How a calorie deficit creates fat loss
Fat tissue is essentially stored energy. Each kilogram of fat holds roughly 7,700 kcal; each pound holds about 3,500 kcal. When your body consistently burns more energy than you consume, it draws on these reserves to make up the shortfall. The rate of fat loss is therefore directly proportional to the size and consistency of your daily calorie deficit — within limits.
The numbers above assume pure fat loss. In practice, large deficits accelerate muscle catabolism, water shifts mask fat changes on the scale, and adaptive thermogenesis (your body reducing its own metabolic rate in response to undereating) shrinks the effective deficit over time. A moderate, sustained deficit of 300–700 kcal/day is generally more effective long-term than an aggressive crash.
The Mifflin-St Jeor equation explained
Dozens of BMR equations exist, but a 2005 comparison by Frankenfield et al. in the Journal of the American Dietetic Association found Mifflin-St Jeor the most accurate for the general US population — within 10% of measured BMR for roughly 82% of people. The formula is:
- •Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
- •Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
TDEE is then calculated by multiplying BMR by an activity factor ranging from 1.2 (sedentary) to 1.9 (very active). These multipliers were validated by doubly-labelled water studies and remain the standard used by registered dietitians.
Protein: the most important macro during a cut
A meta-analysis by Morton et al. (2018, British Journal of Sports Medicine) found that 1.62 g/kg/day of protein maximises muscle protein synthesis across the general population. During a calorie deficit, some researchers recommend going higher — up to 2.2 g/kg — particularly for resistance-trained individuals. This calculator shows a conservative 1.6 g/kg floor as a minimum protein target.
Protein also has practical advantages during a cut: it has the highest thermic effect of food (20–30% of its calories are burned in digestion), it promotes satiety, and it preserves the metabolic rate by maintaining lean mass. A high-protein diet does not damage healthy kidneys — this myth was debunked in a 2016 review by Antonio et al.
Recalculating as you lose weight
Your TDEE is not fixed. Every kilogram of weight you lose lowers your BMR slightly. After losing 5–10% of your body weight, adaptive thermogenesis can reduce your daily energy expenditure by an additional 50–150 kcal beyond what the equation predicts. This is why weight loss tends to slow after the first few weeks even when adherence is perfect. Recalculate your deficit every 4–6 weeks or whenever a two-week stall occurs, and update your calorie target accordingly.
Frequently asked questions
What is a calorie deficit?▾
A calorie deficit occurs when you consume fewer calories than your body burns. One pound of fat stores roughly 3,500 kcal, so a daily deficit of 500 kcal produces about 1 lb (0.45 kg) of fat loss per week in theory. Real-world results vary due to metabolic adaptation, water retention, and diet composition.
How is TDEE calculated?▾
Total Daily Energy Expenditure (TDEE) is your Basal Metabolic Rate (BMR) multiplied by an activity factor. This calculator uses the Mifflin-St Jeor equation for BMR — the most validated for the general population — and multiplies by 1.2–1.9 depending on your activity level.
Is a 1,000 kcal/day deficit safe?▾
For most adults, 500–1,000 kcal/day is the evidence-based range. Larger deficits accelerate muscle loss, reduce metabolic rate, and are difficult to sustain. The minimum recommended intake is 1,200 kcal for women and 1,500 kcal for men; this calculator will not go below 1,200 kcal.
Why is protein important during a deficit?▾
In a calorie deficit, the body may catabolise muscle for energy. Consuming 1.6–2.2 g of protein per kg of bodyweight — the range supported by meta-analyses — preserves lean mass, keeps you fuller, and has a higher thermic effect than carbohydrates or fat.
Will I hit a plateau?▾
Yes. As you lose weight, your TDEE drops because you weigh less and your body reduces energy expenditure (adaptive thermogenesis). Recalculate every 4–6 weeks, or when weight loss stalls for more than two weeks, and adjust your calorie target accordingly.
What is the difference between BMR and TDEE?▾
BMR (Basal Metabolic Rate) is the calories your body needs at complete rest — for organ function, breathing, and cell repair. TDEE adds the energy cost of daily movement and exercise on top. Your actual deficit should be calculated from TDEE, not BMR.
References
- •Mifflin MD, et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr, 51(2), 241–247.
- •Frankenfield D, et al. (2005). Comparison of predictive equations for resting metabolic rate. J Am Diet Assoc, 105(5), 775–789.
- •Morton RW, et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains. Br J Sports Med, 52(6), 376–384.
- •Rosenbaum M & Leibel RL. (2010). Adaptive thermogenesis in humans. Int J Obes, 34, S47–S55.