How your calorie number is calculated
Two steps. First your BMR (basal metabolic rate) — the energy your body burns at complete rest just running organs — using the Mifflin-St Jeor equation, which the American Council on Exercise rates as the most accurate of the common formulas:
Men: 10×weight(kg) + 6.25×height(cm) − 5×age + 5
Women: 10×weight(kg) + 6.25×height(cm) − 5×age − 161
Then BMR is multiplied by an activity factor (1.2 sedentary → 1.9 extra active) to get your TDEE — total daily energy expenditure, the maintenance number at the top of your results. If you enter a body-fat percentage, the calculator switches to the Katch-McArdle formula (370 + 21.6 × lean mass), which is more precise because muscle burns most of the calories.
The only rule of weight change
Eat below your TDEE and you lose; above it and you gain. Roughly 7,700 kcal ≈ 1 kg (3,500 kcal ≈ 1 lb) of body fat, so a 550 kcal daily deficit ≈ 0.5 kg per week. The goal table applies exactly this math. Sustainable beats fast: deficits larger than ~25% of maintenance are hard to stick to, cost muscle, and slow your metabolism as weight drops — which is also why you should recalculate every 5 kg / 10 lb of change.
Pick your activity level honestly
The single biggest error people make is overrating activity. A desk job plus three gym visits a week is "Light", not "Moderate". Training an hour a day but sitting the other fifteen waking hours is "Moderate", rarely "Very active". If you're between two levels, pick the lower one, track your weight for two or three weeks, and adjust — the scale is the final referee, not the formula.
Why the safety floor matters
Below roughly 1,200 kcal/day (women) or 1,500 (men) it becomes genuinely difficult to get enough protein, iron, calcium and essential fats, and the body responds by down-regulating and burning muscle. The calculator greys out any goal that lands under those floors. If the aggressive target is greyed, take the slower lane — you'll keep more muscle and the result lasts.
What the macros card is for
Calories decide weight change; macros decide how it feels and what you keep. Protein preserves muscle in a deficit (and is the most filling macro), carbs fuel training, fat supports hormones. The three presets cover most goals — high-protein is the usual pick while cutting. Grams are computed from the goal you select, at 4 kcal/g for protein and carbs and 9 kcal/g for fat.
Estimates, not gospel
Two people with identical stats can differ by a couple hundred kcal/day. Treat the output as a well-informed starting point: eat the number for two or three weeks, watch the trend, then nudge by 100–200 kcal. That feedback loop beats any formula.