Basal metabolic rate calculator. The most accurate formula: Harris-Benedict 1919, Mifflin-St Jeor 1990, or Lean with bodyfat?

The most accurate formula is the one that relies on real lean mass, not on estimates from weight, height, age and sex. Without bodyfat, Mifflin-St Jeor 1990 remains the best approximation (±10 % on 70 % of profiles). With bodyfat, an equation on lean mass of the form 370 + 21.6 × FFM is more accurate. The proprietary patented Lean model refines further by integrating metabolic adaptation and continuous recalibration.

The original formula dates from 1919, revised in 1984. It was calibrated on 136 subjects, in a nutritional environment and lifestyle very different from ours. On modern populations, it overestimates BMR by 5 to 10 %. Clinical dietitians have considered it obsolete since Mifflin-St Jeor arrived in 1990.

On a man of 80 kg, 180 cm, 32 years, Harris-Benedict gives about 1 855 kcal, Mifflin-St Jeor about 1 780 kcal. Mifflin slightly underestimates Harris-Benedict by 75 kcal. Mifflin is generally more accurate, but neither distinguishes a muscular profile from a sedentary one.

Because lean mass, not total mass, drives expenditure at rest. 1 kg of muscle burns about 13 kcal/day, 1 kg of fat about 4 kcal/day. Two people at the same weight but with 10 bodyfat points apart can have BMRs that diverge by 200 to 400 kcal/day. No formula without bodyfat can capture that.

Lean uses a proprietary patented model. The main input is your bodyfat measured by AI BodyScan (photo, 5 seconds, DEXA-grade precision). Lean derives your real lean mass, computes a BMR on that lean mass, then integrates a metabolic-adaptation function that recalibrates every week based on your weight and composition changes. Lean mass is the dominant factor; age fine-tunes the result (expenditure at rest decreases slightly at constant lean mass over the years).

As long as your body composition doesn’t move (stable weight, stable bodyfat), your BMR doesn’t move either. As soon as you lose or gain 2 kg, or your bodyfat varies by 2 points, you need to recalculate. Lean does it automatically at every BodyScan and every weigh-in.

Yes, for two reasons. First, physical: you weigh less, so you have less lean mass to maintain, so the mechanical BMR drops. Second, physiological: your body activates a metabolic adaptation under prolonged calorie deficit, which slows your BMR by 5 to 20 % below its expected value. Lean models both phenomena.

At equivalent weight, height and age, women have on average 8 to 12 % less lean mass than men (hormonal and morphological difference). Their BMR is therefore structurally lower, on the order of 150 to 250 kcal/day. The two classic formulas integrate a different sex constant. On Lean, it is the measured lean mass that speaks, independently of sex.

BMR is strictly expenditure at rest (lying down, fasted, thermal neutrality). When you walk, you add NEAT (Non-Exercise Activity Thermogenesis). When you work out, you add EAT (Exercise Activity Thermogenesis). When you eat, you add TEF (Thermic Effect of Food). The sum is your TDEE.

A public lean-mass equation, cited in the physiology literature: BMR ≈ 370 + 21.6 × lean mass (kg). Lean mass is derived from your weight and bodyfat. It is a useful approximation to understand the bodyfat effect. The proprietary patented Lean model in the app goes further (metabolic adaptation, water, glycogen, continuous recalibration).