Mifflin-St Jeor Calculator

This calculator estimates resting metabolic rate (RMR) -- the calories the body burns at complete rest just to maintain basic functions -- using the Mifflin-St Jeor equation, published by Mark Mifflin, Sachiko St Jeor, and colleagues in the American Journal of Clinical Nutrition in 1990, which is widely regarded in clinical nutrition practice as more accurate for modern populations than the older Harris-Benedict equation it largely replaced. The formula is 10 x weight (kg) + 6.25 x height (cm) - 5 x age (years), plus a fixed offset of +5 for men or -161 for women -- the offset is where the equation encodes the average difference in body composition (more lean mass relative to total weight) between the sexes at a given weight, height, and age. Total daily energy expenditure (TDEE) then multiplies RMR by an activity factor ranging from 1.2 (sedentary) to 1.9 (very active), reflecting how many calories the body burns beyond resting metabolism through daily movement and exercise -- rather than force a single guess at activity level, the calculator shows all five common tiers at once so you can read off whichever matches your week.

The calculator also suggests a maintenance macronutrient split, anchored to the Moderate-activity TDEE tier: protein at 1.2 g per kg body weight (a moderate, not high-protein, target), fat at 25% of total calories, and the remaining calories allocated to carbohydrates. Mifflin-St Jeor was derived from indirect calorimetry measurements in a generally healthy adult population and, like all predictive RMR equations, carries a real margin of error for any individual -- it tends to be more accurate at typical body weights and less reliable at the extremes of very low or very high body fat, and it does not account for thyroid disease, certain medications, or recent significant weight change, all of which can shift true metabolic rate away from the formula's prediction.