How to Calculate BMR — Mifflin-St Jeor Explained

Introduction

Understanding how to calculate BMR is essential for anyone looking to manage their weight, optimise nutrition, or improve their metabolic health. Your Basal Metabolic Rate (BMR) is the number of calories your body needs to perform fundamental life-sustaining functions while at complete rest.

What Is BMR?

Your BMR represents the energy required for:

Breathing and circulation (cardiovascular function)
Body temperature regulation (thermoregulation)
Cellular repair and growth (metabolism)
Brain and nervous system function
Organ maintenance and basic operations

Why BMR Calculations Matter

Weight management foundation for calories in/out
Nutrition planning for personalised diet strategies
Fitness goals (bulking, cutting, maintenance)
Metabolic health assessment and tracking
Medical applications for treatment planning

Beyond Generic Recommendations

BMR provides a personalised baseline that moves beyond:

One-size-fits-all calorie recommendations
Generic diet plans that ignore individual differences
Guesswork in nutrition and weight management
Ineffective strategies based on assumptions
Metabolic confusion and plateau frustration

Scientific Foundation

This guide teaches you to use the Mifflin-St Jeor equation:

Most accurate method according to research
Manual calculation for complete understanding
Comparison with Harris Benedict formula
TDEE conversion using activity multipliers
Practical application for real-world nutrition

You'll gain the knowledge to create scientifically grounded nutrition plans tailored to your unique metabolic needs.

What Is a BMR Calculation?

A BMR calculation estimates the minimum energy your body requires to stay alive and functioning in a state of rest. Think of it as the number of calories you would burn if you stayed in bed all day. It is the largest component of your Total Daily Energy Expenditure (TDEE), typically accounting for 60-75% of the calories you burn. Knowing your BMR is the first step in creating a scientifically grounded nutrition plan, whether your goal is weight loss, muscle gain, or weight maintenance. It provides a personalised baseline that moves beyond generic calorie recommendations.

The Formula Behind the Calculator

The most accurate formula for the general population is the Mifflin-St Jeor equation. It uses your weight, height, age, and sex to predict your metabolic rate.

The Core Formulas

Mifflin-St Jeor Equation (Most Accurate):

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

Harris-Benedict Equation (Original):

For Men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) - (5.677 × age in years)
For Women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) - (4.330 × age in years)

Breaking Down the Variables

Weight: Your body mass in kilograms. This is a key driver of metabolic rate.
Height: Your stature in centimetres. Taller people have a larger surface area, which can influence energy needs.
Age: Metabolic rate generally decreases with age due to factors like muscle loss.
Sex: Biological sex is a factor because of differences in average body composition (muscle mass and fat distribution).

BMR vs TDEE: Understanding the Difference

BMR (Basal Metabolic Rate): Calories burned at complete rest
TDEE (Total Daily Energy Expenditure): BMR + calories from activity and digestion
Activity Multipliers: Sedentary (1.2), Lightly Active (1.375), Moderately Active (1.55), Very Active (1.725), Extremely Active (1.9)

How to Calculate Your BMR Manually

Calculating your BMR by hand is a simple process of plugging your numbers into the formula. Let's use the Mifflin-St Jeor equation.

Step-by-Step Instructions

Gather Your Metrics. You will need your weight in kilograms (kg), height in centimetres (cm), and age in years. If you know your weight in pounds, divide by 2.205 to get kg. For height in feet/inches, convert to inches and then multiply by 2.54 to get cm.
Apply the Correct Formula. Choose the formula for your sex.
Perform the Multiplication and Addition. Follow the order of operations (multiplication before addition/subtraction).

Manual Calculation Example

Problem: Calculate the BMR for a 30-year-old woman who weighs 65 kg and is 168 cm tall.

Identify the Formula: Use the Mifflin-St Jeor equation for women.
Plug in the Values: BMR = (10 × 65) + (6.25 × 168) - (5 × 30) - 161
Calculate Step-by-Step:
- (10 × 65) = 650
- (6.25 × 168) = 1,050
- (5 × 30) = 150
- Now combine: 650 + 1,050 = 1,700
- 1,700 - 150 = 1,550
- 1,550 - 161 = 1,389
Result: Her BMR is approximately 1,389 calories per day.

Worked Examples

Example 1: Male BMR Calculation

Scenario: Tom is a 45-year-old man, weighing 85 kg and standing 178 cm tall.
Calculation (Mifflin-St Jeor for Men): BMR = (10 × 85) + (6.25 × 178) - (5 × 45) + 5 BMR = 850 + 1,112.5 - 225 + 5 BMR = 1,742.5 calories
Result: Tom's BMR is approximately 1,743 calories.

