What This Calculator Does

"How many calories should I eat in a day?" depends on more than just body size — age, sex, and how active you are all shift the number meaningfully. This tool takes those factors and estimates your Total Daily Energy Expenditure: roughly how many calories your body burns in a day at your current activity level, which is the number most people use as a baseline before deciding whether to eat more, less, or the same.

How to Use It

Fill in your Age, Gender, Weight in kilograms, and Height in centimeters — note that height here is centimeters, not meters like the BMI Calculator, so a 175cm person enters 175, not 1.75. Then choose the Activity Level that best matches your week, from Sedentary up through Super Active, and the result gives your estimated daily calorie needs in kcal, rounded to the nearest whole calorie.

The Formula

This calculator uses the Mifflin-St Jeor equation, generally considered one of the more accurate BMR formulas for the general population. It first estimates your Basal Metabolic Rate — the calories you'd burn lying still all day — and the formula differs slightly by gender: for men, BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5; for women, BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161.

That BMR is then multiplied by an activity factor to reach TDEE: 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active, and 1.9 for super active (very hard exercise or a physical job, on top of intense training). The final TDEE figure is rounded to the nearest whole calorie.

A Worked Example

A 30-year-old male, 70kg, 175cm, moderately active: BMR = (10 × 70) + (6.25 × 175) − (5 × 30) + 5 = 700 + 1093.75 − 150 + 5 = 1648.75. Multiply by the moderate-activity factor of 1.55 and TDEE = 1648.75 × 1.55 = 2555.5625, which rounds to 2,556 kcal per day. For contrast, a 25-year-old female, 60kg, 165cm, sedentary: BMR = (10 × 60) + (6.25 × 165) − (5 × 25) − 161 = 600 + 1031.25 − 125 − 161 = 1345.25, and at the sedentary multiplier of 1.2, TDEE = 1345.25 × 1.2 = 1614.3, rounding to 1,614 kcal per day — noticeably lower, driven by both the smaller body size and the much lower activity multiplier.

Why the Formula Uses These Inputs

Weight, height, age, and sex aren't arbitrary — each one is a practical stand-in for something the equation can't measure directly: how much metabolically active tissue (mostly muscle and organ mass) your body is carrying, since that tissue is what actually burns most of your resting calories. Weight and height together correlate with overall body size and lean mass. Age is subtracted because resting metabolic rate tends to decline gradually as people get older, largely due to a gradual average loss of muscle mass over time. The formula splits by sex (the +5 for men vs. −161 for women) because, on average, men carry a higher proportion of lean muscle mass relative to total body weight than women do at the same weight and height, which raises average BMR even when body size is otherwise similar. None of these are perfect proxies for any one individual — they're population-level averages baked into the equation from the research it was derived from.

The activity multiplier exists because BMR alone only covers energy spent at rest — digestion, breathing, circulation, keeping organs running — and says nothing about movement. Each multiplier is a rough estimate of how much extra energy a typical activity pattern burns on top of that resting baseline: 1.2 for sedentary reflects very little deliberate exercise, while 1.9 for super active reflects intense regular training combined with a physically demanding job or lifestyle. The worked examples above show why this matters as much as body size does: the 70kg male's BMR (1,648.75) is only about 23% higher than the 60kg female's (1,345.25), but his moderately-active 1.55 multiplier versus her sedentary 1.2 multiplier widens the final TDEE gap to about 58% (2,556 vs. 1,614 kcal) — activity level alone accounts for more of that difference than body size does.

Limitations

Mifflin-St Jeor is a population-average formula, and real individual metabolic rates vary meaningfully around that average even for two people with identical age, sex, weight, and height — genetics, body composition (muscle vs. fat ratio at the same total weight), and other individual factors all shift actual BMR up or down from what the formula predicts. This calculator also doesn't account for medical conditions (thyroid disorders being a common example) or medications that are known to raise or lower metabolic rate, since it has no way to know about either. Treat the result as a reasonable starting estimate for general planning, not as personalized medical or dietary guidance — if you have a specific health condition or medical concern, a doctor or registered dietitian can give you guidance this kind of general-purpose calculator simply isn't built to provide.