BMR Calculator

Calculate your Basal Metabolic Rate using the Mifflin St Jeor equation.

What Is the BMR Calculator?

This calculator estimates your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation. BMR represents the number of calories your body needs to perform basic life-sustaining functions like breathing, circulation, and cell production while at complete rest. It is the largest component of your total daily energy expenditure, typically accounting for 60 to 75 percent of calories burned each day.

Knowing your BMR provides a baseline for understanding your daily calorie needs. From this number, you can calculate maintenance calories, create a calorie deficit for weight loss, or determine a surplus for muscle gain.

How the Mifflin-St Jeor Equation Works

The Mifflin-St Jeor equation calculates BMR based on sex, weight, height, and age. It was developed in 1990 and is widely regarded as one of the most accurate predictive equations for the general population.

For males:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5

For females:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

The equation accounts for the fact that muscle tissue is more metabolically active than fat tissue, which is why weight and height are included. Age is factored in because metabolic rate naturally declines over time, largely due to changes in body composition.

How to Use the Calculator

Select your sex.
Enter your weight. You can use kilograms or pounds.
Enter your height. You can use centimeters or feet and inches.
Enter your age in years.
Click the calculate button to see your estimated BMR.

The calculator automatically converts imperial units to metric before applying the equation, so you do not need to convert manually.

Understanding Your Results

Your result is an estimate of the calories your body burns at rest over 24 hours. This is not the total number of calories you should eat each day. To estimate total daily calorie needs, multiply your BMR by an activity factor:

Sedentary (little or no exercise): BMR × 1.2
Lightly active (light exercise 1–3 days/week): BMR × 1.375
Moderately active (moderate exercise 3–5 days/week): BMR × 1.55
Very active (hard exercise 6–7 days/week): BMR × 1.725
Extra active (very hard exercise, physical job): BMR × 1.9

For example, a BMR of 1,500 calories with a moderately active lifestyle results in an estimated maintenance of 2,325 calories per day.

Example Calculation

A 30-year-old woman who weighs 65 kg and is 165 cm tall:

BMR = (10 × 65) + (6.25 × 165) − (5 × 30) − 161

BMR = 650 + 1031.25 − 150 − 161

BMR = 1,370.25 calories per day

This means her body burns approximately 1,370 calories per day at complete rest. If she is moderately active, her estimated maintenance calories would be around 2,124 per day.

Common Mistakes When Using BMR

Treating BMR as total calorie needs. BMR is only the resting component. Always apply an activity multiplier for a realistic daily estimate.
Using incorrect units. Ensure weight and height are entered accurately. A misplaced decimal can significantly skew the result.
Assuming BMR is fixed. BMR changes with weight loss, muscle gain, and aging. Recalculate periodically if your body composition or age changes.
Relying on BMR for precise meal planning. BMR is an estimate, not a lab measurement. Individual metabolic rates can vary by 10 percent or more from the equation prediction.

Limitations of the Mifflin-St Jeor Equation

The Mifflin-St Jeor equation is validated for adults aged 19 to 78 with a BMI between 18 and 40. It may be less accurate for:

Extremely athletic individuals with very high muscle mass
People with a BMI above 40
Adolescents and children
Older adults over 78
Individuals with certain medical conditions that affect metabolism

For these populations, other equations like the Cunningham equation (for athletes) or the Harris-Benedict equation may provide a better estimate. No predictive equation can replace indirect calorimetry for a truly personalized measurement.

Practical Use Cases

Weight loss planning: Subtract 300 to 500 calories from your maintenance level for a sustainable deficit.
Muscle building: Add 300 to 500 calories above maintenance to support muscle growth.
Weight maintenance: Use your maintenance calories as a starting point and adjust based on actual weight trends over two to three weeks.
Understanding metabolic health: A very low BMR relative to your body size may warrant a discussion with a healthcare provider.

Frequently Asked Questions

What is the difference between BMR and RMR?

BMR (Basal Metabolic Rate) is measured under strict conditions: complete rest, after an overnight fast, and in a thermoneutral environment. RMR (Resting Metabolic Rate) is a slightly less strict measurement and is typically 10 to 20 percent higher than BMR. In practice, the terms are often used interchangeably, and the Mifflin-St Jeor equation actually estimates RMR, though it is commonly called BMR.

Can I use this calculator if I am pregnant or breastfeeding?

The Mifflin-St Jeor equation does not account for the increased metabolic demands of pregnancy or lactation. Pregnant and breastfeeding individuals should consult a healthcare provider or dietitian for personalized calorie recommendations.

Why does the calculator ask for sex?

Males and females typically have different body compositions, with males generally having more muscle mass relative to body weight. The Mifflin-St Jeor equation uses separate formulas to account for these differences, resulting in a more accurate estimate.

How often should I recalculate my BMR?

Recalculate whenever your weight changes by 5 kg (11 lbs) or more, if you experience significant changes in muscle mass, or if you have a birthday that changes your age by a decade. For most people, recalculating every few months is sufficient.

Is BMR the same as the number of calories I should eat to lose weight?

No. Eating at your BMR is generally too low for sustainable weight loss and can lead to muscle loss, metabolic slowdown, and nutrient deficiencies. A moderate deficit from your total maintenance calories is safer and more effective.