How a calorie deficit works
Body fat stores roughly 7,700 calories per kilogram (about 3,500 per pound). Eating consistently below the energy you use forces your body to draw on those stores. This calculator estimates your maintenance level with the Mifflin-St Jeor equation and offers three options:
- Gentle (−10%) — slowest, easiest to sustain, minimal disruption to training and mood.
- Moderate (−20%) — the reasonable default for most people.
- Faster (−25%) — quicker, hungrier, and harder to hold. Best used for shorter blocks.
Why the 3,500-calorie rule overpredicts
The familiar arithmetic — cut 500 calories a day, lose a pound a week, indefinitely — assumes your energy expenditure stays constant while you shrink. It does not, and the error compounds.
As you lose weight, three things reduce your expenditure. You are a smaller body, so resting metabolism falls. Moving that smaller body costs less energy per step. And non-exercise activity tends to decline, often without you noticing. Modelling work published in the Lancet by Kevin Hall and colleagues describes this properly: a sustained change in intake moves you toward a new steady weight, approached gradually over roughly a year, rather than producing indefinite linear loss.
The practical consequence is that a fixed deficit yields progressively smaller weekly losses and eventually none at all. The "plateau" people interpret as a broken metabolism is usually just this arithmetic arriving. The fix is to recalculate at your new weight, not to conclude something is wrong with you.
Losing fat rather than weight
Weight loss and fat loss are not the same thing, and an aggressive deficit widens the gap between them. Three things protect your muscle while you lose:
- Adequate protein — around 1.6–2.2 g per kg of body weight, toward the higher end in a deficit.
- Resistance training two to four times a week — this is the signal that tells your body the muscle is still needed.
- A moderate rather than severe deficit — aim to lose about 0.5–1% of body weight per week.
This matters beyond appearance. Muscle is metabolically active tissue, so losing it lowers your maintenance calories and makes the result harder to keep. A great deal of weight regain is set up during the losing phase.
When the scale stalls
Weight is noisy. Day-to-day fluctuations of a kilogram or more come from water retention, glycogen stores, food volume in transit, salt intake, and — for people who menstruate — cycle-related fluid shifts that can mask two weeks of genuine fat loss.
Judge progress from a weekly average across two to three weeks, weighing under consistent conditions: same time of day, after using the bathroom, before eating. If the trend is genuinely flat for three weeks and you are confident in your tracking, recalculate your maintenance for your current weight and adjust.
Honest tracking is the other common culprit. Under-reporting of food intake is well documented and is not usually deliberate — cooking oil, drinks, sauces, and untracked bites add up quickly.
Deficits that are too large
Very aggressive restriction tends to backfire through several routes at once: disproportionate muscle loss, falling energy that reduces daily movement, deteriorating training quality, increased preoccupation with food, and poor adherence. Very-low-calorie diets have a legitimate place under medical supervision for specific clinical situations; they are a poor choice for general fat loss.
If you have a history of disordered eating, calorie targets can be actively harmful. Please read our medical disclaimer and consider working with a professional rather than a calculator.