How daily calorie needs are calculated — BMR, activity multipliers and what a deficit means

Every calorie calculator runs the same two steps: estimate what your body burns at rest, then multiply for how active you are. The formulas are decades old, decent on average and wrong by a few hundred calories for any given person — which is fine, if you know how to correct them. This guide explains the arithmetic behind the calorie calculator and how to use it honestly. General information, not medical advice.

BMR: what your body burns at rest

Basal metabolic rate is the energy used to stay alive lying still: breathing, circulation, cell repair, brain function — typically 60–70% of a sedentary person's total. It scales with lean mass mainly, which is why the formulas use weight, height, age and sex as proxies. The most accurate simple formula is Mifflin-St Jeor (1990): for men, 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5; for women, the same minus 161. A 30-year-old woman of 65 kg and 165 cm: 650 + 1031 − 150 − 161 = 1,370 kcal. Older formulas (Harris-Benedict, 1919) run a few percent higher; the Katch-McArdle formula uses lean mass directly if you know your body fat (How body fat percentage is estimated — and how much to trust each method).

TDEE: adding activity

Total daily energy expenditure multiplies BMR by an activity factor: sedentary 1.2, light 1.375, moderate 1.55, very active 1.725, extremely active 1.9. The 1,370 kcal woman at "light" is about 1,880 kcal a day. The multipliers include the thermic effect of food (roughly 10% of intake) and all movement — exercise and the fidgeting, walking and standing that vary enormously between people (NEAT). The commonest error is picking "moderate" for three gym sessions a week in an otherwise desk-bound life: that is "light". Exercise burns less than watches claim, and a desk job burns less than most people feel it should.

Deficits, surpluses and the 3,500-calorie idea

Weight change follows energy balance: eat below TDEE and the body draws on stores, above it and it adds them. The old rule that a pound of fat is 3,500 kcal — so 500 a day below maintenance loses a pound a week — is roughly right at the start and increasingly wrong over time, because the body adapts: a lighter body burns less, NEAT drops, and appetite rises. Realistic sustained rates are 0.5–1% of body weight per week on a 15–25% deficit; steeper cuts lose muscle and rarely last. Gaining muscle wants a modest surplus (200–300 kcal) plus training and protein (the macro calculator, How much protein do you actually need?).

Why the number is wrong for you

Individual BMR varies about ±10–15% around the formula for the same inputs; activity is guessed; food labels are allowed 20% error and portions are misjudged by more; and metabolism adapts to dieting. So a calculator's TDEE is a starting hypothesis, not a fact. The correction is empirical: eat at the estimated maintenance for two to three weeks, weigh daily and average weekly, and adjust by 100–200 kcal in the direction the trend says. Two weeks of stable weight tells you your maintenance better than any formula.

Using it well

• Choose the honest activity level, usually one below your first instinct.
• Track for a few weeks against the scale trend; adjust the number, not the formula.
• Prefer moderate deficits and adequate protein; count the exercise you actually do, not the watch's number.
• Recalculate as weight changes — every 5 kg or so.
• Very low intakes (under about 1,200 kcal for women, 1,500 for men) belong under medical supervision, as does any plan for someone with a history of disordered eating, pregnancy, or a medical condition.

How to count macros, in plain English covers splitting the calories; the BMI calculator the crude but useful screen; How much water you should drink a day — what the evidence says, and where 8 glasses came from the other intake people over-think.