For informational purposes only. Not a substitute for professional medical advice. Consult a healthcare provider before making health decisions.
Calorie Deficit Calculator — Daily Deficit & Weekly Weight Loss (kcal)
Enter your TDEE (Total Daily Energy Expenditure — get it from the calorie calculator if you don't have it) and a target weight loss in kg or lb. The page returns the daily calorie deficit needed to hit that goal in 4, 8, 12, or N weeks, plus a corresponding intake target. Loss rates that exceed ~1% of bodyweight per week are flagged because the bulk of evidence shows aggressive deficits (≥1%/week) trigger faster muscle loss, hormonal disruption, and rebound risk — sustainable fat-loss-focused ranges sit at 0.5-0.75% of bodyweight per week. Useful for setting a realistic intake target ahead of a cut, projecting how long a goal will actually take, and avoiding the classic crash-diet that destroys 6 months of training in a single brutal month.
About calorie deficit
The standard heuristic is that 1 kg of body fat ≈ 7,700 kcal of stored energy (the often-cited 3,500 kcal/lb), so a 500 kcal/day deficit predicts roughly 0.5 kg/week loss. Real-world results undershoot this prediction over months because of metabolic adaptation: as bodyweight drops, BMR drops with it (every kg of fat-free mass burns ~22 kcal/day, every kg of fat burns ~4 kcal/day, so even pure fat loss reduces TDEE), NEAT (Non-Exercise Activity Thermogenesis) silently down-regulates by 100-300 kcal/day in many subjects, and adaptive thermogenesis adds another 5-15% reduction. The practical implication: linear long-term predictions overshoot — a 12-week deficit usually loses 75-85% of the predicted total. The tool flags loss rates above 1% of bodyweight/week (≈ 0.7-1.0 kg/week for a 70-100 kg person) because that's where evidence-based literature draws the upper safety line. Critical caveat: not medical advice. Eating disorders, pregnancy, breastfeeding, certain medications, and metabolic conditions all change the math; consult a registered dietitian or doctor before starting an aggressive deficit.
- TDEE + target loss → daily calorie target + projection
- Uses the 7,700 kcal-per-kg fat heuristic (3,500 kcal/lb)
- 4 / 8 / 12 / N week timeline mode
- Loss-rate safety cap at 1% bodyweight/week with warning
- Documents metabolic-adaptation undershoot (75-85% of prediction)
- Notes BMR drop + NEAT down-regulation + adaptive thermogenesis
- Reactive — recalcs as you change TDEE or target
- Pure client-side math — no upload
- Educational tool — explicit disclaimer about EDs / pregnancy / medication
- Pairs naturally with calorie calculator + protein calculator for a full cut plan
Free. No signup. Your inputs stay in your browser. Ads via Google AdSense (consent required).
Frequently asked questions
Where does the 7,700 kcal-per-kg-of-fat number come from?
Max Wishnofsky's 1958 paper 'Caloric equivalents of gained or lost weight' (Am J Clin Nutr 6(5):542-546) calculated the energy density of adipose tissue from Bozenrad's chemical analysis. Adipose tissue is ~87% lipid and lipid energy density is ~9 kcal/g. Wishnofsky's calculation: 454 g of adipose × 0.87 lipid fraction × 9 kcal/g ≈ 3,555 kcal per pound (rounded to 3,500). Converted to metric: 1 kg ≈ 7,700 kcal. The number is a planning heuristic, not a metabolic constant — Hall 2011 documents the dynamic deviations.
Why does my weight loss stall after several weeks?
Three biological adaptations conspire (Hall et al. 2011 Lancet 378:826-837; Müller & Bosy-Westphal 2013 Obesity (Silver Spring) 21(2):218-228). First, BMR drops mechanically as bodyweight drops — every kg of fat-free mass burns ~22 kcal/day, every kg of fat ~3.6 kcal/day. Second, NEAT (non-exercise activity thermogenesis) silently down-regulates by 100-300 kcal/day; you fidget less, take fewer steps, sit more. Third, adaptive thermogenesis adds another 5-15% reduction beyond what's predicted by body-composition changes alone. The same caloric intake represents a progressively smaller deficit.
