Calorie Deficit Calculator
Calculate your BMR and TDEE, then find the exact calorie target for your goal. Based on the Mifflin-St Jeor equation — the most accurate formula for most people.
The Science of Calorie Deficits
A calorie deficit occurs when you consume fewer calories than your body expends. This forces your body to use stored energy — primarily body fat — to make up the difference. The fundamental principle is based on the first law of thermodynamics: energy cannot be created or destroyed, only converted.
The Mifflin-St Jeor equation, which this calculator uses, is considered the most accurate formula for estimating Basal Metabolic Rate (BMR) in most populations. A 2005 meta-analysis in the Journal of the American Dietetic Association found it to be more accurate than the Harris-Benedict equation for both normal-weight and obese individuals.
Research consistently shows that a deficit of 500 kcal/day leads to approximately 0.5 kg of fat loss per week. However, very large deficits (over 1000 kcal/day) can lead to muscle loss, hormonal disruption, and metabolic adaptation. The sweet spot for most people is a 20-25% deficit below TDEE.
Important: These calculations are estimates. Individual variation in metabolism, gut microbiome, NEAT (non-exercise activity thermogenesis), and hormonal status all affect actual results. Track your weight over 2-3 weeks and adjust by 100-200 kcal if not seeing expected progress.