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How Are Daily Calorie Needs Calculated?

Your basal metabolic rate (BMR) — the energy your body burns at complete rest just to keep your organs functioning — is estimated with the Mifflin-St Jeor equation: for men, BMR = 10×weight(kg) + 6.25×height(cm) − 5×age(years) + 5; for women, the same formula minus 161 instead of plus 5. Multiplying BMR by an activity factor — 1.2 for a sedentary desk job, 1.375 lightly active, 1.55 moderately active, 1.725 very active, up to 1.9 for physically demanding work or twice-daily training — gives your total daily energy expenditure (TDEE): the calories that keep your weight stable.

Worked example: a 35-year-old woman who is 165 cm tall and weighs 65 kg has a BMR of 10×65 + 6.25×165 − 5×35 − 161 = 650 + 1,031.25 − 175 − 161 ≈ 1,345 kcal/day. With a moderately active lifestyle (factor 1.55), her TDEE is roughly 1,345 × 1.55 ≈ 2,085 kcal/day. To lose about 0.5 kg per week she would eat around 1,585 kcal/day (a 500 kcal deficit); to gain weight she would add roughly 500 kcal on top of 2,085.

What You Should Know

Mifflin-St Jeor is considered more accurate than the older Harris-Benedict equation for most adults, but it is still a population average — individual metabolism can differ by 10% or more due to genetics, muscle mass, hormonal conditions such as thyroid disorders, and even sleep quality. The "500 kcal ≈ 0.5 kg/week" rule of thumb comes from the rough energy density of body fat (about 7,700 kcal per kilogram), but real-world weight change also includes water retention, glycogen stores and digestive contents, so week-to-week scale numbers are noisy even when the underlying fat loss is on track.

Very large deficits — more than about 25% below TDEE — tend to backfire: they are hard to sustain, increase muscle loss alongside fat loss, and can trigger metabolic adaptation that lowers BMR further. The equation also does not fit people with very low or very high muscle mass well, since it is based on total weight rather than lean body mass directly. Treat the calculated number as a starting point: track your actual weight trend over 2–3 weeks and adjust the target up or down by 100–200 kcal if the real-world result does not match the expectation. This tool is informational only and is not a substitute for personalised advice from a doctor or registered dietitian, especially if you have a medical condition affecting metabolism or are pregnant or breastfeeding.