Why does the 1% bodyweight/week cap matter?
Garthe et al. 2011 (Int J Sport Nutr Exerc Metab 21(2):97-104) compared 0.7%/week vs 1.4%/week weight-loss rates in elite athletes during 8-12 weeks of resistance training. The slow group GAINED 2.1% lean body mass; the fast group lost 0.2% lean. Both groups lost similar fat mass — the slow group's body-composition outcome was strictly better. Practical implication: aggressive deficits don't accelerate fat loss, they accelerate muscle loss. The 1%/week cap is the upper bound of the evidence-based safe range for body-composition-focused deficits.
Should I do refeeds or diet breaks?
Mixed evidence, context-dependent. Trexler, Smith-Ryan & Norton (2014, J Int Soc Sports Nutr 11:7) reviewed the metabolic-adaptation literature: planned refeeds (1-2 days higher carbohydrate intake per week) and structured diet breaks (1-2 weeks at maintenance every 6-8 weeks of deficit) appear to attenuate adaptive thermogenesis and preserve adherence. They don't shortcut total fat loss but reduce psychological strain and may preserve metabolic flexibility. Useful for prolonged cuts (>12 weeks); less critical for short ones (<8 weeks).
How aggressive can I go without losing muscle?
Three protective factors per the resistance-training nutrition literature plus Garthe 2011: (1) protein intake at 1.6-2.4 g/kg (cutting upper end); (2) resistance training continued through the deficit; (3) deficit ≤1% bodyweight/week, ≤25% below TDEE. Aggressive cuts (≥1.5%/week, intake <60% of TDEE) trigger faster muscle loss, hormonal disruption (T3, leptin), and rebound risk per Trexler et al. 2014. People with eating disorder history, pregnancy/breastfeeding, or metabolic conditions should work with a registered dietitian rather than self-prescribed deficits.
Sources (5)
- Wishnofsky, M. (1958). Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition, 6(5), 542–546 — origin of the 3,500 kcal/lb (≈ 7,700 kcal/kg) body-fat energy heuristic; static planning rule, not metabolic constant.
- Hall, K. D., Sacks, G., Chandramohan, D., Chow, C. C., Wang, Y. C., Gortmaker, S. L., & Swinburn, B. A. (2011). Quantification of the effect of energy imbalance on bodyweight. The Lancet, 378(9793), 826–837 — BMR drops with bodyweight (~22 kcal/day per kg fat-free mass, ~3.6 kcal/day per kg fat); 12-week deficit predictions undershoot 75–85% of theoretical total.
- Müller, M. J., & Bosy-Westphal, A. (2013). Adaptive thermogenesis with weight loss in humans. Obesity (Silver Spring), 21(2), 218–228 (DOI 10.1002/oby.20027) — adaptive thermogenesis reduces TDEE 5–15% beyond predicted by body-composition changes alone; documented during prolonged caloric restriction.
- Garthe, I., Raastad, T., Refsnes, P. E., Koivisto, A., & Sundgot-Borgen, J. (2011). Effect of two different weight-loss rates on body composition and strength and power-related performance in elite athletes. International Journal of Sport Nutrition and Exercise Metabolism, 21(2), 97–104 — 0.7%/week vs 1.4%/week; slow rate gained 2.1% lean mass, fast rate lost 0.2%; both lost similar fat. Evidence basis for 1%/week cap.
- Trexler, E. T., Smith-Ryan, A. E., & Norton, L. E. (2014). Metabolic adaptation to weight loss: implications for the athlete. Journal of the International Society of Sports Nutrition, 11(1), 7 — review of NEAT down-regulation (100–300 kcal/day in many subjects), refeed/diet-break protocols, leptin/ghrelin/T3 hormonal shifts during prolonged deficits.
These are the original publications the formulas in this tool are based on. Locate them by journal name and year on Google Scholar or PubMed.
By Marco B. ·