⚡

BMR Calculator — Basal Metabolic Rate Calculator

Calculate your Basal Metabolic Rate using multiple formulas

BMR Calculator: Calculate Your Basal Metabolic Rate

Table of Contents

Metabolism Research Transformed in 2021

A landmark study published in Science in August 2021 fundamentally changed our understanding of how metabolism varies across the human lifespan. The research, led by Herman Pontzer at Duke University and involving over 6,400 participants from 29 countries, revealed that adult metabolism is far more stable than previously believed.

Four Metabolic Life Stages

The study identified four distinct phases of metabolic rate adjusted for body size:

Phase 1: Infancy (0-1 year) Energy expenditure similar to adults at birth, then increasing rapidly. By 9-15 months, infants burn calories approximately 50% faster than adults relative to body size—the highest metabolic rate of any life stage.

Phase 2: Childhood Decline (1-20 years) Gradual decline of approximately 3% per year as growth slows and body composition changes.

Phase 3: Adult Stability (20-60 years) Total and basal expenditure remain remarkably stable regardless of sex. The researchers stated unequivocally: "Metabolic rate is really stable all through adult life, 20 to 60 years old."

Phase 4: Later Life Decline (60+ years) Adjusted total expenditure declines by approximately 0.7% per year. By the nineties, adjusted energy expenditure is approximately 26% below middle-aged levels.

Implications for Weight Management

This research challenges the common belief that metabolism crashes in the thirties or forties. Weight gain during middle adulthood cannot be attributed primarily to metabolic decline—factors such as reduced physical activity, changes in diet and lifestyle factors play larger roles.

However, the post-60 decline is real and significant. A 75-year-old genuinely requires fewer calories than a 45-year-old of identical size and activity level.

What BMR Reveals About Energy Balance

Basal Metabolic Rate represents the energy required for fundamental biological processes: breathing, circulation, cell repair, brain function and temperature regulation. These processes never cease and account for 60-75% of total daily energy expenditure.

The Energy Balance Equation

Total Daily Energy Expenditure (TDEE) comprises three components:

TDEE = BMR (60-75%) + Physical Activity (15-30%) + Thermic Effect of Food (10%)

BMR: Energy for basic survival at complete rest Physical Activity: Both exercise and non-exercise activity thermogenesis (NEAT) Thermic Effect of Food (TEF): Energy used to digest and process food

Variables Affecting BMR

Body Size: Larger bodies require more energy to maintain. Weight and height directly influence BMR.

Body Composition: Muscle tissue is metabolically active; fat tissue is relatively inert. Two individuals of identical weight can have different BMRs based on their lean mass percentage.

Age: After age 60, BMR declines approximately 0.7% annually, primarily due to loss of lean tissue.

Sex: Hormonal differences between males and females affect body composition and metabolic rate. Males typically have higher BMR due to greater muscle mass.

Genetics: Individual variation of 10-15% exists even when controlling for all measurable factors.

How to Use This Calculator

Step 1: Select Your Formula Three equations are available:

  • Mifflin-St Jeor (default): Most accurate for the general population; recommended by the Academy of Nutrition and Dietetics
  • Harris-Benedict: The original 1919 formula; tends to slightly overestimate
  • Katch-McArdle: Uses lean body mass; requires body fat percentage

Step 2: Choose Units Toggle between Metric (kg, cm) and Imperial (lbs, ft/in).

Step 3: Enter Your Information Input age, sex, weight and height. For Katch-McArdle, an additional field appears for body fat percentage.

