What is the Janmahasatian formula?

It is a 2005 equation that estimates lean body weight from total weight, height and sex by first calculating BMI. For men LBW equals 9270 times weight divided by 6680 plus 216 times BMI; for women the constants are 8780 and 244. The result is in kilograms.

Why prefer it over the James equation?

The older James equation can paradoxically decrease as body weight rises into the severely obese range, which risks underdosing. The Janmahasatian equation increases smoothly with weight, so it gives safer, more sensible lean body weight estimates in obese patients.

Which drugs are dosed on lean body weight?

Lean body weight is used for several anaesthetic and other agents, including remifentanil and the maintenance phase of propofol, where total body weight would overestimate the dose in an obese patient. Always check the specific drug's recommended dosing weight.

Is lean body weight the same as ideal body weight?

No. Ideal body weight is a height-based target derived from population norms, while lean body weight is an estimate of the patient's actual fat-free mass given their current weight. The two often differ, especially in muscular or obese individuals.

Does this replace clinical judgement or a pharmacist check?

No. It reproduces a published equation but does not know the drug, the indication, or the patient's organ function. Use it as an input and confirm every dose against current prescribing references and, where needed, a pharmacist.

Lean Body Weight Calculator (Janmahasatian)

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, with an example

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. As an example, a 178 cm man weighing 110 kg has a BMI near 34.7 and a Janmahasatian lean body weight of roughly 74 kg, well below his total weight, which is exactly why scaling certain drugs to total weight would overdose him.