Calorie Calculator

What is BMR?

Your Basal Metabolic Rate is the calories your body burns at complete rest - breathing, circulation, cellular repair, temperature regulation. It accounts for 60-70% of total daily energy expenditure. Even lying in bed all day, you still burn your full BMR.

What is TDEE?

Total Daily Energy Expenditure is your actual total daily calorie burn. It combines BMR (~65%) + the Thermic Effect of Food (~10%) + Exercise (~10-30%) + NEAT (non-exercise movement, ~15%). Eating at TDEE maintains your weight.

Deficit & Surplus

1 kg of body fat ≈ 7,700 kcal. A 500 kcal/day deficit produces ~0.45 kg (1 lb) of fat loss per week. A 250-500 kcal surplus supports lean muscle gain. Maximum safe deficit: 1,000 kcal/day. Never go below 1,200 kcal (women) or 1,500 kcal (men) without medical supervision.

The Three BMR Formulas

Mifflin-St Jeor (1990) - gold standard for general use, accurate within ±10% for most adults.

Katch-McArdle - uses Lean Body Mass, more accurate for athletes when body fat % is known.

Harris-Benedict (revised 1984) - historically popular, tends to overestimate slightly in overweight individuals.

Macronutrients Explained

• Protein (4 kcal/g) - builds and repairs muscle. Critical on a deficit. Aim 1.6-2.8 g/kg.
• Carbohydrates (4 kcal/g) - primary fuel for brain and high-intensity exercise. Stored as glycogen (~500g total).
• Fat (9 kcal/g) - essential for sex hormones, brain (60% fat), and vitamins A, D, E, K. Never go below 0.5 g/kg.

What Affects Metabolism

• Muscle mass - 1 kg muscle burns ~13 kcal/day at rest vs 4.5 for fat
• Age - BMR drops ~1-2% per decade after 20
• Thyroid - hypothyroidism can cut BMR by up to 30%
• Sleep - 2 nights of poor sleep raises hunger hormones and can cut BMR ~5%
• Cold & heat - cold activates brown fat thermogenesis
• Medications - steroids, beta-blockers, antidepressants all influence metabolic rate

NEAT - The Hidden Calorie Burner

Non-Exercise Activity Thermogenesis covers all movement outside structured exercise - walking, fidgeting, cooking, typing, cleaning. NEAT can vary by up to 2,000 kcal/day between individuals of the same size. Going from 3,000 to 10,000 steps/day can add more calorie burn than most gym sessions - every single day.

Sleep & Calorie Impact

Sleep deprivation raises ghrelin (hunger) and lowers leptin (satiety), increasing caloric intake by 300-500 kcal/day after just 2 nights. It also reduces insulin sensitivity, elevates cortisol (fat storage), and reduces anabolic hormones. Aim for 7-9 hours - it may be the highest-leverage body composition habit.

Hydration & Metabolism

Drinking 500 ml of cold water temporarily increases metabolic rate by 10-30% for ~40 minutes. Even mild dehydration (1% of bodyweight) reduces exercise performance by 5% and cognitive function by 3%. Guideline: 35 ml/kg/day, rising to 40-45 ml/kg on training days.

Thermic Effect of Food

Digesting food costs energy. Protein has the highest thermic effect at 20-30% - eating 100 kcal of protein costs 20-30 kcal to digest. Carbohydrates cost 5-10%, fats only 0-3%. On a 2,000 kcal diet TEF accounts for ~150-200 kcal/day - a key reason high-protein diets support fat loss beyond just satiety.

Metabolic Adaptation

During prolonged dieting, the body suppresses metabolism beyond weight loss alone - by 100-400 kcal/day extra through reduced NEAT, thyroid output and leptin. This causes plateaus. Diet breaks (2 weeks at maintenance every 8-12 weeks) and refeeds (1-2 high-carb days/week) are the primary countermeasures.

Fat Loss vs Weight Loss

Scale weight includes fat, muscle, bone, organs, water and gut contents. On an aggressive deficit without adequate protein, up to 30-40% of weight lost can be lean mass - leaving a lower BMR. To maximise fat loss while preserving muscle: 300-500 kcal deficit + 1.6-2.4 g protein/kg + resistance training 2-3×/week.