BMR Calculator (Mifflin-St Jeor)

TL;DR: To calculate your BMR using the Mifflin-St Jeor equation, input your weight in kilograms, height in centimeters, age in years, and biological sex into the calculator, which then automatically applies the formula (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5 for men, or -161 for women, delivering an estimate of the calories your body burns at complete rest.

What Is the BMR Calculator (Mifflin-St Jeor)?

The BMR Calculator (Mifflin-St Jeor) estimates your Basal Metabolic Rate (BMR) — the total number of calories your body requires to perform essential life-sustaining functions like breathing, circulating blood, regulating body temperature, and maintaining cellular activity while at complete rest. BMR is the single largest component of your total daily energy expenditure (TDEE), typically accounting for 60–75% of all calories burned in a day. This number is the starting point for any fat loss, muscle gain, or weight maintenance protocol.

This specific calculator uses the Mifflin-St Jeor equation, which was developed in 1990 and has since become the gold standard in clinical and sports nutrition settings. It replaced the older Harris-Benedict equation because it is significantly more accurate, particularly for individuals who are overweight or obese. Research published in the Journal of the American Dietetic Association confirmed that the Mifflin-St Jeor formula predicts resting metabolic rate within ±10% of measured values in the general population, making it the most validated and clinically reliable predictive equation available. This calculator is essential for dietitians, fitness coaches, athletes, and anyone serious about precisely managing their caloric intake.

Unlike other BMR equations that require body fat percentage measurements or more complex variables, the Mifflin-St Jeor calculator requires only four basic data points: weight, height, age, and biological sex. This simplicity, combined with its proven accuracy, makes it the recommended standard for adults of all body types, excluding only highly muscular individuals, children, and pregnant or lactating women, where alternative methods may be more appropriate.

How to Use the Calculator

Using the BMR Calculator (Mifflin-St Jeor) is straightforward. Follow these numbered steps to get an accurate estimate of your resting metabolic rate:

1. Locate the weight input field. Enter your current body weight in kilograms (kg). If you know your weight in pounds (lbs), divide that number by 2.2046 to convert it to kilograms before entering it into the calculator.
2. Locate the height input field. Enter your height in centimeters (cm). If you know your height in feet and inches, convert it by multiplying your total inches by 2.54. For example, 5'10" equals 70 inches, which equals 177.8 cm.
3. Locate the age input field. Enter your current age in years. Use your whole years (e.g., 30, 35, 42) — do not enter months or fractional years.
4. Select your biological sex. Choose either 'Male' or 'Female' from the provided selector. This is critical because the formula uses a constant of +5 for males and -161 for females.
5. Click the calculate button. The calculator will automatically apply the Mifflin-St Jeor equation to your entered values and display your estimated BMR in calories per day.
6. Review your result. The output will be a number representing your daily caloric requirement at rest. Use this number alongside your activity factor to determine your total daily energy expenditure (TDEE).

Formula and Calculation Method

The Mifflin-St Jeor equation is a linear regression model that predicts resting energy expenditure based on three primary variables: body mass, height, and age. The core logic is that larger bodies with more metabolically active lean tissue require more energy, while aging is associated with a natural decline in metabolic rate due to loss of muscle mass and hormonal changes.

The Mifflin-St Jeor formula is as follows:

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

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

Concrete Worked Example: Consider a 30-year-old male who weighs 75 kg and is 175 cm tall. Plugging these values directly into the male formula yields:

Step 1: Multiply weight by 10 → 10 × 75 = 750
Step 2: Multiply height by 6.25 → 6.25 × 175 = 1093.75
Step 3: Multiply age by 5 → 5 × 30 = 150
Step 4: Apply the equation → 750 + 1093.75 - 150 + 5

Step 5: Add the first two results → 750 + 1093.75 = 1843.75
Step 6: Subtract the age factor → 1843.75 - 150 = 1693.75
Step 7: Add the male constant → 1693.75 + 5 = 1698.75 kcal/day

This result of approximately 1,699 calories means this specific 75 kg, 175 cm, 30-year-old male burns roughly 1,700 calories per day just to keep his body alive and functioning while lying completely still. The calculator performs all of these steps automatically in a fraction of a second, eliminating the risk of arithmetic errors.

Practical Examples

To illustrate how the BMR calculator responds to different profiles, here are three realistic scenarios with worked results and interpretation of what they mean for daily nutrition planning.

Scenario 1 — Sedentary Office Worker: For the 68 kg, 165 cm, 28-year-old female, the calculation is (10×68) + (6.25×165) - (5×28) - 161 = 680 + 1031.25 - 140 - 161 = 1,461.25 kcal/day. Her BMR is 1,461 kcal/day. If she is entirely sedentary (little to no exercise) and uses a standard activity multiplier of 1.2, her total daily maintenance calories are 1,461 × 1.2 = 1,753 kcal. Eating below this number would result in gradual fat loss.