Step 4: Review Results The calculator displays:

  • Daily BMR in calories
  • Category label based on typical ranges
  • Weekly and monthly calorie baseline
  • Comparison across formulas (if applicable)

BMR Category Reference:

| Category | Approximate Range | Typical Profile | |----------|------------------|-----------------| | Very Low | Below 1,200 kcal | Small frame, low activity | | Low | 1,200 – 1,500 kcal | Smaller adults, sedentary | | Normal | 1,500 – 1,800 kcal | Average adults | | High | 1,800 – 2,100 kcal | Larger adults, muscular | | Very High | Above 2,100 kcal | Very tall, highly muscular |

The Evolution of BMR Equations

Harris-Benedict (1919)

The original predictive equation, developed by James Arthur Harris and Francis Gano Benedict, has been used for over a century. Based on direct calorimetry measurements of 239 subjects, it was the standard until modern reassessment suggested it overestimates BMR by 5-15% for contemporary populations.

Formulas:

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

Mifflin-St Jeor (1990)

Developed from 498 subjects and validated extensively since publication. The Academy of Nutrition and Dietetics recommends this equation as the most accurate for most populations.

Formulas:

Men: BMR = (10 × weight kg) + (6.25 × height cm) - (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) - (5 × age) - 161

The sex constant (+5 for men, -161 for women) reflects typical body composition differences rather than biological sex per se.

Katch-McArdle (1996)

Accounts for body composition by using lean body mass rather than total weight. Theoretically more accurate for individuals with atypical body composition (very lean or very high body fat), but requires accurate body fat measurement.

Formula:

BMR = 370 + (21.6 × lean body mass kg)

Where lean body mass = weight × (1 - body fat percentage/100)

Cunningham Equation (1991)

An alternative lean mass equation sometimes used for athletes:

BMR = 500 + (22 × lean body mass kg)

Research suggests this may be slightly more accurate for highly trained athletes.

Current Dietary Guidelines and Calorie Needs

US Dietary Guidelines 2025-2030

Released in January 2026, the 2025-2030 Dietary Guidelines for Americans emphasise minimally processed, nutrient-dense foods. Key calorie-related recommendations:

General Adult Ranges:

  • Most adults: 1,600 to 3,000 calories per day
  • Females minimum: 1,600 calories
  • Males minimum: 2,000 calories

Protein Update: The new guidelines suggest 1.2 to 1.6 grams of protein per kilogram of body weight daily—50-100% higher than previous minimum recommendations. This has implications for meal planning and thermic effect calculations.

Added Sugar: The guidelines state that "no amount of added sugars or non-nutritive sweeteners is recommended or considered part of a healthy or nutritious diet," with a practical limit of no more than 10g added sugars per meal.

UK NHS Recommendations

The NHS maintains general daily calorie recommendations:

Standard Guidelines:

  • Women: approximately 2,000 kcal per day (8,400 kJ)
  • Men: approximately 2,500 kcal per day (10,500 kJ)

Weight Loss Guidelines: The NHS Weight Loss Plan recommends:

  • Women: 1,400 kcal per day maximum
  • Men: 1,900 kcal per day maximum

Meal Distribution (400-600-600 Rule): Public Health England suggests approximately 400 calories for breakfast, 600 for lunch and 600 for dinner, with healthy snacks bringing totals to recommended levels.

Individual Variation

These guidelines represent population averages. Individual needs vary based on:

  • Age (younger adults typically require more)
  • Height and weight
  • Activity level
  • Metabolic rate
  • Health conditions

BMR calculation provides a personalised baseline from which to determine individual requirements.

Equation Accuracy: What 2025 Research Shows

Mifflin-St Jeor Performance

A 2025 study in the Asia Pacific Journal of Clinical Nutrition found that the Mifflin-St Jeor equation demonstrated the best performance among tested equations, with the smallest bias of -9.73 kcal/day—indicating predicted values were closest to measured resting energy expenditure.

A separate 2025 study in Archives of Public Health examining Emirati female young adults confirmed Mifflin-St Jeor as most accurate (mean difference: -15.8 to +83.8 kcal/day), whilst Harris-Benedict was least accurate.

