Daily calorie needs from the Mifflin-St Jeor equation, adjusted for activity.
First the basal metabolic rate is worked out with the Mifflin-St Jeor equation: for men 10 × kg + 6.25 × cm − 5 × years + 5, for women the same minus 161. That figure is multiplied by an activity factor, from 1.2 for sedentary to roughly 1.9 for very active. The result is an approximate daily maintenance requirement.
Basal metabolic rate is the energy the body uses at complete rest — heartbeat, breathing, maintaining body temperature and running the brain. In an adult it usually accounts for most of total daily energy use. This calculator estimates it with the Mifflin-St Jeor equation, one of the more accurate estimates available without laboratory measurement.
A common guide is a deficit of 500 kilocalories a day, which corresponds to roughly half a kilogram a week. Deficits beyond about 20–25% of maintenance more often cost muscle mass and are harder to sustain. The Mifflin-St Jeor result is an estimate rather than a measurement, so real expenditure can differ by several hundred kilocalories.
The Mifflin-St Jeor equation takes height, age and sex as well as weight, and more still sits outside the formula: the ratio of muscle to fat, thyroid function, genetics and the daily movement that is not exercise. That is why the equation gives an estimate with a spread of several hundred kilocalories, and is meant as a starting point rather than a measurement.