Scenario 2 — Athletic CrossFitter: For the 90 kg, 183 cm, 25-year-old male, the calculation is (10×90) + (6.25×183) - (5×25) + 5 = 900 + 1143.75 - 125 + 5 = 1,923.75 kcal/day. His BMR is 1,924 kcal/day. Because he is highly active with an activity factor of 1.725, his TDEE is 1,924 × 1.725 = 3,319 kcal/day, meaning he needs roughly 3,300 calories daily just to maintain his weight given his intense training schedule.

Scenario 3 — Retired Senior: For the 80 kg, 178 cm, 70-year-old male, the calculation is (10×80) + (6.25×178) - (5×70) + 5 = 800 + 1112.5 - 350 + 5 = 1,567.5 kcal/day. His BMR is 1,568 kcal/day. This is notably lower than his 25-year-old counterpart at the same weight, demonstrating the significant ~5 kcal per year reduction in BMR that aging causes.

Tips for Accurate Results

To ensure the BMR Calculator (Mifflin-St Jeor) produces the most reliable output, you must pay close attention to unit consistency and avoid common calculation errors. A small mistake in the input values can lead to a clinically significant error in your estimated daily caloric needs.

• Verify weight is in kilograms, not pounds. The most common mistake is entering pounds directly into the formula. If your scale measures in pounds, divide by 2.2046. For example, 165 lbs ÷ 2.2046 = 74.84 kg. Entering 165 instead of 74.84 will inflate your BMR result by over 900 calories.
• Verify height is in centimeters, not inches or feet. Height must be in centimeters. A 5'11" individual is 71 inches tall, which converts to 180.34 cm (71 × 2.54). Entering 71 cm instead of 180 cm will massively deflate your result.
• Never round intermediate values before the final result. If you calculate manually, keep all decimal places until the final step. Rounding 1093.75 to 1094 and 1693.75 to 1694 is acceptable, but rounding aggressive values to whole numbers throughout the equation (e.g., rounding 6.25 × 175 to 1000) will introduce cumulative errors.
• Use your current age, not your birth year's result. BMR decreases via the -5 per year factor. Using an age that is even one year off shifts your BMR by 5 calories per day, which accumulates to 4,550 calories over 91 days — enough to affect weight loss progress by roughly 1.3 lbs.
• Do not apply the result blindly to special populations. The Mifflin-St Jeor equation has a ±10% accuracy margin for the general population. For individuals with high muscle mass (e.g., bodybuilders), the equation may underestimate BMR. For severely obese individuals (BMI > 50), it may overestimate. These results are a strong starting point, not a biological certainty.
• Repeat the measurement under consistent conditions. If you are tracking changes over time, always measure your weight at the same time of day (e.g., first thing in the morning, post-bathroom, pre-food) to avoid daily fluctuations from hydration and food mass skewing the BMR output.

Frequently Asked Questions

Is the Mifflin-St Jeor BMR formula accurate for everyone?

No formula is perfectly accurate for every individual, but the Mifflin-St Jeor equation is the most reliable predictive method for the general adult population. Clinical validation studies have demonstrated it predicts resting metabolic rate within ±10% of indirect calorimetry measurements approximately 80% of the time. The equation was designed for healthy adults aged 19 to 78 with a BMI typically between 18 and 40. It is not accurate for children, pregnant or nursing women, competitive bodybuilders with extremely low body fat and high lean mass, or individuals with severe chronic conditions like hyperthyroidism or cachexia. In these populations, the equation can be off by 10–20%, and direct measurement via metabolic testing is recommended. For lean body mass extremes, the Katch-McArdle formula, which uses body fat percentage, may yield better results.

How does the Mifflin-St Jeor BMR calculator compare to the Harris-Benedict formula?

Mifflin-St Jeor is generally recognized as superior to the original Harris-Benedict equation because it was developed in 1990 using a more representative sample of the modern population, which has higher average body weight than the 1919 cohort that Harris-Benedict used. When the two equations are compared to actual measured resting metabolic rates, Mifflin-St Jeor is accurate in about 82% of subjects, while Harris-Benedict is accurate in only about 62% of subjects. The primary difference lies in how the formulas weigh variables. Harris-Benedict assigns a different coefficient to weight depending on sex, whereas Mifflin-St Jeor uses the same 10 × weight coefficient for both men and women, only changing the sex-specific constant at the end. For an average adult, Mifflin-St Jeor typically produces a BMR value that is approximately 5–10% lower than Harris-Benedict, which aligns more closely with measured reality.

How many calories should I eat per day based on my Mifflin-St Jeor BMR result?

The BMR result is only your resting metabolic rate — it does not include calories burned through daily activity, exercise, or digestion. To find your maintenance calories (the amount you need to eat to stay at the same weight), you must multiply your BMR by an activity factor. For someone who is sedentary (little or no exercise), multiply BMR by 1.2. For lightly active (1–3 days of exercise per week), use 1.375. For moderately active (3–5 days per week), use 1.55. For very active (6–7 days of hard training), use 1.725. For extremely active (physically demanding job + twice-daily training), use 1.9. To lose weight at a safe rate of 0.5–1 kg per week, create a calorie deficit of 500–1000 kcal below your maintenance number. To gain muscle, add 250–500 kcal above maintenance. Never eat below your BMR number itself, as this is the floor your body needs for fundamental survival.