How it works, what it calculates, and why the math is the way it is.
What's My Deficit? is a free, browser-based calorie deficit calculator designed to help you understand exactly how many calories you need to eat each day to reach your goal weight by a specific date — safely and sustainably.
No account required. No data sent to a server. All calculations happen entirely in your browser.
This calculator uses the Mifflin-St Jeor equation (1990), which is the formula recommended by the Academy of Nutrition and Dietetics for estimating resting metabolic rate. It is considered the most accurate of the commonly available predictive equations for most adults.
Your BMR (Basal Metabolic Rate) is then multiplied by an activity factor to estimate your TDEE — Total Daily Energy Expenditure — the total calories your body burns on an average day. Your daily calorie target is set below that number to create a deficit.
Because one pound of fat contains approximately 3,500 kilocalories (one kilogram ≈ 7,700 kcal), your required daily deficit is calculated by dividing your total caloric shortfall by the number of days to your goal date.
The calculator enforces a minimum calorie floor of 1,200 cal/day for women and 1,500 cal/day for men, consistent with standard clinical guidelines. Going below these levels without medical supervision is generally not recommended.
It also flags deficits that would require losing more than 2.0 lbs per week as too aggressive, and offers to automatically recalculate a safer goal date at a 1.5 lbs/week pace.
This tool is for informational purposes only. It is not a substitute for advice from a registered dietitian, physician, or other qualified healthcare provider. Always consult a professional before making significant changes to your diet or exercise routine.