Example 2: Converting BMR to TDEE

Scenario: Maria has a BMR of 1,400 calories. She exercises moderately 3-5 days per week.
Activity Multiplier: Moderately Active = 1.55
Calculation: TDEE = BMR × Activity Multiplier = 1,400 × 1.55 = 2,170 calories
Result: To maintain her weight, Maria should eat about 2,170 calories per day.

Example 3: Formula Comparison

Scenario: Compare BMR for a 25-year-old woman (62 kg, 165 cm) using both equations.
Mifflin-St Jeor: (10×62) + (6.25×165) - (5×25) - 161 = 1,385 calories
Harris-Benedict: 447.593 + (9.247×62) + (3.098×165) - (4.330×25) = 1,413 calories
Interpretation: The Harris-Benedict formula often gives a slightly higher estimate, which is why Mifflin-St Jeor is generally preferred for accuracy.

How to Use the Online Calculator

Our online BMR calculator automates this process for speed and accuracy.

Enter Your Details. Input your age, sex, weight, and height. The calculator can handle both metric and imperial units.
Click Calculate. The tool instantly computes your BMR using the Mifflin-St Jeor equation.
Find Your TDEE. For a more practical number, use the activity multiplier section to estimate your Total Daily Energy Expenditure (TDEE).

Practical Applications and Real-Life Uses

Your BMR is the foundation for:

Weight Loss: Creating a sustainable calorie deficit (eating 500 calories less than your TDEE).
Muscle Gain: Ensuring a slight calorie surplus to support new muscle growth.
Weight Maintenance: Finding the precise calorie intake to maintain your current weight.
Metabolic Health: Understanding your body's baseline energy needs can be a indicator of metabolic efficiency.

Pro Tips and Common Mistakes

Pro Tips:

Use the Katch-McArdle formula if you know your body fat percentage: It can be more accurate as it's based on lean body mass: BMR = 370 + (21.6 × Lean Body Mass in kg).
Be honest about your activity level: Overestimating your activity is the most common mistake when calculating TDEE from BMR.
Recalculate regularly: Your BMR changes as your weight, age, and body composition change.

Common Mistakes:

Using the wrong formula for your sex: Ensure you select the male or female version of the equation.
Not recalculating after significant weight change: Your BMR changes as your weight changes.
Confusing BMR with TDEE: Remember that BMR is your resting metabolic rate; you need to multiply by an activity factor for daily calories.

Frequently Asked Questions

Q: What is the difference between BMR and RMR? A: BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) are very similar. BMR is measured under more strict conditions (complete rest, fasted, and in a thermoneutral environment). RMR is measured under less restrictive conditions and is typically about 10% higher. For practical purposes, the terms are often used interchangeably, and online calculators estimate RMR.

Q: How can I increase my BMR? A: You can increase your BMR by building lean muscle mass through resistance training, as muscle is more metabolically active than fat. Other factors include eating adequate protein (due to the thermic effect of food), staying hydrated, and getting enough sleep.

Q: How accurate are BMR formulas? A: Formulas like Mifflin-St Jeor are estimates and are generally within 10% of your true metabolic rate for most people. Individual variations due to genetics, hormones, and other factors mean it should be used as a starting point. Adjust your calorie intake based on real-world results (e.g., weight change over time).

Q: Should I eat below my BMR for weight loss? A: Generally, no. Eating significantly below your BMR can slow your metabolism and make weight loss harder long-term. Instead, aim for a moderate deficit from your TDEE (typically 500-750 calories below maintenance).

Q: How does age affect BMR? A: BMR typically decreases by about 2-3% per decade after age 30, primarily due to muscle mass loss. This is why maintaining muscle through resistance training becomes increasingly important as we age.

Q: Can medications affect my BMR? A: Yes, certain medications (especially thyroid medications, stimulants, and some antidepressants) can affect your metabolic rate. If you're on medication, consult your healthcare provider about how it might impact your BMR calculations.

Q: What's the Katch-McArdle formula and when should I use it? A: The Katch-McArdle formula (BMR = 370 + (21.6 × Lean Body Mass in kg)) is based on lean body mass rather than total weight. It's more accurate for people who know their body fat percentage, especially athletes or very lean individuals.

Conclusion

Learning how to calculate BMR empowers you to take control of your nutrition with a science-based, personalised approach. By using the Mifflin-St Jeor equation outlined in this guide, you can establish a solid caloric baseline that's specific to your body's needs. Remember that BMR is just the starting point—factor in your activity level to find your TDEE, and use that number to guide your dietary choices for weight loss, maintenance, or muscle gain. Understanding your metabolic rate removes the guesswork from nutrition planning and helps you create sustainable, effective eating patterns. Try our precise BMR Calculator to discover your metabolic rate and start planning your personalised nutrition strategy today.