What this calorie calculator does
The calorie calculator estimates how many calories your body needs in a day, then adjusts the number for your goal — maintain your current weight, lose weight at a sustainable pace, or gain weight steadily. The math runs in two steps. First it computes your basal metabolic rate (BMR) using the Mifflin-St Jeor equation, the most validated modern BMR formula. Then it multiplies by an activity factor to get total daily energy expenditure (TDEE), and applies a sensible 500-calorie deficit for fat loss or 300-calorie surplus for lean gain.
The Mifflin-St Jeor formula
BMR = 10 × kg + 6.25 × cm − 5 × age + 5 (men)BMR = 10 × kg + 6.25 × cm − 5 × age − 161 (women)
Multiply BMR by an activity factor — 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 athlete — and you get your maintenance calories. Subtract 500 for weight loss (≈ 0.45 kg / 1 lb per week), or add 300 for lean gain.
Pick your activity level honestly
This is where most calorie calculators go wrong, because most people overshoot. "Moderately active" means structured exercise 3-5 days a week plus a fair amount of incidental movement. If you sit most of the day and train three times a week, "lightly active" is usually closer to reality. Overstating activity inflates the calorie number, which means a fat-loss plan stalls or a maintenance plan slowly drifts up.
What a healthy rate looks like
Mainstream guidance — from the CDC, NHS, and American College of Sports Medicine — recommends losing 0.5-1% of body weight per week. For a 70 kg adult that is around 350-700 g per week. Faster than that for more than a couple of weeks typically comes from water and lean tissue rather than fat, and rebounds when you stop. Slow and consistent beats fast and fragile every time. On the gain side, 0.25-0.5 kg per week is enough for new muscle to actually attach to your frame; faster gains add mostly fat.
Tracking and adjusting
Eat to the calorie target for two to four weeks, weigh in two or three times a week under similar conditions, and look at the 14-day average rather than a single day. If the trend doesn't match your goal, nudge by 100-150 calories and reassess. Body weight bounces 1-2 kg from food volume, salt and water, so any single day means very little.
Use with the other tools
For protein, carb and fat targets in grams, use the macro calculator. For BMR only without the goal layer, the BMR calculator compares Mifflin-St Jeor and Harris-Benedict. For an in-depth TDEE breakdown across activity levels, the TDEE calculator is purpose-built.