Every diet starts with a number — but the number most people use is a guess, a friend's number, or a round figure from a magazine. Your body burns a specific amount of energy each day, and knowing that figure turns dieting from guesswork into arithmetic. Our free Calorie Calculator works it out in seconds: it uses the Mifflin-St Jeor equation — the formula dietitians rate most accurate — to find your BMR and TDEE, then shows the daily targets for losing, maintaining or gaining weight, with a protein, carb and fat split for each. This guide explains how to calculate your daily calories with it, the equation behind it, the five activity levels that make or break the answer, and the targets that actually work.
Step one: enter your sex, age, weight and height. Pick metric (kg / cm) or imperial (lb / ft-in) — the calculator converts either way. Step two: pick the activity level that honestly matches a normal week — not your best week. That honesty matters more than any other input, because the activity multiplier is the biggest source of error in every calorie calculator. If you work a desk job and train three evenings a week, you are lightly to moderately active, even though the training itself feels hard. Step three: press Calculate My Calories. You get two headline numbers: your BMR, the calories your body burns doing nothing at all, and your TDEE, the maintenance level at which your weight stays stable. Everything else on the results table is simple arithmetic on the TDEE: the targets use the standard, evidence-based deficits and surpluses — eating about 500 kcal below maintenance produces roughly one pound of fat loss per week, 250 below produces about half a pound, and the mirror-image surpluses gain at the same gentle rates.
Of the equations used to estimate resting metabolism, the Mifflin-St Jeor equation, published in 1990, has repeatedly tested as the most accurate for the general population — within about 10% of measured values for most adults, and more reliable than the older Harris-Benedict formula it replaced. It needs only your sex, weight, height and age. Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5. Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161. Your TDEE is then your BMR multiplied by an activity factor that accounts for everything else you do — walking, working, training, fidgeting. Sedentary (desk job, driving, no structured exercise) multiplies by 1.2. Lightly active (light exercise 1–3 days a week, or a job with some walking) multiplies by 1.375. Moderately active (training 3–5 days a week) multiplies by 1.55. Very active (hard training 6–7 days a week) multiplies by 1.725. Extra active (physical labour plus daily training, or elite athletes in season) multiplies by 1.9. A moderately active person burns 55% more than their BMR across a full day — everyday movement, walking, stairs, standing and chores add more than most people guess.
Take a 30-year-old man, 80 kg, 180 cm, who trains four days a week. His BMR is (10 × 80) + (6.25 × 180) − (5 × 30) + 5 = 800 + 1,125 − 150 + 5 = 1,780 kcal. Multiplied by 1.55 for his activity, his maintenance TDEE is 2,759 kcal a day. To lose about a pound a week he eats near 2,259 kcal; to gain slowly, near 3,009 kcal. At maintenance, the 30/40/30 macro split works out to roughly 207 g protein, 276 g carbs and 92 g fat — protein is deliberately set generously, because adequate protein is what protects muscle while you lose fat, and it is the macro most people under-eat. Two practical notes before you treat the targets as a plan. Protein is set first in the split — 30% of calories — because it drives satiety and protects muscle in a deficit; if your calculated protein lands below about 1.6 g per kilogram of body weight while dieting, favour protein-rich foods first and let the carb and fat numbers flex around it. And weigh the trend, not the day: body weight swings by a kilogram or more with water, salt and meal timing, so judge your intake by the average of two to three weeks, never by a single morning.
Faster is rarely better — deficits beyond 500 kcal a day cost muscle, energy and adherence, which is why this calculator does not offer them. The 500-kcal-equals-a-pound rule is also an approximation that flattens out over time as your body adapts: your TDEE falls as you lose weight — roughly 20–30 kcal per day for every kilogram lost — so a deficit that worked at 90 kg will stall at 78 kg unless the target is refreshed. Recalculate every 5 kg or so, and treat a two-week plateau at a verified intake as the signal to recalculate. The safety floor matters too: eating below 1,200 kcal a day (women) or 1,500 kcal a day (men) is not recommended without medical supervision, and the calculator flags targets that fall below it. For gaining, the +250 kcal target suits most lifters: it supports roughly half a pound of gain per week with minimal fat, provided protein stays near the level shown in the macro split (about 30% of calories, roughly 1.6–2.2 g per kg of body weight for most trainees) and the training is progressive resistance work. Faster surpluses mostly add fat.
Choosing your best week as your activity level. The multiplier is the biggest source of error in the whole calculation — if your calculated TDEE looks high but your weight has been stable at a lower intake, choose the activity level one step down; your personal multiplier may sit below the average for that band. Eating at your BMR instead of your TDEE. BMR is what you burn lying still; eating at your BMR while living a normal active life is a substantial deficit, not maintenance. Chasing a deficit bigger than 500 kcal a day. It costs muscle, energy and adherence, and the rate slows anyway as your body adapts. Judging by one morning's weight. Water, salt and meal timing swing body weight by a kilogram or more — judge by the two-to-three-week average. Ignoring protein. Protein is set first in the split for a reason: it drives satiety and protects muscle in a deficit, and it is the macro most people under-eat.
The honest limits are three. First, every calorie equation is an estimate: individual metabolic rates vary by roughly ±10% around any formula, because muscle mass, genetics, hormones, sleep and medications all move the true figure. Treat the result as a starting point, hold your intake steady for two to three weeks, and adjust by 100–200 kcal based on what your weight actually does — that feedback loop beats any equation. Second, the calculator is general information, not medical advice: it is not designed for children, pregnancy, eating-disorder recovery, or medical conditions such as diabetes or thyroid disease, and it will not offer targets below the recognised safety floors without flagging them. If in doubt, a registered dietitian can measure what a formula can only estimate. Third, the numbers assume a stable routine — a new job, a training block or a long diet all change the answer, which is why recalculation every few months is part of the method, not an afterthought. Within those limits, the tool answers the question every diet starts with: how many calories do I actually need? Two companion guides cover the numbers around the result: our BMR guide works the Mifflin-St Jeor formula in full on its own, and our BMI chart guide explains what your weight-for-height number really means — and when it misleads.
Mifflin-St Jeor worked in full — 1,699 calories at rest becomes a 2,633 TDEE, with the ±500 ladder for losing or gaining half a kilo a week.
What the standard international cut-offs are, when they mislead, and how to read your number correctly.
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