Calorie Deficit Calculator

This calculator runs in two steps. First it estimates your maintenance calories (TDEE) from the Mifflin-St Jeor equation and the activity level you select — the same calculation behind the TDEE calculator. Second, it converts your chosen weekly rate into a daily deficit using a common rule of thumb: about 7,700 kcal is stored as one kilogram of body mass, so losing 0.5 kg a week works out to 0.5 × 7,700 ÷ 7 ≈ 550 kcal below maintenance each day. The target shown is maintenance minus that deficit.

The table lines up four weekly rates against the daily calories each one implies. A faster rate on paper is not automatically a better outcome: the deficit needed for 1 kg a week is roughly double the one for 0.5 kg, and a bigger daily shortfall is harder to sustain, more likely to be interrupted, and more likely to include muscle loss along with fat. The rates in the table are common reference points, not a ranking from best to worst.

Two approximations sit inside this number, and both carry real individual error. The Mifflin-St Jeor equation is a population regression — it predicts resting energy expenditure well on average but scatters around any one person's true value, for reasons covered in detail on the BMR calculator (body composition, genetics, thyroid function). The 7,700 kcal per kilogram conversion is itself a simplification: early weight loss includes water and glycogen alongside fat, the ratio of fat to lean tissue lost shifts with how large the deficit is, and energy expenditure tends to drop somewhat as body weight falls — a pattern often called metabolic adaptation. None of this makes the number useless, only a starting estimate to check against real results, not a guarantee.

If the calculated target lands below roughly 1,200 kcal for women or 1,500 kcal for men, the page flags it — targets that low are difficult to sustain, tend to shortchange nutrient intake, and raise the risk of losing muscle along with fat. That flag is a cue to talk to a registered dietitian or physician about the approach, not a target to push through alone. This calculator also is not built for pregnancy or breastfeeding, for growing children and teenagers, or for anyone with a current or past eating disorder, all of which change energy needs or make a calorie deficit the wrong tool entirely. Anyone managing a medical condition that affects weight or metabolism should work from a clinician's guidance rather than this generic formula.

Day-to-day weight moves with water, sodium, hormonal cycles and gut contents far more than with fat, so a single morning's reading proves little either way. The more useful habit is holding a target for two to three weeks and watching the trend across that window. If the trend does not move the way the math predicted, the gap is at least as likely to be in food tracking as in the formula — portions and labels are both easy to underestimate. This page explains how the number is built and where its assumptions can be wrong; it does not tell you which rate to pick or how low to go, and a registered dietitian or physician can weigh your history, health conditions and goals in a way a calculator cannot.