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What is a calorie deficit?
A calorie deficit means eating fewer calories than you burn (TDEE — your total daily energy expenditure). To cover the missing energy, your body taps into stored fat and you lose weight. That's the fundamental rule of weight loss: whatever diet you follow, if it ends up creating a calorie deficit, you lose weight. This tool first calculates your daily energy needs, then shows how big a deficit you should create for your goal.
How is a calorie deficit calculated?
Roughly 1 kilogram of fat tissue ≈ 7,700 kcal of energy. If you want to lose a certain amount per week, we convert that amount to calories, divide by 7 to get the daily deficit, and subtract it from your TDEE:
Daily calories = TDEE − daily deficit
Example: Someone with a TDEE of 2,336 kcal who wants to lose 0.5 kg per week:
Daily deficit = (0.5 × 7,700) ÷ 7 ≈ 550 kcal → Daily target ≈ 1,786 kcal. (The tool also warns you if this value drops below a healthy floor.)
Method: TDEE via Mifflin-St Jeor (1990) plus an activity factor; calorie-to-weight conversion using the ~7,700 kcal/kg approximation (Wishnofsky, 1958 — the metric equivalent of the '3,500 kcal ≈ 0.45 kg' rule). Safety floor: 1,200 kcal for women / 1,500 kcal for men (a common clinical lower limit).
A safe and sustainable deficit
A bigger deficit may look like faster results, but it's risky. Overly aggressive deficits lead to muscle loss, a slowed metabolism, fatigue, and the yo-yo effect. General guidelines:
- 0.5–1% of your body weight per week is considered safe (0.5–1 kg for most people).
- Don't drop calorie intake below 1,200 kcal for women or 1,500 kcal for men.
- Enough protein and resistance training preserve muscle.
Frequently asked questions
A common target is a 500 kcal deficit per day, which gives about 0.45 kg of fat loss per week. You can increase the deficit to lose faster, but don't go below a healthy floor (1,200 kcal for women / 1,500 kcal for men), and keep protein high to prevent muscle loss.
As you lose weight your body gets smaller and burns fewer calories, and your metabolism adapts somewhat too. That's why the old deficit becomes too small over time. The fix: recalculate your calories for your current weight, increase activity, or take a short 'diet break'.
Three ways: (1) enough protein (1.6–2.2 g/kg), (2) regular resistance training, and (3) a deficit that isn't too aggressive (0.5–1% per week). This combination lets your body burn fat while preserving muscle.
Yes. When 'gain weight' is selected, the tool adds a calorie surplus on top of your TDEE. For clean mass, 0.25–0.5 kg per week (a small surplus) is recommended; going faster raises your body fat percentage.
This is very common and usually doesn't mean your deficit isn't working. The most frequent causes: water retention (stress, salt, your menstrual cycle, or a new workout can hold water and keep the scale flat for days), underestimating your intake (most people undercount oils, sauces, and snacks), and metabolic adaptation (as weight drops, expenditure falls). The fix: read the scale as a weekly average rather than daily, weigh your portions for a while, and gather a few weeks of data. Measurements (like waist circumference) often change before the scale does.
For most adults, a deficit of 500–750 kcal per day (about 0.5–0.7 kg per week) is both effective and sustainable. More aggressive deficits (e.g. 1,000+ kcal per day) give fast results but raise the risk of muscle loss, fatigue, hair loss, gallstones, and quitting the program. Never drop your calorie intake below your basal metabolic rate (BMR). If your starting weight is very high, larger deficits can be used under medical supervision; but the general rule is 'the smallest deficit you can sustain'.