Lean body mass calculator

This tool computes LBM three ways:

• Boer (1984) — modern default, most commonly cited in fitness contexts. Derived from a healthy adult population.
• James (1976) — referenced in the Garrow nutrition handbook. Uses height in metres explicitly to adjust the weight contribution.
• Hume (1966) — older formula still used in clinical pharmacology to scale drug doses (avoiding over-dosing in obese patients where total body weight overstates active drug volume).

For healthy adults the three formulas typically agree within ±2 kg. Wider disagreements suggest you're at the edge of the populations they were derived from (very obese, very lean, or very tall/short).

Body weight is a noisy metric — it bundles muscle, fat, water, glycogen and gut contents into one number. Two adults at 80 kg can have wildly different metabolic profiles depending on how that 80 kg splits.

LBM gives you:

• A better BMR estimate — Katch-McArdle uses LBM directly and beats Mifflin-St Jeor for very lean or very muscular individuals.
• A protein target — research recommends 1.6–2.2 g protein per kg lean body weight for active adults; using total weight overstates the target for higher-fat individuals.
• A drug-dosing reference — many clinical drugs (anaesthetics, antibiotics, oncology agents) scale their dose by LBM rather than total body weight.
• A sarcopenia signal in older adults — LBM declines ~1% per year after 50; tracking it flags functional decline earlier than weight does.

Predictive LBM formulas carry roughly ±5–10% error vs DEXA scan, the research reference for body composition. They're best for:

• Trend tracking — measurement bias is consistent for a given person, so changes in LBM over weeks and months are reliable even when the absolute number isn't.
• Population-level planning — calorie or protein recommendations for groups.

For individual precision, get a DEXA scan (most accurate, research standard), BodPod (air-displacement plethysmography) or a calibrated multi-frequency BIA scale. Skinfold calipers in trained hands are also accurate.