How does BMR work?
Your basal metabolic rate (BMR) is the energy your body uses at complete rest: breathing, circulation, body temperature, brain function and the constant repair of cells and tissues. Even a full day in bed would still burn roughly this amount, and for most adults it makes up 60 to 70 percent of total daily energy use — typically somewhere between about 1,200 and 2,000 kcal.
This calculator uses the Mifflin–St Jeor equation, generally found to be the most accurate of the common formulas. For men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5. For women, the same equation ends in −161 instead of +5. It works because body size is the strongest driver of resting energy use: more tissue costs more to maintain, and BMR drifts down with age mainly because muscle mass does. Two people with identical stats can still differ by 5 to 10 percent, so treat the result as a starting point, not a lab measurement.
From BMR to TDEE: what gets added on top?
Your total daily energy expenditure (TDEE) is BMR plus everything else. The biggest addition for most people is NEAT — non-exercise activity thermogenesis: walking, standing, chores and other daily movement that is not formal exercise. It varies enormously, from a few hundred calories for a desk worker to a thousand or more for someone on their feet all day, which is why the activity level you pick matters so much.
Structured training adds a second layer, usually a few hundred calories per session — less than most trackers claim. The third component is the thermic effect of food (TEF): digesting what you eat costs roughly 10 percent of intake on a mixed diet, with protein at the high end (20 to 30 percent of its calories) and fat at the low end. Calculators roll all of this into one multiplier, roughly 1.2 for sedentary up to 1.9 for very heavy days. Each component is an estimate, so the final TDEE can be off by 10 percent or more — which is why the adjustment step below matters more than the first calculation.
How do you choose a calorie target for your goal?
For fat loss, subtract a percentage of TDEE so the deficit scales to your size. About 15 percent is gentle and easy to sustain, around 20 percent is a standard moderate choice, and around 25 percent is aggressive and best kept short. A sensible deficit produces loss of roughly 0.5 to 1 percent of body weight per week; faster usually costs muscle and adherence, slower is hard to distinguish from normal fluctuation.
For muscle gain the surplus should stay small: about 5 percent above TDEE for experienced lifters, around 10 percent as a middle ground, and up to 15 percent for lean beginners, who build muscle fastest. Even under good conditions, realistic gain is roughly 0.5 to 1 kg of muscle per month in the first training year, slowing each year after. A bigger surplus mostly adds fat, not extra muscle. For maintenance, eat at TDEE and trust the trend: a flat three-to-four-week average means you have found your true maintenance, whatever the calculator said.
How should you split calories between protein, fat and carbs?
Set protein first. About 1.6 to 2.2 grams per kilogram of body weight per day suits people who train or who are dieting, with the higher end helping to protect muscle in a deficit — roughly 120 to 165 g a day for a 75 kg person. Set fat second, at roughly 0.6 to 1.0 g/kg per day. Fat is essential for hormone production and vitamin absorption, so going below about 0.5 g/kg is a poor idea; above that floor, the exact amount is largely taste.
Carbohydrate then takes whatever calories remain. This remainder approach is deliberate: once protein and minimum fat are covered, shifting calories between fat and carbohydrate has little measurable effect on fat loss or muscle gain when total calories and protein are equal. Hard trainers often perform better with more carbohydrate; others prefer fattier food. Adherence beats fine-tuning.
How do you adjust when reality disagrees with the calculator?
Weigh yourself several times a week, ideally in the morning, and compare weekly averages rather than single days, which swing with water, salt and digestion. Give any new target two to three weeks before judging it, because the first week usually includes water shifts that hide the real trend.
Then adjust in small steps of 100 to 200 kcal. Losing slower than about 0.25 percent of body weight per week? Trim 100 to 200 kcal, or add daily steps for an equivalent deficit. Losing faster than about 1 percent per week? Add 100 to 200 kcal back to protect muscle and energy. For muscle gain, if your average weight is flat after three weeks, add 100 to 200 kcal; if it is climbing much faster than about 0.25 to 0.5 percent per week, trim a similar amount. Small corrections from your own data always beat any formula.