For the same adult, the three main lean body mass formulas can land about 5 kg apart. For an 80 kg, 180 cm man, Boer gives 61.42 kg, James 62.72 kg, and Hume 57.79 kg. Boer (1984) is the common default in most tools and clinics.
- Three published formulas estimate lean body mass from sex, weight, and height: Boer (1984), James (1976), and Hume (1966).
- They use different coefficients, so they disagree. For one 80 kg, 180 cm man the spread is 4.93 kg (James high, Hume low).
- Boer is the usual default because it is the most widely used clinically and sits in the middle for many adults.
- All three are estimates, not body-composition scans, and they lose accuracy at very high or very low body fat.
- This is an educational estimate, not a diagnosis. Discuss your own results with a qualified health professional.
Which Lean Body Mass Formula Should You Use?
The short answer is Boer (1984), unless you have a reason to match a specific clinical protocol. Boer is the most widely used formula and is the default in most calculators, including our lean body mass calculator. James (1976) tends to read higher, and Hume (1966) tends to read lower, for the same adult.
Each formula was built from a different study group decades ago, so each fits “the average adult” a little differently. None is the single true answer. They are three reasonable estimates of the same thing, and the gap between them tells you how much uncertainty sits in any single number.
This article compares the three. For the plain step-by-step method of working one out, see our guide on body composition basics and the tool itself, rather than re-reading the math here.
How the Three Formulas Differ (Side by Side)
They differ in their coefficients, not their inputs. All three take sex, weight in kilograms, and height in centimeters, then apply a different weighting. Here are the adult equations, with W in kg and H in cm.
| Formula (year) | Men’s equation | Women’s equation | Notes |
|---|---|---|---|
| Boer (1984) | 0.407 W + 0.267 H – 19.2 | 0.252 W + 0.473 H – 48.3 | Common default; most widely used clinically. |
| James (1976) | 1.1 W – 128 (W/H)^2 | 1.07 W – 148 (W/H)^2 | Uses a weight-to-height-squared term; often reads highest. |
| Hume (1966) | 0.32810 W + 0.33929 H – 29.5336 | 0.29569 W + 0.41813 H – 43.2933 | Oldest of the three; often reads lowest. |
Put a real person through all three and the pattern shows up fast. The table below uses one sample man and one sample woman.
| Formula | Man, 80 kg / 180 cm | Woman, 65 kg / 165 cm |
|---|---|---|
| Boer (1984) | 61.42 kg | 46.13 kg |
| James (1976) | 62.72 kg | 46.58 kg |
| Hume (1966) | 57.79 kg | 44.92 kg |
| Spread (high minus low) | 4.93 kg | 1.66 kg |
For this man, James sits about 4.93 kg above Hume, with Boer in between. For this woman the spread is smaller, about 1.66 kg. The gap is not fixed. It changes with your weight, height, and sex, which is exactly why the choice of formula matters.
Why Do the Estimates Disagree?
They disagree because each was fitted to a different group of people with different tools. Boer (1984) came from research on body fluid volumes. James (1976) came from UK obesity research and uses a squared weight-to-height term instead of a flat height term. Hume (1966) is the oldest, built from a small mid-century sample.
Because the coefficients differ, each formula responds differently as weight and height change. James leans harder on raw weight, so for a heavier, average-height adult it often climbs above the others. Hume weights height and weight more modestly, so it often lands lowest. Boer usually falls in the middle, which is one reason it became the default.
What Lean Body Mass and Fat Mass Actually Mean
Lean body mass is everything in your body that is not stored fat: muscle, bone, organs, and water. Fat mass is the rest. Add them together and you get your total body weight. A formula estimates the lean part, then the fat part is simply what is left over.
Using Boer for the sample man, lean body mass is 61.42 kg out of 80 kg, so estimated fat mass is 18.58 kg, or about 23% of body weight. Switch to Hume and the lean estimate drops to 57.79 kg, which pushes the estimated fat mass up to 22.21 kg. Same man, same scale weight, different split, just from changing the formula.
When the Formula Choice Matters Most
The choice matters most at the edges of body composition and when a clinical rule names a specific formula. For an average-sized adult near a healthy weight, all three land close enough that the default is fine for general tracking.
Two cases call for more care. First, at very high or very low body fat, all three formulas lose accuracy, because they were built around typical adults. Second, some medical settings, such as drug or contrast dosing, specify one formula by name, so matching it matters more than picking a “better” one. These are decisions for a clinician, not a web tool.
For everyday context, lean body mass pairs well with other simple measures. It helps explain your basal metabolic rate, since lean tissue burns more at rest, and it adds nuance that a single BMI figure cannot show on its own.
Enter your height, weight, and sex in the Lean Body Mass Calculator to get the Boer estimate instantly, then compare it with the James and Hume results for the same inputs.
Lean Body Mass Formulas: Frequently Asked Questions
Which lean body mass formula is most accurate?
No single formula is proven most accurate for everyone. Boer (1984) is the most widely used default. For a true measurement, a body-composition scan such as DEXA is far more precise than any formula.
Why do Boer and James give different numbers?
They use different coefficients from different studies. James leans more on raw weight and a weight-to-height-squared term, so it often reads higher. For one 80 kg, 180 cm man, James was 1.3 kg above Boer.
How big is the gap between the formulas?
It varies by person. For a sample 80 kg, 180 cm man the high-to-low spread was 4.93 kg. For a 65 kg, 165 cm woman it was smaller, about 1.66 kg.
Do these formulas work for children?
No. Boer, James, and Hume were all derived and validated in adults. For anyone under 18, these estimates do not apply, and you should ask a pediatric health professional instead.
Is lean body mass the same as muscle mass?
No. Lean body mass includes muscle plus bone, organs, and body water. Muscle is only part of it. That is why your lean mass is much larger than your muscle mass alone.
Should I worry if two calculators disagree?
Usually not. Different tools may use different formulas, so a few kilograms of difference is expected. Pick one formula and track the trend over time rather than comparing single numbers.
Can I use these formulas to diagnose a health problem?
No. These are educational estimates, not diagnostic tools. They cannot confirm a condition. Share any concerns and your actual numbers with a qualified health professional.
Sources and Further Reading
References Used in This Article
- Boer P. (1984), Estimated lean body mass as an index for normalization of body fluid volumes, Am J Physiol 247(4):F632-636
- Hume R. (1966), Prediction of lean body mass from height and weight, J Clin Pathol 19(4):389-391
- Caruso et al. (2018), Boer versus James Formula, BioMed Research International (restates James 1976)
Explore related tools in our health calculators hub, including body fat and BMI estimators that use the same measurements.
This article compares published lean body mass formulas for general education. It is an educational estimate, not medical advice or a diagnosis. These are screening estimates, not a body-composition scan; speak with a qualified health professional about your own results. Reviewed for accuracy by Dr. Abdullah Khalil, MBBS. Last updated October 4, 2026.
Author
Shakeel Muzaffar is the Founder and Editor-in-Chief of MultiCalculators.com, bringing over 15 years of experience in digital publishing, product strategy, and online tool development. He leads the platform's editorial vision, ensuring every calculator meets strict standards for accuracy, usability, and real-world value. Shakeel personally oversees content quality, formula verification workflows, and the platform's commitment to publishing tools that are genuinely useful for students, professionals, and everyday users worldwide.
