BMR Estimator with Body Fat Option

It calculates a standard BMR from your sex, age, height, and weight using Mifflin-St Jeor. When body fat % is provided, it estimates lean body mass as Lean mass = weight × (1 − body fat%/100), then computes BMR using Katch-McArdle: BMR = 370 + 21.6 × lean mass. If both are available, it can show the difference between the two estimates (kcal/day and percent).

Body fat % is often estimated (not measured), so errors in body fat can carry over into lean mass and the Katch-McArdle result. This body-fat-aware approach tends to be more meaningful when your body composition is far from average—such as very lean or very higher body-fat ranges—because weight-only formulas can be less representative. For most people, the standard Mifflin-St Jeor estimate is still a solid starting point for calories.

If you enter body fat %, the calculator assumes lean mass must stay positive; extremely low weight with high body fat % may be flagged as invalid. Body fat % is constrained between 3% and 60%—outside that range, results aren’t calculated. If body fat % is left blank, the tool uses only Mifflin-St Jeor and explains that adding body composition may improve accuracy if your build is atypical.

BMR is estimated for complete rest and does not directly equal your daily calorie needs. Genetics, thyroid function, recent dieting, illness, and training history can shift real metabolism beyond these equations. Also, you shouldn’t use this tool as medical advice—if you have metabolic/thyroid concerns, consult a professional.