Guide · Health
Lean Body Mass Calculator: Boer, James, and Hume Explained
Updated 2026-09-03 · 6 min read
Lean body mass sounds like something a scan prints after you lie still on a table. On most websites, including DevOkk, it is a regression: height and weight (and sex) fed into equations published years apart by different authors. Boer, James, and Hume will not agree to the gram. That disagreement is the point.
DevOkk’s Lean Body Mass Calculator runs all three sex-specific forms and optionally computes LBM from a body-fat percentage you supply. Default male 175 cm, 80 kg yields roughly 63.5 kg (Boer), 64.0 kg (James), and 63.8 kg (Hume)-a narrow band here, wider on other heights and weights. None of those numbers is DEXA. None diagnoses sarcopenia. None replaces a clinician’s nutrition or dosing decision.
This guide explains what each equation assumes, when the optional fat-percent path helps, how LBM relates to Body Fat Calculator tape estimates and Protein Calculator g/kg math, and why BMR Calculator is yet another regression family that should not be chained blindly. Tape versus lab methods: Navy body fat vs DEXA. BMI screening: Why BMI is not a diagnosis.
What “lean body mass” means on a calculator page
In gym language, lean body mass (LBM) is everything that is not stored fat: muscle, bone, water, organs. In research, fat-free mass (FFM) is close but not always identical in strict four-compartment models. DevOkk uses the teaching label lean body mass for outputs.
A regression LBM never measured your muscle directly. It predicted from population averages. A short, heavy person and a tall, lean person can share a weight and get different implied composition from BMI alone-which is why LBM equations exist as a step beyond kg/m².
They are still not a scan.
Boer, James, and Hume on DevOkk
All three use weight W in kg and height H in cm with sex-specific constants.
Boer (1984) is common in clinical calculators:
- Male: LBM = 0.407 × W + 0.267 × H − 19.2
- Female: LBM = 0.252 × W + 0.473 × H − 48.3
James (1976) uses height-to-weight ratio squared:
- Male: LBM = 1.1 × W − 128 × (W/H)²
- Female: LBM = 1.07 × W − 148 × (W/H)²
Hume (1966) is older, still cited:
- Male: LBM = 0.32810 × W + 0.33929 × H − 29.5336
- Female: LBM = 0.29569 × W + 0.41813 × H − 43.2933
DevOkk prints all three so you see spread. If Boer and Hume differ by 4 kg at your inputs, that interval is more honest than treating any single equation as truth.
Worked default male: W = 80, H = 175, (W/H)² ≈ 0.002612. James: 1.1 × 80 − 128 × 0.002612 ≈ 88 − 0.33 ≈ 64.0 kg (page rounding may show 64.0). Boer: 0.407 × 80 + 0.267 × 175 − 19.2 ≈ 63.5 kg. Hume lands near the same band.
Optional path: LBM from body-fat percent
If you enter body-fat %, DevOkk computes:
LBM = weight × (1 − fat/100)
Example: 80 kg at 20% fat → LBM = 64 kg. That arithmetic is exact given the percent. The weak link is the percent source. Tape Hodgdon–Beckett on Body Fat Calculator is an estimate. A scale bioimpedance reading is another. A number you guess is fiction.
Chain rule honesty: LBM from tape fat compounds two estimates. Use it for direction (“roughly mid-60s kg lean”) not for medication math.
How LBM differs from tape and BMI
| Tool | Inputs | Output | Blind spots |
|---|---|---|---|
| Lean Body Mass Calculator | Height, weight, sex | Three LBM kg estimates | No direct measurement; equations disagree |
| Body Fat Calculator | Height, neck, abdomen/waist/hip | % fat | Tape placement; log sensitivity |
| BMI Calculator | Height, weight | kg/m² WHO band | Fat vs muscle invisible |
Athletes often care about LBM because protein and calorie planning sometimes reference fat-free mass. Even then, total-body g/kg is simpler and common in sports nutrition literature-see Protein grams per kg.
Protein, BMR, and why not to chain everything
Protein Calculator defaults to total body weight × g/kg by activity. Multiplying LBM × g/kg is defensible if you trust LBM, but DevOkk does not auto-switch-you reason explicitly.
BMR Calculator uses Mifflin–St Jeor (or similar) from weight and height, not LBM from Boer. Feeding Boer output into BMR without a formula designed for LBM is DIY physiology. Energy needs belong in Calorie Calculator and BMR vs TDEE vs calories burned-still estimates.
Clinical and sport use without overclaiming
Drug dosing historically used ideal body weight and adjusted body weight; some protocols reference LBM for specific agents. That is prescriber math, not a public regression page.
Sarcopenia screening in older adults uses grip strength, gait speed, and clinical criteria-not Boer alone.
