How the common formulas differ
Basal metabolic rate estimates the energy the body uses at rest. The Harris-Benedict equation was an early standard but tends to run high by modern standards. The Mifflin-St Jeor equation, which uses weight, height, age, and sex, is widely regarded as more accurate for the general population and is a common default. The Katch-McArdle equation takes a different approach, basing the estimate on lean body mass rather than total weight.
Each formula encodes assumptions, so the same person can get noticeably different numbers depending on which is used.
Why body-fat-aware methods can edge ahead
Because metabolically active lean tissue drives most resting energy use, an equation built on lean mass (Katch-McArdle) can be more accurate for people far from average body composition, such as very muscular or very lean individuals, provided their body fat is measured reasonably well. For typical bodies, Mifflin-St Jeor performs well without needing a body fat figure.
All of these are estimates with meaningful error, best treated as a starting point and adjusted from real-world results. This is general educational information, not medical advice.