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Lean body weight (Janmahasatian)

Fat-free mass from sex, weight and height (Janmahasatian 2005)

Sources last checked: 2026-11-03

Lean body weight (Janmahasatian)

Fat-free mass from sex, weight and height (Janmahasatian 2005)

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Derived against DXA fat-free mass in 373 adults aged 18–82 with BMI up to about 70 kg/m²; not validated in children, amputees, marked oedema or ascites. A body-composition estimate, not a dosing recommendation.

Source: Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. Quantification of lean bodyweight. Clin Pharmacokinet 2005;44(10):1051-1065.Open

What is the Lean body weight (Janmahasatian)?

Lean body weight approximates fat-free mass. Janmahasatian and colleagues (2005) derived a semi-mechanistic equation from DXA measurements in 373 adults across a wide size range (weight 41–217 kg, BMI 17–70 kg/m²), so that it does not fall paradoxically at high BMI as the older James equation does.

When to use it

  • When a pharmacokinetic protocol or study uses lean body weight (e.g. some anaesthetic agents in obesity)
  • Body composition estimates in research or nuclear medicine (SUV normalised to lean body mass)
  • Comparing weight descriptors in obese patients

When not to use it

  • Children and adolescents
  • Marked fluid overload, ascites or amputation, which distort the weight input
  • As a dose recommendation on its own

How it is calculated

Men: LBW = 9270 × weight / (6680 + 216 × BMI) Women: LBW = 9270 × weight / (8780 + 244 × BMI)

Clinical pearls and limitations

  • Derived against DXA fat-free mass in 373 adults aged 18–82 with BMI up to about 70 kg/m²; not validated in children, amputees, marked oedema or ascites. A body-composition estimate, not a dosing recommendation.
  • Unlike the James formula, the Janmahasatian equation keeps rising with weight and does not collapse at very high BMI.
  • In older adults (60–65 years) it performed about as well as the Gallagher equation against DXA.
  • Lean body weight rises only slowly with increasing fat mass, so it can be far below actual weight in severe obesity.

Frequently asked questions

What is the Janmahasatian formula?

Men: 9270 × weight / (6680 + 216 × BMI); women: 9270 × weight / (8780 + 244 × BMI), weight in kg and BMI in kg/m².

Lean body weight vs ideal body weight — what is the difference?

Ideal body weight depends only on height and sex; lean body weight also rises with actual weight, because heavier people carry more fat-free mass too.

Why not use the James formula?

James' equation gives falling, even negative, values at very high BMI; Janmahasatian was developed to avoid that.

Evidence and guidelines

  1. Primary source: Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. Quantification of lean bodyweight. Clin Pharmacokinet 2005;44(10):1051-1065.
  2. Puri T, Blake GM. Comparison of ten predictive equations for estimating lean body mass with dual-energy X-ray absorptiometry in older patients. Br J Radiol 2022;95(1133):20210378 (equation constants restated; validation in 60–65-year-olds).

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