This calculator computes lean body weight (LBW) with the Boer formula from weight, height, and sex. LBW estimates body mass excluding fat and is used in some dosing and physiology calculations.
What Is Lean Body Weight?
Lean body weight (LBW) is an estimate of body mass excluding fat: the muscle, bone, organ, and water compartment. Unlike ideal body weight, which is a fixed reference from height and sex, LBW is calculated from a person's measured weight and height, so it changes with body composition. It matters for dosing because some drugs distribute mainly into lean tissue rather than fat. For those agents, LBW is a better size measure than total body weight.
The Boer Formula
This calculator uses the Boer formula, which takes weight in kilograms and height in centimetres:
- Men: 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
- Women: 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
In the formula, weight is the measured (total) body weight and height is standing height in centimetres. The sex-specific coefficients reflect the different average proportions of lean to fat mass in men and women. If your figures are in pounds and inches, convert first: multiply pounds by 0.4536 for kilograms and inches by 2.54 for centimetres.
Worked Example
For a man weighing 90 kg at 180 cm: LBW = 0.407 × 90 + 0.267 × 180 − 19.2 = 36.6 + 48.1 − 19.2 ≈ 65.5 kg. The remaining ~24.5 kg is the estimated fat mass. For a woman of the same weight and height, LBW = 0.252 × 90 + 0.473 × 180 − 48.3 ≈ 59.5 kg, which reflects the higher average fat proportion.
Lean vs Ideal vs Adjusted Weight
Lean, ideal, and adjusted body weight are easy to confuse but have different roles. The table shows how each is derived and where it is used:
| Weight | Derived from | Typical use |
|---|---|---|
| Lean body weight (Boer) | measured weight + height + sex | drugs distributed into lean tissue |
| Ideal body weight (Devine) | height + sex | Cockcroft-Gault when actual weight is from IBW up to 130% of IBW |
| Adjusted body weight | IBW + 0.4 × (actual − IBW) | Cockcroft-Gault when actual weight is more than 130% of IBW |
For Cockcroft-Gault creatinine clearance, the dosing weight is actual weight when it is below ideal, ideal body weight from ideal up to 130% of ideal, and adjusted body weight above 130% of ideal. Lean body weight is a separate concept used for certain agents, not for the creatinine clearance estimate itself unless a drug label or protocol names it.
Limitations and Edge Cases
- Not for creatinine clearance: Cockcroft-Gault uses actual, ideal, or adjusted body weight depending on actual weight, not LBW.
- Estimate, not measurement: the Boer formula approximates fat-free mass; it does not replace body-composition testing such as DEXA.
- Extreme obesity: formula-based LBW can become unreliable at very high body weights, where alternative equations may fit better.
- Marked fluid shifts: oedema or dehydration changes measured weight and therefore the estimate.
- Drug-specific guidance: always follow the individual drug label, which specifies which weight to use.
What Lean Body Weight Is Used For
Lean body weight matters because not every drug distributes evenly through the whole body. Some agents stay largely within lean tissue and water rather than dissolving into fat, so their dose tracks lean mass more closely than total body weight. For those drugs, dosing on total weight in a person carrying a lot of fat would overshoot, while dosing on a fixed height-based reference would ignore real differences in build. Lean body weight, calculated from measured weight and height, captures the size of the active compartment. It is also used in physiology and body-composition work as an estimate of fat-free mass.
Why It Is Not Used for Creatinine Clearance
The weight rule for the Cockcroft-Gault creatinine clearance equation rests on studies that compared actual, ideal, and adjusted body weight against measured creatinine clearance. In the 2012 evaluation of 3,678 patients by Winter and colleagues, actual weight was unbiased in underweight patients and ideal body weight was unbiased in patients of normal weight. Adjusted body weight was the least biased option in overweight and obese patients. The FDA 2024 renal impairment guidance likewise names ideal or adjusted body weight, not lean body weight, as the alternatives to actual weight in overweight or obese people. The standard practice is therefore to keep actual weight, IBW, or AjBW as the creatinine-clearance weight term. Lean body weight answers a different question (how much fat-free mass a person has) and is used for the drugs and calculations that specify it.
Units and Inputs
The Boer formula expects weight in kilograms and height in centimetres. If your measurements are in pounds and inches, convert before entering: multiply pounds by 0.4536 to get kilograms and inches by 2.54 to get centimetres. The output is an estimate of fat-free mass in kilograms; the difference between total weight and lean body weight approximates fat mass. As with any weight-based formula, accurate, recently measured height and weight give the most reliable result.
Other Lean Body Weight Formulas
The Boer formula used here is one of several published estimates of fat-free mass. The Hume and James formulas are also widely cited and use the same inputs (measured weight, height, and sex). They assign slightly different coefficients, so they can return values a kilogram or two apart for the same person. None is a direct measurement; each is a regression fitted to a study population, and all become less reliable at the extremes of body size. For most clinical purposes the differences between them are small, and the choice of formula matters less than using consistent, recently measured height and weight. When a drug label or protocol names a specific lean-body-weight method, follow that one. For creatinine clearance, remember the standard rule uses actual, ideal, or adjusted body weight instead.
Why Sex Changes the Result
The Boer formula uses different coefficients for men and women because, at the same height and weight, the two sexes carry different average proportions of lean and fat mass. Women tend to have a higher fraction of body fat and a lower fraction of muscle than men of identical build, so applying the male coefficients to a woman would overestimate her fat-free mass. The sex-specific constants correct for this, which is why the worked examples above return a lower lean body weight for the woman than the man at the same 90 kg and 180 cm. The same logic underlies the sex term in ideal body weight and in Cockcroft-Gault itself.