Accuracy Rates by Population

| Population | Mifflin-St Jeor Accuracy | Notes | |------------|-------------------------|-------| | Non-obese adults | 87% within 10% | Best performance | | Obese adults | 75% within 10% | Reduced accuracy | | Ultra-endurance athletes | Acceptable | Best among tested equations | | Underweight | Variable | Limited validation data |

Limitations Identified

A 2025 Nutrients study found that existing predictive equations show limited accuracy in patients with BMI over 35, often leading to overestimation or underestimation of resting energy expenditure. For severely obese individuals, measured calorimetry may be preferable when available.

Older adults and ethnic minorities remain underrepresented in both equation development and validation studies. Caution is warranted when applying standard equations to these populations.

Individual Variation

Even the most accurate equation predicts within 10% for only 70-87% of individuals. The remaining 13-30% fall outside this range due to genetic variation, hormonal differences, health conditions and other unmeasured factors.

This underscores that BMR calculations are estimates—starting points for nutritional planning rather than precise measurements.

Worked Calculations for Different Profiles

Scenario 1: Active Young Professional

Profile: 28-year-old female, 165 cm (5'5"), 62 kg (137 lbs), exercises 4 times weekly

Mifflin-St Jeor Calculation:

BMR = (10 × 62) + (6.25 × 165) - (5 × 28) - 161
BMR = 620 + 1,031.25 - 140 - 161
BMR = 1,350 kcal/day

TDEE Calculation (Moderately Active: ×1.55):

TDEE = 1,350 × 1.55 = 2,093 kcal/day

For Weight Loss (500 kcal deficit): Target intake: approximately 1,600 kcal/day

Scenario 2: Middle-Aged Office Worker

Profile: 52-year-old male, 178 cm (5'10"), 88 kg (194 lbs), sedentary desk job, walks occasionally

Mifflin-St Jeor Calculation:

BMR = (10 × 88) + (6.25 × 178) - (5 × 52) + 5
BMR = 880 + 1,112.5 - 260 + 5
BMR = 1,738 kcal/day

TDEE Calculation (Lightly Active: ×1.375):

TDEE = 1,738 × 1.375 = 2,390 kcal/day

For Gradual Weight Loss (300 kcal deficit): Target intake: approximately 2,090 kcal/day

Scenario 3: Muscular Athlete Using Katch-McArdle

Profile: 30-year-old male, 185 cm (6'1"), 92 kg (203 lbs), 12% body fat, trains daily

Lean Body Mass Calculation:

Lean mass = 92 × (1 - 0.12) = 92 × 0.88 = 81 kg

Katch-McArdle Calculation:

BMR = 370 + (21.6 × 81) = 370 + 1,750 = 2,120 kcal/day

Comparison with Mifflin-St Jeor:

Mifflin-St Jeor BMR = (10 × 92) + (6.25 × 185) - (5 × 30) + 5
                    = 920 + 1,156.25 - 150 + 5 = 1,931 kcal/day

The Katch-McArdle formula produces a higher estimate (2,120 vs 1,931 kcal/day) because it accounts for this individual's above-average muscle mass. For lean individuals, Katch-McArdle typically provides more accurate results.

TDEE Calculation (Very Active: ×1.725):

TDEE = 2,120 × 1.725 = 3,657 kcal/day

Scenario 4: Older Adult Post-60

Profile: 68-year-old female, 160 cm (5'3"), 65 kg (143 lbs), light daily walking

Mifflin-St Jeor Calculation:

BMR = (10 × 65) + (6.25 × 160) - (5 × 68) - 161
BMR = 650 + 1,000 - 340 - 161
BMR = 1,149 kcal/day

Context: The 2021 Pontzer research confirms that post-60 metabolism declines approximately 0.7% annually. At 68, this individual's BMR is genuinely lower than it was at 58, not due to lifestyle but biological decline.

TDEE Calculation (Lightly Active: ×1.375):

TDEE = 1,149 × 1.375 = 1,580 kcal/day

This lower TDEE explains why older adults often gain weight whilst eating "the same as always"—they genuinely require fewer calories.

