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BMR Calculator (Mifflin-St Jeor)

Calculate your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation, the most accurate BMR formula validated by the American Dietetic Association.

Reviewed by Chase FloiedUpdated

This free online bmr calculator (mifflin-st jeor) provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the BMR Calculator (Mifflin-St Jeor). Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.

2

Review your inputs

Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.

3

Read the results

The BMR Calculator (Mifflin-St Jeor) instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.

4

Explore parameter sensitivity

Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.

Formula Reference

BMR Calculator (Mifflin-St Jeor) Formula

See calculator inputs for the governing equation

Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.

When to Use This Calculator

  • •Use the BMR Calculator (Mifflin-St Jeor) for personal health tracking and wellness monitoring, establishing a baseline and tracking changes over time.
  • •Use it when recording fitness metrics to track progress toward health or athletic goals.
  • •Use it to compare measurements before and after a lifestyle, diet, or training change to quantify the impact.
  • •Use it as a conversation starter before a doctor's appointment, bringing objective data to discuss with a healthcare professional.

About This Calculator

The BMR Calculator (Mifflin-St Jeor) is a free health and wellness calculation tool designed for personal use and general informational purposes. Calculate your Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation, the most accurate BMR formula validated by the American Dietetic Association. This calculator provides reference values based on established health screening formulas and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Results may vary based on individual factors not captured by the calculation. Consult a qualified healthcare provider for personalized health guidance. All calculations are performed locally in your browser — no personal health data is transmitted or stored.

About BMR Calculator (Mifflin-St Jeor)

The BMR Calculator computes your Basal Metabolic Rate — the minimum number of calories your body needs to perform essential physiological functions at complete rest. This includes breathing, circulating blood, cell production, nutrient processing, protein synthesis, and ion transport. BMR accounts for 60-75% of total daily energy expenditure in sedentary individuals. Understanding your BMR is the foundation for all weight management: it's the baseline from which you calculate how many calories you need to maintain, lose, or gain weight.

The Math Behind It

Basal metabolic rate is measured under strict laboratory conditions: complete rest, 12 hours after eating, after sleeping, in a thermoneutral environment, without physical or emotional stress. In practice, these conditions are hard to achieve, so BMR is typically estimated using predictive equations. The Mifflin-St Jeor equation was developed in 1990 by Mark Mifflin and Sachiko St Jeor after they identified the Harris-Benedict equation (1919) was systematically overestimating BMR, especially in obese individuals. They measured BMR in 498 individuals using indirect calorimetry and derived a new equation with 10% greater accuracy. Factors affecting BMR: - **Muscle mass**: Each kg of muscle burns ~13 kcal/day at rest vs. ~4 kcal/day for fat - **Age**: BMR decreases ~1-2% per decade after 20, primarily due to muscle loss - **Sex**: Men have higher BMR due to greater muscle mass - **Genetics**: Can cause ±10% variation - **Hormones**: Thyroid hormone, cortisol, and growth hormone affect BMR - **Temperature**: Cold exposure increases BMR (non-shivering thermogenesis) - **Diet**: Severe calorie restriction can lower BMR by 10-20%

Formula Reference

Mifflin-St Jeor

BMR = 10W + 6.25H - 5A + S

Variables: W=weight(kg), H=height(cm), A=age, S=+5(M)/-161(F)

Worked Examples

Example 1: Male BMR

Male, 70 kg, 175 cm, 30 years

Step 1:BMR = 10(70) + 6.25(175) - 5(30) + 5
Step 2:= 700 + 1,093.75 - 150 + 5
Step 3:= 1,648.75 kcal/day

BMR is approximately 1,649 kcal/day.

Example 2: Female BMR

Female, 60 kg, 165 cm, 40 years

Step 1:BMR = 10(60) + 6.25(165) - 5(40) - 161
Step 2:= 600 + 1,031.25 - 200 - 161
Step 3:= 1,270.25 kcal/day

BMR is approximately 1,270 kcal/day.

Common Mistakes & Tips

  • !Confusing BMR with TDEE. BMR is only the resting metabolic rate; TDEE includes activity.
  • !Eating below BMR for extended periods. This triggers metabolic adaptation and muscle loss.
  • !Assuming BMR is constant. It can vary ±5-10% day-to-day due to hormonal fluctuations, sleep quality, and stress.

Related Concepts

TDEE Calculator

Total Daily Energy Expenditure = BMR × activity factor.

Calorie Calculator

A more complete tool that includes activity level and weight management targets.

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Calorie Calculator

TDEE Calculator

Water Intake Calculator

Frequently Asked Questions

How accurate is the Mifflin-St Jeor equation?

It predicts BMR within 10% of measured values for most people. It's the most accurate predictive equation available without lab testing.

Why does BMR decrease with age?

Primarily due to sarcopenia (age-related muscle loss). Starting around age 30, adults lose approximately 3-8% of muscle mass per decade, which directly reduces BMR. Regular resistance training can slow this decline.