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Lean Body Weight Calculator

Enter weight, height and sex to estimate lean body weight.

How it is calculated

The engine first computes body mass index, BMI = W / (H/100)², with weight W in kilograms and height H in centimetres. It then applies the Janmahasatian equation, which differs by sex. For men: LBW = 9270 × W / (6680 + 216 × BMI). For women: LBW = 9270 × W / (8780 + 244 × BMI). Any sex value other than female uses the male constants. Fat mass is simply W − LBW. Weight and height must be above zero, and results are given to two decimals in kilograms. The formula is a population estimate, so a measured body composition always takes precedence, and the output is not a dose.

Worked example

A man weighs 70 kg and is 175 cm tall. BMI = 70 / 1.75² = 70 / 3.0625 = 22.86. The denominator is 6680 + 216 × 22.86 = 6680 + 4937 = 11 617. The numerator is 9270 × 70 = 648 900. Lean body weight = 648 900 / 11 617 = 55.86 kg. Fat mass is 70 − 55.86 = 14.14 kg. For a woman with the same weight and height the female constants give 45.20 kg lean mass and 24.80 kg fat mass.

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Common mistakes

  • Entering height in metres. The field takes centimetres: 1.75 is read as 1.75 cm, giving a BMI of 228 571 and a lean weight of 0.01 kg.
  • Entering weight in pounds. The engine expects kilograms, so 154 lb must be entered as 70.
  • Leaving the sex on male for a woman. The constants differ a lot: the same 70 kg at 175 cm gives 55.86 kg for a man and 45.20 kg for a woman.
  • Confusing lean body weight with ideal body weight. Lean weight is weight minus estimated fat, so it rises and falls with total weight, while ideal weight depends only on height.
  • Using the result as a drug dose. It is only one input to a dosing method, and the product label or a clinician decides whether lean weight is the right basis.

Sources

About this calculator

The Janmahasatian formula (2005) estimates lean body weight — total body mass minus fat mass — from total body weight, height and sex, derived by correlating dual-energy X-ray absorptiometry (DEXA) measurements against body mass index across a large population, which makes it more accurate across a wider BMI range, including obese patients, than older formulas like Hume's or Boer's. Lean body weight matters clinically because many drugs distribute mainly into lean tissue rather than fat, so dosing by total body weight in an obese patient can significantly overshoot the effective dose relative to how the drug actually distributes.

This calculator takes weight, height and sex and returns the estimated lean body weight using the Janmahasatian equation. Anesthesiologists and pharmacists calculating weight-based drug dosing, clinical researchers standardizing dosing across a study population, and dietitians estimating fat-free mass for metabolic calculations use it in place of total body weight when a drug's pharmacokinetics are better predicted by lean mass — this tool provides the estimate only and is not dosing advice.

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Comments (1)

  • Editorial team

    Lean body weight here comes from a population equation based on weight, height and sex, so it moves up and down with body weight and is not a body-composition measurement. If you have a DEXA or similar scan, trust that over this number. The result is useful as a rough reference but is not a dose, and any dosing question belongs with a clinician or pharmacist. Which values do you compare it against, and what do they show?

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