Lean Body Weight Calculator (Janmahasatian)
Lean Body Weight Calculator (Janmahasatian)
Lean body weight by the Janmahasatian 2005 equation, from sex, weight and height. Unlike ideal body weight it responds to actual weight — but it climbs towards a ceiling rather than rising without limit, which is what makes it a better size descriptor than total weight for some drugs in obesity.
Lean body weight (Janmahasatian)
Sex, weight, height → LBWMan, 110 kg, 178 cm
Janmahasatian 2005
Women: LBW = 9270 × weight ÷ (8780 + 244 × BMI)
- weight
- actual body weight, kg
- BMI
- weight ÷ height² in kg/m²
- ceiling
- substituting BMI = weight ÷ height² shows LBW approaches 9270 × height² ÷ 216 in men (÷ 244 in women) and never passes it
Worked example
Man, 110 kg, 178 cm
BMI = 110 ÷ 1.78² = 110 ÷ 3.1684 = 34.72
216 × 34.72 = 7,499.1; 6,680 + 7,499.1 = 14,179.1
9,270 × 110 = 1,019,700
1,019,700 ÷ 14,179.1 = 71.9 kg
Three weights for the same man (178 cm)
| Actual weight | Ideal (Devine) | Adjusted (0.4) | Lean (Janmahasatian) |
|---|---|---|---|
| 70 kg | 73.2 | 70.0 | 56.7 |
| 90 kg | 73.2 | 79.9 | 65.1 |
| 110 kg | 73.2 | 87.9 | 71.9 |
| 150 kg | 73.2 | 103.9 | 82.2 |
| 200 kg | 73.2 | 123.9 | 91.3 |
Why a weight that ignores the patient’s weight is not enough
Ideal body weight has one striking property: it does not use the patient’s weight. Two people of the same height and sex get the same number whether they weigh 55 kg or 155 kg. That was deliberate — Devine wanted a weight for gentamicin that ignored fat — but it throws away real information, because heavier people of the same height also carry more lean tissue, more blood volume and larger organs. Total body weight makes the opposite mistake and treats every extra kilogram of fat as if it cleared drug like muscle.
Janmahasatian and colleagues built a lean body weight equation in 2005 from body composition measured in adults across a wide range of size. It uses sex, weight and BMI, and its shape is the useful part. At normal weight it rises almost in step with actual weight. As weight climbs it bends over, approaching a ceiling set by height — about 136 kg for a man of 178 cm — which it never passes. That is the behaviour a size descriptor for obesity needs: it keeps responding to the patient, but stops crediting fat as if it were muscle.
It is used as a dosing scalar for some drugs in obesity, and in one study of morbidly obese patients (Demirovic 2009) lean weight in Cockcroft-Gault was the most accurate estimate of measured creatinine clearance, where total weight overestimated by about two-fold and ideal weight underestimated by about 22%. Other work favours the adjusted weight, and protocols differ, so lean weight is one of several defensible conventions rather than the answer; the dosing weight selection page lays them out. The ideal body weight calculator and adjusted body weight calculator give the other two weights for the same patient, and the Cockcroft-Gault creatinine clearance calculator takes whichever one your protocol names.
Frequently asked questions
What is the Janmahasatian lean body weight formula?
For men, LBW = 9270 × weight ÷ (6680 + 216 × BMI); for women, LBW = 9270 × weight ÷ (8780 + 244 × BMI), with weight in kg and BMI in kg/m². A man of 110 kg and 178 cm has a BMI of 34.7 and a lean body weight of 71.9 kg.
Why use lean body weight instead of ideal body weight?
Ideal body weight depends only on height and sex, so it ignores the extra lean tissue heavier people carry. Lean body weight responds to actual weight but levels off towards a ceiling set by height, so it tracks the tissue that clears most drugs without counting fat as if it were muscle. For some drugs in obesity that makes it the better size descriptor.
Is lean body weight the same as fat-free mass?
They are closely related. The Janmahasatian equation was developed against measured body composition and estimates the fat-free component of weight. For dosing it is used as a size descriptor, not as a precise measurement of any one individual’s body composition.
Should I use lean body weight in Cockcroft-Gault?
It is one of several conventions. One study in morbidly obese patients found it gave the most accurate estimate of measured creatinine clearance, but other work supports the 0.4-adjusted weight and many protocols use ideal or adjusted weight. Use the weight your protocol names and record which one you used.
Why does lean body weight stop rising?
Because the equation contains BMI, which rises with weight, in the denominator. Substituting BMI = weight ÷ height² shows lean weight approaching 9270 × height² ÷ 216 in men, or ÷ 244 in women, as weight increases. For a man of 178 cm that ceiling is about 136 kg of lean mass, however heavy he becomes.
Related calculators
References
- Janmahasatian S, Duffull SB, Ash S, Ward LC, Byrne NM, Green B. Quantification of lean bodyweight. Clin Pharmacokinet. 2005;44(10):1051–1065. Men: LBW = 9270 × weight ÷ (6680 + 216 × BMI); women: LBW = 9270 × weight ÷ (8780 + 244 × BMI).
- Barletta JF. Drug dosing in special populations: obesity and geriatrics. In: CCSAP 2020 Book 2, Issues in Critical Care Practice. Lenexa, KS: American College of Clinical Pharmacy; 2020. Transcribes the Janmahasatian and Devine equations; names Bauer 1983 as the source of the 0.4 factor and reports published correction factors from 0.14 to 0.98; summarises Demirovic 2009 and the heparin literature.
- Demirovic JA, Pai AB, Pai MP. Estimation of creatinine clearance in morbidly obese patients. Am J Health Syst Pharm. 2009;66(7):642–648. Against measured clearance, total body weight overestimated by about two-fold, ideal body weight underestimated by about 22%, the 0.4-adjusted weight overestimated by about 30%, and lean body weight was the most accurate.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.