Calorie calculator

Where daily expenditure comes from

Almost everything you burn in a day sits in your basal metabolic rate: the energy your body spends simply functioning — heart beating, kidneys filtering, temperature held, cells renewing. In sedentary people that is somewhere between 60% and 70% of the total.

The rest splits three ways. Digestion costs energy to process what you eat, roughly 10% of daily intake, with protein costing considerably more than the other macronutrients. Structured exercise comes next. And then there is everything that is not training: walking, taking stairs, gesturing, standing, that background restlessness which varies enormously from person to person.

It is that last portion that makes two people of the same weight, height and age burn quite different amounts — and it is exactly the one no five-option questionnaire can measure properly.

The equation used here

Basal metabolic rate is estimated with the Mifflin-St Jeor equation, published in 1990. Among the predictive equations in common use it performs best in adults, and it is the one recommended by the Academy of Nutrition and Dietetics. The Harris-Benedict equation, from 1919, still circulates on many sites but overestimates expenditure in contemporary populations.

When you enter a body fat percentage, the maths switches to the Katch-McArdle equation, which starts from lean mass rather than total weight. When body composition has actually been measured, it explains expenditure better — but it only makes sense with a figure from bioimpedance, calipers or DEXA. A guess makes the result worse, not better.

An activity factor is then applied on top of the basal rate, and that is where most of the error lives. The 1.2 to 1.9 scale is a coarse approximation by nature. If you sit between two descriptions, picking the lower one and adjusting later tends to cost less trouble than the reverse.

Deficit, surplus and the real pace

The adjustment for losing or gaining is applied as a percentage of expenditure rather than a fixed number of calories. The reason is simple: 500 kcal less is a moderate cut for someone burning 3,000 and a severe one for someone burning 1,600. The same proportion treats both fairly.

The projected kilos per week uses the classic rule of 7,700 kcal per kilo of body tissue. It is useful for the first weeks and misleading after that: as weight comes down, expenditure comes down with it, and the same plate produces a smaller deficit. Loss that slows is not a failure of discipline or a "stalled" metabolism — it is arithmetic.

The deficit ceiling offered here is 20%. Sharper cuts increase lean mass loss without proportionally speeding up fat loss, and adherence collapses. Below 1,200 kcal for women and 1,500 kcal for men the calculator raises the target and says so: in that range, covering vitamin and mineral needs gets hard, and supervision becomes clinical.

How the macro split is built

Protein is set first, in grams per kilo, because it is the only one with a well-established absolute target: around 1.6 g/kg supports muscle gain, and about 2.0 g/kg in a deficit is what best preserves lean mass while weight comes down.

It is calculated on a reference weight, not the number on the scale. When current weight sits above the healthy range for your height, the maths uses the top of that range — fat tissue does not demand protein the way muscle does, and using total weight would inflate the target for no benefit.

Fat comes next at 25% of calories, the middle of the usual 20% to 35% recommendation, with a floor per kilo so it never falls to levels that would compromise fat-soluble vitamin absorption and hormone production. Carbohydrate takes what remains — which is what makes the three add up to exactly the target.

The suggested fibre follows the Institute of Medicine reference of 14 g per 1,000 kcal.

Activity factors