How the calorie deficit is worked out
The calculator first finds your maintenance calories — your TDEE — using the Mifflin-St Jeor equation for your basal metabolic rate multiplied by your activity level. That is the number of calories that keeps your weight steady. Your daily target is simply your maintenance minus the deficit you choose, and the weekly loss comes from the classic estimate that a pound of body fat holds about 3,500 calories (7,700 per kilogram). So a 500-calorie daily deficit projects to roughly a pound a week. Add a goal weight and it also estimates how long the plan takes. Want the maintenance number by itself, or the full lose/gain table? See the TDEE Calculator.
Choosing a deficit you can stick to
Faster is not better if you cannot sustain it. A 250–500 calorie deficit loses fat steadily while leaving enough food to train hard, protect muscle and stay sane socially. Larger deficits of 750–1,000 work for a while but get harder as hunger, fatigue and cravings build, and they raise the risk of losing muscle alongside fat. Keep protein high while cutting — the Protein Calculator shows your target — and consider dividing your calories across the day with the Macro Calculator. If your target ever drops below about 1,200 calories (women) or 1,500 (men), shrink the deficit instead.
Why the scale lies week to week
Fat loss is slow and steady; the scale is neither. Day to day it swings with water, sodium, carbohydrate stores, hormones and digestion, easily masking a real week of fat loss or faking a gain that vanishes in days. That is normal. The deficit math is a projection, not a promise — judge it over two to three weeks against a consistent tracking habit, and adjust your target by 100–150 calories if the trend genuinely stalls.