From BMR to Total Daily Expenditure

Activity Multipliers

| Activity Level | Multiplier | Description | |----------------|------------|-------------| | Sedentary | 1.2 | Little or no exercise, desk job | | Lightly Active | 1.375 | Light exercise 1-3 days/week | | Moderately Active | 1.55 | Moderate exercise 3-5 days/week | | Very Active | 1.725 | Hard exercise 6-7 days/week | | Extremely Active | 1.9 | Very hard exercise, physical job |

Non-Exercise Activity Thermogenesis (NEAT)

NEAT—fidgeting, walking, standing, household tasks—varies dramatically between individuals and can account for 200-900 calories daily. This hidden variable often explains why some people maintain weight more easily than others despite similar exercise habits.

Thermic Effect of Food (TEF)

Digesting food requires energy:

  • Protein: 20-30% of calories consumed
  • Carbohydrates: 5-10% of calories consumed
  • Fat: 0-3% of calories consumed

This is one reason higher-protein diets can support weight management—more energy is used to process protein compared to other macronutrients.

Practical Application

For Weight Loss: Eat 500 kcal below TDEE for approximately 0.5 kg (1 lb) weekly loss For Weight Gain: Eat 250-500 kcal above TDEE for lean muscle building For Maintenance: Match calorie intake to TDEE

Never eat below BMR for extended periods—this risks muscle loss, nutrient deficiency and metabolic adaptation that makes future weight management more difficult.

Sources

FAQs

What is the difference between BMR and TDEE?

BMR is calories burned at complete rest—the energy required for basic survival. TDEE (Total Daily Energy Expenditure) is BMR plus calories from physical activity and food digestion. TDEE represents maintenance calories; BMR represents the minimum below which you should not restrict.

Which formula is most accurate?

Mifflin-St Jeor is generally most accurate for the general population, predicting within 10% of measured values for 82-87% of non-obese adults. Katch-McArdle may be more accurate if you have reliable body fat percentage measurement. Harris-Benedict tends to overestimate by 5-15%.

Does metabolism really slow down with age?

Research shows metabolism remains stable from ages 20-60, then declines approximately 0.7% annually after 60. The common belief that metabolism crashes in the thirties or forties is not supported by current evidence.

Should I eat at my BMR to lose weight?

No. Eating at BMR creates too severe a deficit, risking muscle loss, fatigue and metabolic adaptation. Calculate your TDEE and eat 500-750 calories below that level for sustainable weight loss.

Why does the Katch-McArdle formula require body fat percentage?

Katch-McArdle calculates based on lean body mass rather than total weight. This requires knowing your body fat percentage to determine how much of your weight is metabolically active tissue versus fat tissue.

How accurate are BMR formulas?

For most people, formulas estimate within 10-15% of actual measured BMR. Individual variation due to genetics, hormones and health conditions can push actual BMR outside this range. Use calculations as starting points, then adjust based on real-world results.

Can I increase my BMR?

The most effective method is building muscle through resistance training. Each kilogram of muscle tissue burns approximately 13 calories daily at rest, compared to 4.5 calories for fat tissue. Adequate sleep and protein intake support metabolic health but have smaller effects.

How often should I recalculate my BMR?

Recalculate after significant weight change (every 5-7 kg) or annually if your weight is stable. As you age, periodic recalculation accounts for age-related metabolic changes.

Why do men have higher BMR than women?

The difference primarily reflects body composition rather than biological sex. Men typically have greater muscle mass and lower body fat percentage. Women with equivalent muscle mass would have similar BMR values.

What is metabolic adaptation?

Extended calorie restriction can lower metabolic rate beyond what formulas predict—your body adapts to conserve energy. This is one reason very low-calorie diets often fail long-term. Gradual calorie reduction and resistance training help minimise metabolic adaptation.