Calorie Calculator

Resting rate, activity, and the two multipliers that matter

The calculation has two halves. The first is the resting metabolic rate: what the body burns at rest, estimated from weight, height, age and sex. Mifflin-St Jeor is the equation most dietitians use; Harris-Benedict is older and usually a little higher. Both are close for most people and both are off by more than 10% for plenty of them, which is why the page shows the pair.

The second half is the activity multiplier, applied to the resting rate: 1.2 for sedentary up to 1.9 for physical work plus twice-daily training. This is where the estimate usually goes wrong, because it is self-reported and most people overestimate how much they move. The page prints the level it used so that changing it is an obvious thing to try.

Two weeks of keeping to a target and watching what the scale does is worth more than any refinement of the equations: that is the measurement, and these are the starting estimates.

Deficits, surpluses and the 7700 kcal figure

A deficit of 500 kcal a day is about half a kilogram a week, because 7700 kcal is approximately the energy in a kilogram of body mass: 500 × 7 = 3500 kcal a week, which is 0.45 kg. The number comes from a 1950s estimate of the energy content of body tissue and it is used everywhere, including here.

It is also a simplification. As weight falls, resting expenditure falls with it, and adherence rarely holds exactly, so a 500 kcal deficit produces less than half a kilogram a week after a few months. The page prints the straight-line figure and says that the line bends.

Gaining weight is the same arithmetic in the other direction, with the same caveat - and a smaller one to hit, because a surplus large enough to add a kilogram a week mostly adds fat rather than muscle.

The floors, and why they are on the page

Commonly cited minimum intakes are around 1200 kcal a day for women and 1500 for men. The page prints the floor for the sex used by the equation and warns when the target is below it, rather than quietly producing a number that a person might try to live on.

The warning exists because very low intakes make it hard to get enough protein, fiber, vitamins and minerals, and because they are a poor fit for this kind of arithmetic: the equations and multipliers are population estimates for ordinary intakes, and pushing them far outside their range makes the answer less reliable rather than more precise. Anything below the floor needs supervision by somebody who can look at the whole picture.

Macros, and what a split does and does not do

The gram figures come from the Atwater factors: protein and carbohydrate are about 4 kcal per gram, fat about 9, alcohol about 7. So a 2000 kcal day that is 30% protein is 150 g of protein, and the page shows the arithmetic rather than only the total.

The split itself is a preference. For weight change, the total is what matters; for body composition and appetite, protein and fiber help more than the ratio of the other two; for health, the quality of the food matters more than any split. Protein is also given per kilogram of body weight on the page, because that is how the guidance is written: the RDA is 0.8 g/kg as a minimum, and muscle-building trials converge around 1.6 g/kg.

No split is offered as a recommendation, and the page says so.

What this calculator cannot know

It does not know your body composition, so it cannot tell muscle from fat among the kilograms it was given. It does not know your medical history, your medication, your sleep, your stress or your appetite, all of which affect energy balance more than the fourth decimal place of a multiplier.

It is not appropriate for pregnancy, for recovery from an eating disorder, or for anybody managing diabetes, kidney disease or another condition where intake is prescribed. Those need a registered dietitian rather than a calculator, and the page says so rather than qualifying everything into meaninglessness.