Both estimates update as you type and show the formula with your own numbers.
How the daily calorie estimate works
First the Mifflin-St Jeor equation estimates your basal metabolic rate (BMR), the energy your body uses at complete rest: 10 × weight in kg + 6.25 × height in cm − 5 × age, then + 5 for the male formula or − 161 for the female one. That is multiplied by an activity factor, from 1.2 for little exercise to 1.9 for very hard daily exercise or physical work, to give your total daily energy expenditure (TDEE), the rough number of calories that keeps weight steady.
How accurate is it?
Mifflin-St Jeor is one of the better formulas for adults, but it is still an average. Many people land within about 10% of a measured value; some are further off, especially very muscular people, older adults and anyone with a medical condition. Picking an activity level is guesswork too, so treat the result as a starting point to compare with how your weight actually changes over a few weeks. It is not meant for children, during pregnancy, or as medical advice.
The US Navy body fat method
It estimates body fat from height and the circumference of your neck and waist, plus your hips for the female formula. Measure on bare skin with a flexible tape, snug but not pressing in: the neck just below the larynx, the waist at the navel for the male formula or at the narrowest point for the female one, and the hips at their widest. Measure two or three times and use the average. There is no single right number for everyone; reference ranges differ by source, age and sex.
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