Weight loss is fundamentally an energy balance equation. The calculation uses the widely-established estimate that 1 kg of body fat contains approximately 7,700 calories (3,500 per pound):
Example: You weigh 85 kg and want to reach 75 kg with a 500-calorie daily deficit. Total to lose = (85 − 75) × 7,700 = 77,000 kcal. Days = 77,000 ÷ 500 = 154 days = 22 weeks (about 5.1 months). Weekly loss rate = 500 × 7 ÷ 7,700 = 0.45 kg/week.
In practice, weight loss isn't perfectly linear. The first 1–2 weeks often show rapid loss due to water weight and glycogen depletion (not fat loss). Then metabolic adaptation kicks in — as you lose weight, your TDEE (Total Daily Energy Expenditure) decreases because your lighter body burns fewer calories. A 10 kg loss might reduce TDEE by 150–200 calories/day, slowing progress unless you adjust your deficit. Use a TDEE calculator to recalculate every 5–10% of body weight lost.
| Daily Deficit | Weekly Loss | Time to Lose 10 kg | Sustainability | Best For |
|---|---|---|---|---|
| 250 kcal | 0.23 kg | 43 weeks | Very high | Small adjustments, lean individuals |
| 500 kcal | 0.45 kg | 22 weeks | High | Most people — optimal balance |
| 750 kcal | 0.68 kg | 14.5 weeks | Moderate | Higher starting weight (>90 kg) |
| 1,000 kcal | 0.91 kg | 11 weeks | Low-moderate | Medically supervised, >100 kg |
Minimum safe calorie intake: Women should generally not go below 1,200 kcal/day; men below 1,500 kcal/day without medical supervision. Below these levels, nutrient deficiency risk increases significantly. Very low calorie diets (VLCDs below 800 kcal) should only be used under medical supervision and for limited periods.
Body fat percentage affects appropriate loss rates. Leaner individuals (<20% body fat) should use smaller deficits (250–400 kcal) to minimize muscle loss, while those with higher body fat (>30%) can safely use larger deficits (500–1,000 kcal) with less risk of muscle loss.
Sarah (70 kg, 165 cm, 35 years old) wants to reach 62 kg. Her TDEE is approximately 2,100 kcal/day.
Marco (82 kg) is training for a marathon in 16 weeks. He wants to reach 78 kg for race day.
A 95 kg person wants to reach 80 kg (15 kg to lose). How long at different deficits?
| Deficit | Intake (TDEE 2,800) | Weeks to Goal | End Date (starting Jan 1) |
|---|---|---|---|
| 300 kcal | 2,500 kcal | 55 weeks | ~Jan next year |
| 500 kcal | 2,300 kcal | 33 weeks | ~Aug |
| 750 kcal | 2,050 kcal | 22 weeks | ~Jun |
| 1,000 kcal | 1,800 kcal | 16.5 weeks | ~Apr |
| Factor | Weight Loss | Fat Loss |
|---|---|---|
| Definition | Any decrease in body weight | Specifically losing adipose tissue |
| Includes | Fat, muscle, water, glycogen | Fat only |
| Measurement | Scale weight | Body fat % (calipers, DEXA, BIA) |
| Speed possible | Very fast (water loss) | Limited to ~1 kg fat/week |
| Strategy | Any calorie deficit | Moderate deficit + high protein + resistance training |
| Long-term result | Often regained | Sustainable body composition |
The goal should always be fat loss, not just weight loss. Losing muscle reduces your metabolic rate, making it harder to maintain weight long-term. This is why crash diets fail: they produce rapid scale weight loss (water + muscle) that rebounds when normal eating resumes. The combination of a moderate calorie deficit, high protein intake, and strength training 2–3× per week produces superior body composition changes compared to diet alone.
Track progress with multiple metrics: scale weight (weekly average), waist circumference (monthly), progress photos (monthly), and how your clothes fit. The BMI calculator provides a rough health indicator, while body fat percentage gives a more accurate picture of body composition.
Approximately 7,700 calories per week, or 1,100 calories per day. This is aggressive for most people. A 500 kcal/day deficit (producing 0.45 kg/week loss) is more sustainable and preserves muscle better. Those with higher starting weights (90+ kg) can safely lose 0.75–1 kg/week initially because a larger proportion comes from fat.
The most common reason is inaccurate calorie tracking — research shows people underestimate intake by 20–40%. Other causes: water retention masking fat loss (especially during menstrual cycles or after starting exercise), metabolic adaptation reducing TDEE, inconsistent adherence (strict on weekdays, overeating on weekends), or medications that affect metabolism. Track food precisely with a food scale for 2 weeks before concluding the deficit isn't working.
0.5–1.0 kg per week (1–2 lbs) is generally the fastest rate that preserves muscle mass and is sustainable. Faster rates risk nutrient deficiency, muscle loss, gallstones, hair loss, and metabolic slowdown. Exception: obese individuals (BMI 35+) can safely lose faster initially under medical supervision. Very low calorie diets (<800 kcal) should only be done with medical monitoring.
Both help, but through different mechanisms. Cardio burns calories during exercise. Strength training builds and preserves muscle, increasing resting metabolic rate. The optimal combination: strength train 2–3× per week, do moderate cardio 2–3× per week, and manage diet for the calorie deficit. If choosing only one: strength training, because it preserves muscle during restriction.
Yes — running is one of the highest calorie-burning activities per minute. An 80 kg runner burns approximately 80 kcal per km (1 kcal/kg/km). Running 5 km daily burns ~400 kcal, equivalent to a meal-sized calorie deficit. However, running alone without dietary changes often disappoints because increased appetite compensates for some of the calories burned. Combine running with moderate calorie restriction for best results.
Maintenance requires continuing most behaviors that produced the loss. Research from the National Weight Control Registry shows successful maintainers: exercise 60+ minutes daily, track food periodically, weigh themselves regularly, eat consistent meals (including breakfast), and treat it as a permanent lifestyle change rather than a temporary diet. Reverse diet gradually (increase calories by 100–200/week) rather than immediately eating at full maintenance.
Metabolic adaptation: your lighter body burns fewer calories (lower TDEE), adaptive thermogenesis reduces non-exercise activity expenditure, and hormonal changes (lower leptin, higher ghrelin) increase hunger. The solution: recalculate TDEE after every 5–10% weight loss, maintain high protein intake, add strength training, and consider a 1–2 week diet break at maintenance every 8–12 weeks.
For most women of average height and activity, 1,200 kcal is the minimum recommended without medical supervision. For taller, more active women and most men, it's too low. Below 1,200, it becomes very difficult to meet nutritional needs for vitamins, minerals, and protein. Never go below 1,000 kcal without medical guidance. The focus should be on an appropriate deficit relative to your TDEE, not an arbitrary calorie target.