Calorie Deficit: How to Set One You Can Actually Keep

The one thing that has to be true

Weight loss requires energy expenditure to exceed energy intake. Every diet that works — low carb, keto, intermittent fasting, calorie counting, Mediterranean — works through that mechanism, whatever else it claims. The differences between them are about adherence, satiety and health markers, not about a separate physical route to fat loss.

This matters because it tells you where to spend effort: on the approach you can sustain for a year, not on the one with the best mechanism story.

Sizing the deficit

A kilogram of body fat stores roughly 7,700 kcal, so a 500 kcal daily deficit predicts about 0.45 kg (1 lb) of loss a week. In practice the real rate is slower than the arithmetic predicts, because expenditure falls as you get lighter and because non-exercise activity drops unconsciously when you eat less.

A workable range is 15–25% below maintenance. For someone maintaining at 2,400 kcal that is 1,800–2,040 kcal. Below about 1,200 kcal for women or 1,500 for men it becomes hard to meet micronutrient needs without careful planning.

Protein at 1.6–2.2 g per kg of body weight, plus resistance training twice a week, is what keeps the loss weighted towards fat rather than muscle. In a deficit this is not optional if body composition matters to you.

Aggressive versus moderate

A large deficit produces faster results and higher dropout. It also increases the share of weight lost as lean tissue, worsens training performance, and tends to be followed by a rebound — the pattern is well documented in the weight-management literature.

A moderate deficit is slower and duller and works more often. The honest comparison is not 'lose 1 kg a week versus 0.5' — it is 'lose 1 kg a week for six weeks and stop, versus 0.5 kg a week for six months'.

What the first month looks like

Week one shows a large drop that is mostly water and glycogen: each gram of stored glycogen holds about three grams of water. Weeks two and three often show nothing at all while fat loss continues underneath, which is where most people quit.

Weigh daily, average across seven days, and judge only the trend of those averages. A single morning's weight is noise — a day of higher salt or carbohydrate can move it a kilogram with no change in body fat whatsoever.

Stop cutting when performance in the gym falls consistently, sleep worsens, or the deficit starts driving your mood. Those are signals, not weakness, and a two-week return to maintenance is a normal part of a long plan.

Find your own number

The calorie calculator estimates maintenance from the Mifflin-St Jeor equation and your activity level; the weight-loss variant applies the deficit for you. Treat the output as a starting hypothesis and adjust it against two weeks of real weight data.