Calorie Deficit Calculator
Turn a daily deficit into a weekly rate and a realistic timeline.
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Why the arithmetic overstates what happens
A kilogram of body fat stores roughly 7,700 kcal, so a 500 kcal daily deficit predicts about 0.45 kg a week. That arithmetic is the basis of every deficit calculator, and it is reliably optimistic — real losses fall short of it, and the gap widens the longer you continue.
Two things cause the drift. As you lose weight you become smaller, so maintenance calories fall and a fixed deficit shrinks in relative terms; a target set at the start is no longer a deficit by the time you are several kilograms down. Separately, the body adapts — non-exercise movement drops, and metabolic rate falls slightly more than the size change alone predicts. Recalculate every few kilograms rather than following one number to the end.
Rate matters for what you keep. A deficit of 0.5-1% of bodyweight per week preserves lean mass reasonably well; much faster loss costs proportionally more muscle, which lowers maintenance calories further and makes the weight easier to regain. Adequate protein and resistance training are what protect lean mass in a deficit — not the size of the deficit itself. Very low targets also make adequate micronutrient intake difficult.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. Very low calorie targets, rapid loss, and deficits during pregnancy, breastfeeding, adolescence or alongside an eating disorder need clinical supervision rather than a calculator.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
How big should a calorie deficit be?
A deficit producing 0.5-1% of bodyweight per week is a common recommendation — roughly 250-500 kcal a day for most people. Larger deficits lose weight faster but cost more lean mass and are harder to sustain.
Why am I losing less than the calculator predicts?
Three usual reasons. Maintenance calories fall as you get smaller, so a fixed deficit shrinks over time. The body adapts by reducing spontaneous movement. And intake is very commonly underestimated — tracking error alone often accounts for the gap.
Is 1200 calories a day safe?
It is below the level generally considered safe without supervision for most adults, and meeting micronutrient needs at that intake is difficult. The commonly cited floors are about 1200 kcal for women and 1500 for men, and going below either warrants clinical input.
Why did my weight not move this week?
Body weight fluctuates by a kilogram or more from water, glycogen, sodium and digestive contents, which easily masks genuine fat loss over short periods. Judge progress on a trend across two to four weeks rather than on single readings.
Do I need to eat back exercise calories?
If your maintenance figure already includes your activity level, then no — exercise is already counted, and adding it again removes the deficit. Only add exercise calories if your baseline was calculated as sedentary.