Skip to content
WKG
v2.90.2

WorkoutGen method BMI

Understand what makes up your weight beyond BMI.

Units

Body composition calculation

Formula sex

Free, no account: nothing is saved or sent.

Your body composition

Complete to see your result

  • Weight
  • Height
  • Sex
  • Waist circumference
  • Neck circumference
  • Confirmation

Enter your measurements and confirm the conditions to see your result.

Understand the calculation

The WorkoutGen method is a cross-analysis, not a new medical formula. It keeps classic BMI and breaks it down into fat mass index (FMI) and fat-free mass index (FFMI). You can enter an already measured body fat percentage or estimate it with the Hodgdon and Beckett equations, known as the US Navy method, using height and waist and neck circumferences, plus hips for the female coefficient. Lean mass also includes water, bones, and organs, not just muscle. No health category is inferred from FFMI.

BMI = FMI + FFMI. Fat mass = weight × body fat percentage / 100. FMI = fat mass / height². FFMI = (weight − fat mass) / height². Weight in kg, height in m. US Navy estimate: fat (%) = 495 / density − 450. Male density = 1.0324 − 0.19077 × log10(abdominal circumference − neck) + 0.15456 × log10(height). Female density = 1.29579 − 0.35004 × log10(abdominal circumference + hips − neck) + 0.221 × log10(height). Measurements in cm.

Fictional calculation example

At 100 kg and 1.80 m, with a measured body fat percentage of 15%, classic BMI is 30.86. It breaks down into an FMI of 4.63 and an FFMI of 26.23, or 15 kg of fat mass and 85 kg of lean mass. This explains body weight composition without concluding anything about health status.

Scientific sources

  1. Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center reports 84-11 and 84-29; 1984. Equations reproduced in: Body fat percentage and infarct size in patients with non-ST segment elevation myocardial infarction. 2023.
  2. Merrill Z, Chambers A, Cham R. Development and validation of body fat prediction models in American adults. Obes Sci Pract. 2020;6:189-195. doi:10.1002/osp4.392. Independent comparison and limitations of circumference estimates.

Your questions

Is the WorkoutGen method more accurate than BMI?
It provides information that BMI does not: the estimated distribution between fat mass and lean mass. Its accuracy depends on the measurements and method used. It is not a clinically validated formula and does not replace a professional body composition assessment.

How do I measure my waist, neck, and hip circumferences?
Use a flexible tape without pulling it tight and breathe normally. For the male formula, measure the abdomen at the navel. For the female formula, measure the waist at its narrowest point and the hips at their widest point. Measure the neck just below the larynx. Repeat each measurement to reduce errors.

Is this suitable for very muscular people?
The breakdown helps explain why a high BMI alone does not prove excess fat. Measurements remain imperfect, especially with a very muscular neck. If you have a percentage obtained from a body composition measurement, choose “Already measured percentage.” Neither measurement nor estimation alone provides a diagnosis.
All free tools