What this calculator does
This calculator estimates how long it will take to reach your target weight given your current stats and a chosen daily calorie deficit. It computes your basal metabolic rate (BMR) using Mifflin-St Jeor (the most accurate formula by ADA review), applies an activity multiplier for your total daily energy expenditure (TDEE), subtracts your deficit to get your target intake, and then divides total kcal-to-lose by daily deficit to estimate the timeline. Use it for realistic goal-setting, NOT as a substitute for a dietitian's plan.
BMR, TDEE, and the deficit
BMR is what your body burns at complete rest - kept-alive calories (organs, brain, basic cellular function). TDEE adds activity: sedentary multiplier 1.2 (BMR × 1.2), light 1.375, moderate 1.55, active 1.725, very active 1.9. To lose weight, eat below TDEE - the gap is your deficit. A safe deficit is 500 kcal/day (~0.5 kg/week) for most adults. Don't drop your intake below your BMR for extended periods - your body slows metabolism, you lose muscle, and you crash.
The 7,700 kcal-per-kg rule
The 7,700 kcal/kg-of-fat conversion comes from Wishnofsky (1958) - 1 lb of fat ≈ 3,500 kcal, scaled to metric. It's a useful approximation but has limits: it assumes pure fat loss (real weight loss includes glycogen, water, and muscle), it ignores metabolic adaptation (your BMR drops as you shrink), and it doesn't account for individual variation in efficiency. Plan for 70-85% of the predicted rate after the first 2 weeks; weekly fluctuations of ±0.5-1 kg from water are normal.
Realistic, sustainable weight-loss rates
0.25-0.5 kg/week: very sustainable, minimal muscle loss, good for the last 5-10 kg. 0.5-1.0 kg/week: standard recommendation, sustainable for 8-16 weeks of cutting. 1.0-1.5 kg/week: aggressive but possible for highly overweight individuals under medical supervision. >1.5 kg/week: rebound risk, muscle loss, gallbladder stress, nutrient deficiencies - not recommended without bariatric or medical oversight. Plateau weeks happen - your body resets hormones over 5-7 days.
Beyond calorie counting
Calorie deficit is necessary but not sufficient. Protein intake (1.6-2.2g per kg body weight) preserves muscle during a cut. Strength training 2-3x/week prevents 60-80% of muscle loss. Sleep (7-9 hours) regulates hunger hormones (leptin, ghrelin); under-sleeping increases hunger ~15%. Stress raises cortisol which promotes abdominal fat. Hydration (2.5-3 L/day) supports metabolism. Refeed weeks every 8-12 weeks (1 week at maintenance kcal) restore hormones and prevent adaptation.
Frequently asked questions
At a healthy 0.5 kg/week rate, ~20 weeks (5 months). At 1 kg/week (aggressive), ~10 weeks (2.5 months). The calculator above estimates this precisely based on your BMR, TDEE, and chosen deficit. Real-world note: the first 1-2 weeks include faster glycogen + water loss; expect a plateau around week 4-6 as your body adapts.
Generally not sustainable for >4-6 weeks. A 1,000 kcal deficit creates ~1 kg/week loss but at this rate you typically lose muscle (10-30% of weight loss instead of <10%), suffer fatigue, and risk metabolic adaptation. Recommended deficit: 300-600 kcal/day for most adults. Aggressive deficits should be supervised by a dietitian or doctor.
Three reasons: (1) Metabolic adaptation - your BMR drops 10-15% over 6 months of deficit. (2) Daily weight fluctuates ±1-2 kg from water, sodium, and digestion - use weekly averages. (3) Compliance - tracked calories are often 20-30% lower than actual intake. To match estimates: weigh food, log every bite, and average your weight over 7 days.
Sources
- A new predictive equation for resting energy expenditure in healthy individuals— Mifflin et al., American Journal of Clinical Nutrition, 1990;51(2):241-247
- 2011 Compendium of Physical Activities: A Second Update of Codes and MET Values— Ainsworth et al., Medicine & Science in Sports & Exercise, 2011;43(8):1575-1581
- Physical Activity Guidelines for Americans (2nd edition)— US Department of Health and Human Services
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