BMR Calculator UK

How to Use the BMR Calculator

This calculator uses the scientifically-validated Mifflin-St Jeor equation (1990) - the most accurate formula for modern populations.

Step-by-Step Instructions:

1. Enter Your Age: BMR decreases with age due to muscle loss and hormonal changes
2. Select Gender: Men typically have higher BMR due to greater muscle mass
3. Input Weight: Use kg (or convert: 1 stone = 6.35kg, pounds ÷ 2.2 = kg)
4. Input Height: Use cm (or convert: feet × 30.48 = cm)
5. Choose Activity Level: Be honest - overestimating leads to weight gain
6. Click Calculate: Get your BMR and Total Daily Energy Expenditure (TDEE)

Complete Guide to BMR and Metabolism

What is Basal Metabolic Rate (BMR)?

BMR is the number of calories your body burns at complete rest to maintain essential functions:

• Breathing and circulation: ~20% of BMR
• Brain and nervous system: ~20% of BMR
• Kidney and liver function: ~20% of BMR
• Cell production and protein synthesis: ~20% of BMR
• Temperature regulation: ~10% of BMR
• Other metabolic processes: ~10% of BMR

BMR represents 60-75% of your total daily calorie expenditure. Even if you did absolutely nothing all day, your body would still burn these calories just to stay alive.

BMR Formula Comparison

1. Mifflin-St Jeor Equation (1990) - MOST ACCURATE

Developed in 1990, this is the most accurate formula for modern populations with ±10% error margin.

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

2. Harris-Benedict Equation (Revised 1984)

Original equation from 1919, revised in 1984. Less accurate than Mifflin-St Jeor for modern populations.

• 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)

3. Katch-McArdle Formula (Body Fat Based)

Most accurate IF you know your body fat percentage. Doesn't differentiate by gender.

• BMR = 370 + (21.6 × lean body mass in kg)
• Lean body mass = weight × (1 - body fat %)
• Example: 70kg person with 20% body fat = 70 × 0.8 = 56kg lean mass
• BMR = 370 + (21.6 × 56) = 1,580 calories

Understanding TDEE (Total Daily Energy Expenditure)

TDEE is your total daily calorie burn including all activity. It consists of:

TDEE Components:

• BMR (60-75%): Calories for basic body functions
• NEAT (15-30%): Non-Exercise Activity Thermogenesis (walking, fidgeting, standing)
• EAT (5-10%): Exercise Activity Thermogenesis (planned workouts)
• TEF (10%): Thermic Effect of Food (calories burned digesting food)

Activity Multipliers:

How BMR Changes with Age, Gender & Body Composition

Age-Related Decline:

• Ages 20-30: Peak metabolic rate
• Ages 30-40: -2% per decade (BMR drops ~50-100 cal/decade)
• Ages 40-50: -2-3% per decade
• Ages 50-60: -3-5% per decade (accelerated due to menopause in women)
• Ages 60+: -5-8% per decade

By age 70, BMR may be 20-30% lower than at age 30 (300-500 fewer calories/day).

Gender Differences:

• Men have 5-10% higher BMR than women (same age, height, weight)
• Difference primarily due to muscle mass and testosterone
• Average 30-year-old: Male BMR ~1,680 cal vs Female BMR ~1,520 cal (160 cal difference)
• Women's BMR increases 5-10% during menstruation (100-150 extra calories)
• Pregnancy increases needs by 300-500 cal/day
• Breastfeeding increases needs by 500 cal/day

Body Composition Impact:

• Muscle burns 6 cal/lb/day at rest
• Fat burns 2 cal/lb/day at rest
• Gaining 5kg muscle = +66 cal/day BMR increase
• Losing 5kg muscle = -66 cal/day BMR decrease
• This is why crash dieting (which loses muscle) damages metabolism

How to Increase Your BMR

1. Build Muscle Mass (Most Effective)

• Strength training 3-4x per week
• Progressive overload (gradually increase weight/reps)
• Compound exercises: squats, deadlifts, bench press, rows
• Potential BMR increase: 7-8% over 6 months (~100-150 cal/day)

2. Eat Adequate Protein

• Protein has 20-30% thermic effect (you burn 20-30% of protein calories digesting it)
• Supports muscle growth and retention
• Recommended: 1.6-2.2g per kg bodyweight (112-154g for 70kg person)
• Distribute protein across meals (30-40g per meal)

3. Stay Hydrated

• Dehydration can slow metabolism by 2-3%
• Drink 2-3 liters water daily
• Cold water may provide small additional thermogenic effect

4. High-Intensity Interval Training (HIIT)

• Creates "afterburn effect" (EPOC - Excess Post-exercise Oxygen Consumption)
• Burns additional 50-150 calories for 24-48 hours post-workout
• Example: 20 min HIIT = 200 cal during + 100 cal afterburn = 300 total

5. Get Adequate Sleep

• Poor sleep (< 7 hours) reduces BMR and increases hunger hormones
• Sleep deprivation can reduce BMR by 5-10%
• Aim for 7-9 hours quality sleep

6. Avoid Crash Dieting

• Very low calorie diets (<1,200 cal) cause metabolic adaptation
• Can reduce BMR by 10-25% (200-400 calories)
• Effect can last months or years
• Safe deficit: 500-750 cal below TDEE for weight loss

Using BMR for Weight Goals

For Weight Loss:

• Calculate TDEE (BMR × activity level)
• Create calorie deficit of 500-750 cal/day
• Expected loss: 0.5-1kg per week
• Never eat below BMR for extended periods (causes metabolic damage)
• Minimum safe intake: Women 1,200 cal, Men 1,500 cal

For Muscle Gain:

• Calculate TDEE
• Add 250-500 cal surplus
• Expected gain: 0.25-0.5kg per week (half muscle, half fat)
• Ensure adequate protein (1.6-2.2g per kg)
• Progressive strength training essential

For Weight Maintenance:

• Eat at your TDEE
• Track weight for 2-4 weeks
• Adjust by 100-200 cal if gaining/losing unintentionally

Expert Tips & Common Mistakes

Top 10 BMR & Metabolism Tips:

1. Don't eat below BMR: This causes metabolic damage and muscle loss. Always eat at least BMR calories.
2. Track for 2-4 weeks: Use BMR as starting point, then adjust based on real weight changes.
3. Be honest about activity level: Most people overestimate - leads to weight gain.
4. Prioritize protein: High thermic effect and preserves muscle during weight loss.
5. Lift heavy things: Muscle is metabolically expensive - burns calories 24/7.
6. Don't fear carbs: Your body burns same calories regardless of macro split (for same total calories).
7. Meal timing doesn't matter much: Total daily calories matter more than when you eat them.
8. Expect plateaus: As you lose weight, BMR decreases. Recalculate every 5kg loss.
9. Don't rely on exercise alone: Can't out-train a bad diet. Exercise burns less than you think.
10. Be patient: Sustainable fat loss is 0.5-1kg/week. Faster = muscle loss and metabolic damage.

Common Mistakes to Avoid:

Frequently Asked Questions

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