What Is a Calorie Deficit?

If your body burns 2,400 kcal a day and you eat 2,000, you are in a 400 kcal deficit. Your body makes up the difference from stored energy — mostly body fat, plus some muscle if protein and training are neglected.

Keto, fasting, low-carb and "clean eating" all work through the same mechanism: they make you eat less than you burn. Find your own numbers with the calorie deficit calculator.

For most people, 300–500 kcal per day (about 15–20% below maintenance) is the sweet spot: fast enough to see progress within a month, small enough to keep training quality, sleep and mood intact.

Very large deficits accelerate the scale but cost muscle, raise hunger sharply and almost always end in rebound eating.

A kilogram of body fat stores roughly 7,700 kcal, so a 500 kcal daily deficit works out to about 0.45 kg a week in theory. In practice water balance, glycogen and digestion make the scale jump around — judge progress on a 2–3 week trend, never on one morning.

Two reasons dominate. First, you get lighter, so maintenance drops — a 5 kg loss can shrink TDEE by roughly 60–100 kcal. Second, logging drifts: oil, sauces, drinks and untracked bites quietly close the gap.

Fix the second before touching the first, then recalculate your target and keep protein high — see the protein calculator.