Lean body weight is an estimate of a person’s fat-free mass, the part of body weight made up of muscle, bone, organs and water rather than fat. Because many drugs distribute into lean tissue rather than fat, dosing on total body weight can dangerously overshoot in obese patients, which is where a lean body weight estimate earns its place.
How it works
The Janmahasatian equation first computes BMI and then scales total weight by a sex-specific function of that BMI:
BMI = weight(kg) / height(m)^2
male LBW = (9270 x weight) / (6680 + 216 x BMI)
female LBW = (9270 x weight) / (8780 + 244 x BMI)Subtracting LBW from total weight gives an estimated fat mass, and dividing LBW by total weight gives the fat-free fraction, both of which this tool reports.
Why it beats the James equation
The historical James equation has a well-known flaw: as weight climbs into the severely obese range its predicted lean weight starts to fall, so a heavier patient can be assigned a smaller lean weight and therefore too little drug. The Janmahasatian model rises monotonically with weight and was validated against measured fat-free mass, so it is the current default for lean-weight dosing.
Worked examples
Example 1 — obese male patient. A 178 cm man weighing 110 kg has a BMI of approximately 34.7. The Janmahasatian formula gives: (9270 × 110) / (6680 + 216 × 34.7) ≈ 74 kg lean body weight. Fat mass is estimated at 110 − 74 = 36 kg, and the fat-free fraction is about 67%. Dosing a drug like remifentanil on total body weight (110 kg) rather than lean body weight (74 kg) would represent an overdose of approximately 49%.
Example 2 — normal-weight female patient. A 165 cm woman weighing 62 kg has a BMI of approximately 22.8. Her Janmahasatian LBW is roughly (9270 × 62) / (8780 + 244 × 22.8) ≈ 43 kg. In a patient near ideal weight, lean body weight and total body weight are closer together, so the choice of dosing weight matters less — but for agents where every kilogram counts, using LBW is still the safer and more accurate approach.
When to use lean body weight vs other dosing weights
| Weight used | Typical context |
|---|---|
| Lean body weight | Remifentanil, propofol maintenance, some chemotherapy agents |
| Ideal body weight | Aminoglycosides, many antibiotics |
| Adjusted body weight | Obese patients where partially distribute into fat (e.g. some opioids) |
| Total body weight | Drugs that freely distribute into adipose tissue |
Always confirm the appropriate dosing weight against the current prescribing information for the specific drug. This calculator reproduces a published formula only — it does not know the drug, the indication, or the patient’s organ function. For clinical decisions, verify against the BNF, Summary of Product Characteristics, or local pharmacy guidance.