63% for Harris-Benedict. The key difference lies in the coefficients: Mifflin-St Jeor uses a lower multiplier for weight (10 vs 13.397 for men) and a higher multiplier for height (6.25 vs 4.799). This makes Mifflin-St Jeor particularly preferred for overweight individuals because it reduces the tendency to overestimate calorie needs, which in turn prevents prescribing excessively high calorie intakes during weight-loss diets. If you are actively trying to lose weight, use the Mifflin-St Jeor result to set your calorie deficit—eat 500 calories below your BMR × activity factor for a sustainable loss of about 0.5 kg per week.

Question 2: What is the difference between BMR and TDEE, and do I need both?

BMR is your resting calorie burn with zero activity—imagine lying motionless for 24 hours. TDEE (Total Daily Energy Expenditure) includes all activity above rest: walking, exercise, digestion, fidgeting, and even standing. To calculate TDEE, multiply your BMR by an activity factor: 1.2 for sedentary (desk job, no exercise), 1.375 for light activity (1–3 days of exercise per week), 1.55 for moderate activity (3–5 days), 1.725 for very active (6–7 days), and 1.9 for extremely active (manual labour plus intense training). For example, if your BMR is 1,500 kcal and you exercise three times weekly, your TDEE is 1,500 × 1.375 = 2,062 kcal. You need TDEE to determine maintenance calories; BMR alone is insufficient because eating at BMR would create a large deficit that causes muscle loss and metabolic slowdown. Use the Mifflin-St Jeor calculator for BMR, then apply the activity factor to get your practical daily target.

Question 3: Can I use this calculator for a child or adolescent, and what are the limitations?

The Mifflin-St Jeor equation was validated primarily in adult populations aged 19 to 78. For children and adolescents, the formula tends to underestimate BMR because growth spurts and hormonal changes increase metabolic demands beyond what a linear age coefficient can capture. If you apply it to a 15-year-old, the result may be 200–300 calories lower than their actual resting burn, potentially leading to underfeeding. For teens, alternative equations like the Schofield or FAO/WHO formulas are more appropriate, as they incorporate age-specific and sex-specific constants for puberty. Additionally, the Mifflin-St Jeor formula assumes a normal body composition distribution; it does not account for genetic variations in thyroid function or thermogenesis. For medical guidance on a growing child, always consult a paediatrician or registered dietitian rather than relying solely on this tool.