Free Calculator · BMR · TDEE · Macros · 3 Formulas
BMR Calculator
Estimate your Basal Metabolic Rate using Mifflin-St Jeor, Harris-Benedict and Katch-McArdle formulas. Calculate TDEE, daily calorie targets and macronutrient splits for your goals.
How to Use the BMR Calculator
Enter your weight, height, age and sex to calculate your Basal Metabolic Rate using three established formulas. Furthermore, select your activity level to compute Total Daily Energy Expenditure. Additionally, choose a weight goal and macro split to get personalised daily calorie and macronutrient targets.
- Enter your measurementsChoose metric or imperial. Enter weight, height and age accurately.
- Add body fat percentageOptional but enables the Katch-McArdle formula. Estimate from skinfold calipers or DEXA scan.
- Select activity levelHonestly assess your typical weekly activity. This multiplier converts BMR to TDEE.
- Choose your goalSelect maintain, lose or gain weight. The calculator adjusts daily calories accordingly.
- Review and copy resultsCompare three BMR formulas, see TDEE at all activity levels, and get macro targets.
What Is Basal Metabolic Rate (BMR)?
Basal Metabolic Rate is the number of calories your body burns at complete rest. It represents the energy required to maintain vital functions: breathing, blood circulation, cell production and temperature regulation. Furthermore, BMR typically accounts for 60-75% of your total daily calorie expenditure. Additionally, it is the single largest component of your energy budget.
BMR is measured in a clinical setting after an overnight fast, in a darkened room, at a comfortable temperature. Furthermore, the subject must be fully rested and awake but motionless. Additionally, because clinical measurement requires specialised equipment (indirect calorimetry), predictive equations are used as practical alternatives.
The Three BMR Formulas Compared
This calculator uses three well-established equations. The Mifflin-St Jeor equation (1990) is considered the most accurate for most adults. Furthermore, the Harris-Benedict equation (revised 1984) is the classic alternative. Additionally, the Katch-McArdle formula uses lean body mass and is most accurate for lean individuals.
A 2005 review in the Journal of the American Dietetic Association found the Mifflin-St Jeor equation predicted BMR within 10% of measured values in more subjects than any other formula. Furthermore, it was accurate for normal weight, overweight and obese individuals. Additionally, the Katch-McArdle formula outperformed others for athletic populations with known body composition.
TDEE and Activity Multipliers
Total Daily Energy Expenditure equals BMR multiplied by an activity factor. This accounts for the calories burned through daily movement, exercise, digestion and non-exercise activity thermogenesis (NEAT). Furthermore, NEAT includes fidgeting, walking, standing and other incidental movement that varies significantly between individuals.
| Activity level | Factor | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise |
| Lightly active | 1.375 | Light exercise 1-3 days per week |
| Moderately active | 1.55 | Moderate exercise 3-5 days per week |
| Very active | 1.725 | Hard exercise 6-7 days per week |
| Extra active | 1.9 | Athlete or very physical job |
Macronutrient Targets and Splits
Macronutrients provide different amounts of energy per gram. Protein and carbohydrates provide 4 calories per gram. Fat provides 9 calories per gram. Furthermore, each macro serves distinct physiological functions. Additionally, the optimal ratio depends on your goals, activity level and metabolic health.
| Split | Protein | Carbs | Fat | Best for |
|---|---|---|---|---|
| Balanced | 30% | 40% | 30% | General health, moderate exercise |
| High protein | 40% | 30% | 30% | Muscle building, fat loss |
| Low carb | 35% | 25% | 40% | Weight loss, insulin sensitivity |
| Keto | 20% | 5% | 75% | Ketogenic diet, epilepsy management |
Factors That Affect BMR
Muscle mass is the strongest modifiable factor. Each kilogram of muscle burns approximately 13 calories per day at rest, compared to about 4.5 calories per kilogram of fat. Furthermore, strength training increases muscle mass and raises BMR over time. Additionally, this is why resistance exercise is recommended alongside calorie restriction for sustainable weight management.
Age reduces BMR by approximately 1-2% per decade after age 20. Furthermore, this decline is partly due to loss of lean muscle mass (sarcopenia) and partly due to hormonal changes. Additionally, thyroid function significantly influences metabolic rate. Hypothyroidism can reduce BMR by 20-30%.
Using BMR for Weight Management
To lose weight, consume fewer calories than your TDEE. A deficit of 500 calories per day produces approximately 0.45 kg (1 lb) of fat loss per week. Furthermore, deficits larger than 1,000 calories per day are generally not recommended without medical supervision. Additionally, very low calorie intake can trigger metabolic adaptation, reducing BMR and slowing weight loss.
To gain muscle mass, consume a surplus of 200-500 calories above TDEE combined with resistance training. Furthermore, protein intake should be 1.6-2.2 grams per kilogram of body weight to support muscle protein synthesis. Additionally, distribute protein intake across 3-5 meals for optimal absorption.
Limitations of BMR Equations
All BMR formulas are estimates based on population averages. Individual variation due to genetics, hormones, gut microbiome and metabolic history can cause actual BMR to differ by 5-15%. Furthermore, these equations were validated primarily on Caucasian populations and may be less accurate for other ethnic groups.
Conditions that affect accuracy include pregnancy, lactation, fever, extreme temperatures, certain medications (beta-blockers, thyroid hormones) and previous dieting history. Furthermore, metabolic adaptation from prolonged calorie restriction can reduce actual BMR below predicted values. Additionally, indirect calorimetry remains the gold standard for precise BMR measurement.
BMR vs RMR: Understanding the Difference
BMR and Resting Metabolic Rate (RMR) are often used interchangeably, but they differ in measurement conditions. BMR requires strict clinical conditions: overnight fast, complete rest and controlled environment. Furthermore, RMR is measured under less restrictive conditions and typically runs 10-20% higher than true BMR.
Most predictive equations, including Mifflin-St Jeor and Harris-Benedict, actually estimate RMR rather than strict BMR. Furthermore, this distinction rarely matters for practical applications like meal planning and weight management. Additionally, the terms are used interchangeably throughout the fitness and nutrition industry.