Weight Loss Calculator

This calculator works out the daily calorie deficit needed to reach your target weight by a chosen date. One kilogram of body fat stores approximately 7,700 kcal, so a deficit of 500 kcal per day produces roughly 0.5 kg of weight loss per week. The NHS recommends a safe rate of 0.5–1.0 kg per week.

The calculator estimates your Total Daily Energy Expenditure (TDEE) using the Mifflin-St Jeor equation and your activity level, then subtracts the required daily deficit. It flags any target that requires a deficit larger than 1,000 kcal per day or a calorie intake below 1,200 kcal (women) or 1,500 kcal (men), as these can be unsustainable.

Enter your current weight, target weight, goal date and activity level. The calculator shows daily calories, expected weekly loss rate and whether the timeline is realistic. Extending the deadline by a few weeks can often make the difference between a healthy deficit and an extreme one.

Calorie deficit — the science of weight loss. 7,700 kcal deficit = ~1kg fat loss. NHS recommended safe rate: 0.5-1kg/week = 3,500-7,700 kcal weekly deficit (500-1,100/day). Sample: 75kg moderately active woman TDEE ~2,000 kcal/day. 1,500 kcal/day diet = 500 deficit = 0.5kg/week loss. Faster (2,000 deficit/day) sustainable for 4-8 weeks max before metabolic adaptation, muscle loss, plateau. Calorie counting accuracy: ±15-20% even with food scales — overestimate slightly to be safe.

BMR and TDEE — your true calorie needs. BMR (Basal Metabolic Rate): energy used just existing. Mifflin-St Jeor formula — Men: 10×weight + 6.25×height − 5×age + 5. Women: same minus 161. Sample 30-year-old woman, 70kg, 165cm: BMR = 1,401 kcal. TDEE = BMR × activity multiplier. Sedentary 1.2; lightly active 1.375; moderately active 1.55; very active 1.725. Most underestimate activity. Use 1.2-1.4 for office workers, 1.5-1.7 for hands-on jobs. TDEE 1,680-1,960 for example woman.

Why crash diets fail — metabolic adaptation. Very low calorie (<1,200 women, <1,500 men): body adapts within 2-4 weeks. Thyroid downshifts. NEAT (Non-Exercise Activity Thermogenesis) drops 200-400 kcal/day. Muscle loss reduces BMR further (each kg muscle = 13 kcal/day at rest). Result: 4-6 weeks in, weight loss stalls despite same calorie intake. Solution: moderate deficit 500-700 kcal/day, refeed days every 1-2 weeks at maintenance, strength training 2-3×/week to preserve muscle.

Protein protects muscle during weight loss. Target 1.6-2.2g protein per kg bodyweight daily. 70kg person: 112-154g protein/day. Without enough protein, 30-50% of weight lost can be muscle (worst outcome — 'skinny fat'). High protein also: increases satiety (longer feeling full), thermic effect of food (25-30% of protein calories used in digestion vs 5-10% for carbs/fat). Protein sources: chicken 31g/100g; eggs 13g/100g; Greek yogurt 10g/100g; whey protein powder 20-25g/scoop; lentils 9g/100g cooked.

Sustainable weight loss strategies. Strength training 2-3×/week: preserves muscle, increases BMR long-term. Cardio for energy expenditure but doesn't shift body composition without strength work. Sleep 7-9 hours: under 6 hours sabotages weight loss (cortisol drives visceral fat storage; hunger hormones disrupted). Walking 8,000-10,000 steps/day: adds 200-400 kcal/day expenditure. Diet break every 8-12 weeks: 2 weeks at maintenance calories prevents metabolic adaptation. Long-term: lifestyle change, not 'diet' — 95% of dieters regain within 5 years vs 20% of lifestyle changers.