Calorie calculators all follow the same three steps: estimate the energy your body burns at rest, scale it up for activity, then adjust for your goal. This guide walks one example through all three so you can see where each calculator's number comes from. These are estimates for general information, not medical advice.
Step 1: basal metabolic rate (BMR)
BMR is the energy you burn at complete rest, to run your heart, lungs and brain. The most widely used estimate is the Mifflin-St Jeor equation: 10 × kg + 6.25 × cm − 5 × age + 5 for men, − 161 for women.
For a 30-year-old man weighing 80 kg and standing 180 cm: 800 + 1125 − 150 + 5 = 1,780 kcal a day. Other equations give nearby but different answers: Harris-Benedict gives 1,854 and Katch-McArdle, which uses body fat (20% here), gives 1,752. They agree within roughly 100 kcal, which is a reminder that the output is an estimate. The BMR Calculator shows all three.
Step 2: total daily energy expenditure (TDEE)
TDEE multiplies BMR by an activity factor: 1.2 for a desk job with little exercise, 1.375 lightly active, 1.55 moderately active, 1.725 very active and 1.9 extra active. At 1.55 the same man needs about 2,759 kcal to maintain his weight. The TDEE Calculator and Calorie Calculator do this multiplication.
Step 3: adjust for a goal
A kilogram of body fat holds roughly 7,700 kcal, a rule of thumb the Calorie Deficit Calculator uses. Losing 0.5 kg a week therefore needs a deficit of 0.5 × 7700 / 7 = 550 kcal a day, a target of about 2,209 kcal, and 5 kg takes 10 weeks. The rule is only approximate: as you lose weight your BMR falls, so the real result slows. Very large deficits, or targets below about 1,200 kcal a day, should be discussed with a clinician first.
Splitting calories into macros
Protein and carbohydrate give 4 kcal per gram, fat 9 and alcohol 7. A target of 2,000 kcal at 30% protein, 40% carbohydrate and 30% fat is 150 g, 200 g and 67 g. Protein is often set per kilogram of body weight instead: at 1.6 g/kg this man would eat 128 g. The Macro Calculator, Protein Calculator, Carbohydrate Calculator and Fat Intake Calculator cover each version.
Energy out: exercise, steps and drinks
- Exercise:
kcal = MET × kg × hours. Five METs (brisk cycling) for an hour at 70 kg burns 350 kcal. See the Calories Burned Calculator and Steps to Calories & Miles Calculator. - Alcohol: a standard US drink holds 14 g of ethanol, which at 7 kcal per gram is 98 kcal before any mixer. See the Alcohol Units Calculator.
- Caffeine: about 400 mg a day is the commonly cited limit for healthy adults; the Caffeine Calculator tracks how it decays through the day.
- Water: a common starting point is about 33 mL per kg plus extra for exercise, from the Daily Water Intake Calculator.
- Units: 1 kcal is 4.184 kJ, so 2,000 kcal is 8,368 kJ; the Calories to Joules Converter converts both ways.
Common mistakes
- Treating the estimate as exact. Weigh yourself over two or three weeks and adjust the target by the trend.
- Choosing an activity factor that is too high. Most people overestimate their activity; start lower.
- Eating below BMR for long periods. It is hard to sustain and can cost muscle; seek medical guidance for very low-calorie plans.
- Forgetting that the formulas assume adults. Children, pregnancy and medical conditions change the numbers.