Bodybuilding tracks LBM to separate muscle gain from fat gain. Regressions smooth away day-to-day glycogen and water noise poorly; repeat DEXA or consistent tape may track trends better than switching equations weekly.
In all cases: online LBM is educational, not diagnostic.
Health disclaimers that matter
- Kidney disease, liver failure, cancer cachexia, eating disorders: protein and weight targets need medical supervision. A lean mass number does not clear high protein or justify restriction.
- Pediatrics: these adult regressions are not pediatric body composition.
- Pregnancy: total mass and composition change for reasons regressions ignore.
- Amputation, edema, ascites: weight on the scale is not the weight the equations imagined.
If LBM feeds into self-judgment or compulsive tracking, step back and seek qualified help. Calculators do not know your history.
Tracking trends without pretending you have a scan
If you weigh weekly at the same time of day and height is stable, Boer LBM can move when scale weight moves even when the real muscle change is small. Water, glycogen, and sodium shift kg faster than lean tissue grows. Treat month-to-month direction as noisy unless a reference method agrees.
A practical pattern: pick one equation (Boer is common) and stick to it; note when James or Hume diverge by more than your usual spread-that is a signal to check inputs, not to celebrate the lowest line. Pairing monthly tape on Body Fat Calculator with regression LBM gives two estimates that can disagree; disagreement is expected.
Do not compute LBM daily for anxiety. Weekly or monthly is enough for educational tracking. Clinical sarcopenia work uses different instruments entirely.
Units, sex selection, and privacy
Enter height and weight in metric or imperial tabs. Sex selects which coefficient set runs-biological sex as used in the original papers, not identity erasure; clinical care may use different frameworks.
Computation is local JavaScript. localStorage may hold drafts 30 days. No account. Not medical advice.
Which tool to open next
Use Lean Body Mass Calculator when you want three regression LBM estimates and optional fat-percent LBM side by side.
Use Body Fat Calculator when you have tape measurements and want % fat first.
Use Protein Calculator when the job is daily protein grams from g/kg.
Use BMR Calculator when the job is resting energy estimate, knowing it is not LBM-based unless you deliberately choose a LBM-aware formula elsewhere.
Then stop. Scans, labs, and clinicians exist because regressions from height and weight cannot see inside you.
Frequently asked questions
What equations does DevOkk use for lean body mass?
Boer, James, and Hume sex-specific regressions from height (cm) and weight (kg). Default male 175 cm, 80 kg → Boer about 63.5 kg, James about 64.0 kg, Hume about 63.8 kg. Lean Body Mass Calculator shows all three plus optional LBM from body-fat percent.
Why do Boer, James, and Hume disagree?
They were fit on different samples and forms. Spread of a few kg is normal. None is ground truth without a reference method like DEXA. Use the range as humility, not as picking the flattering equation.
Can I enter body-fat percent for LBM?
Yes. Optional percent fat computes LBM = weight × (1 − fat/100). That chain is only as good as the fat estimate-tape, scale, or guess. Body Fat Calculator is Hodgdon–Beckett tape, not DEXA.
Is lean body mass the same as fat-free mass?
In conversation, often yes. Strict lab language sometimes separates bone mineral and essential fat. DevOkk’s page uses the teaching label lean body mass for the regression outputs and the optional fat-percent path.
Should protein grams use LBM or total weight?
Protein Calculator multiplies total kg by default. Research often uses total weight for simplicity. If you trust LBM, compare g/kg on lean mass mentally-but clinical renal and disease cases need a clinician, not a regression.
Does DevOkk upload height and weight for LBM?
No. Math runs in the browser. Drafts can sit in localStorage up to 30 days. No account. LBM outputs are educational estimates, not sarcopenia diagnosis or prescription dosing.
Related guides
More reading that links back to the same tools and workflows.
Navy Body Fat vs DEXA: What a Tape Measure Can and Cannot Claim
Hodgdon–Beckett / US Navy circumference estimates are not DEXA, not a Bod Pod, and not an eating-disorder diagnosis. How DevOkk’s body-fat and Army tape tools work, where they fail, and what to open instead of BMI.
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Army Body Fat Calculator vs Navy Tape: Same Math, Different Job
Hodgdon–Beckett circumference equations power both DevOkk tape pages. What Army AR 600-9 framing adds, what the Navy label means, and why neither is DEXA or a diagnosis.
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Protein Grams per kg: What the Calculator Can and Cannot Say
Sedentary 0.8 g/kg vs strength 1.6 g/kg, lean mass vs total weight, and kidney caveats. DevOkk's protein calculator is a range estimate - not a meal plan or medical nutrition prescription.
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Why BMI Is Not a Diagnosis (Athletes, Age, and WHO Bands)
Adult BMI is kg/m² and a WHO screening label. It does not diagnose disease, malnutrition, or an eating disorder. How the formula works, who it misses, and which DevOkk tool to open.
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