Lean Body Mass Calculator

Calculate your lean body mass (LBM) and fat-free mass percentage.

Lean Body Mass Calculator

A lean body mass calculator estimates the weight of everything in your body except fat — muscle, bone, organs, and water. Enter your height, weight, and gender above to instantly see your LBM, fat-free mass percentage, and FFMI using the Boer, Hume, and James formulas — free, with no signup required.

What Is Lean Body Mass (LBM)?

Lean body mass (LBM) is the portion of your total body weight that isn't stored fat. It includes your muscle, bones, organs, blood, and water — in short, everything except adipose (fat) tissue. LBM is closely related to fat-free mass (FFM), a near-identical measure used in clinical and sports-science literature.

Because muscle is metabolically active tissue, LBM is a strong driver of your resting metabolic rate — two people with the same total weight but different LBM will typically burn a different number of calories at rest.

Lean Body Mass Formulas: Boer, Hume & James

This calculator estimates LBM using three validated formulas from the medical and sports-science literature. Each takes your height, weight, and gender — no body fat percentage required.

Boer Formula (1984)

Men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
Women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3

Hume Formula (1966)

Men: LBM = 0.3281 × weight(kg) + 0.33929 × height(cm) − 29.5336
Women: LBM = 0.29569 × weight(kg) + 0.41813 × height(cm) − 43.2933

James Formula (1976)

Men: LBM = 1.10 × weight(kg) − 128 × [weight(kg) / height(cm)]²
Women: LBM = 1.07 × weight(kg) − 148 × [weight(kg) / height(cm)]²

All three formulas are estimates derived from population averages. Individual results can vary, particularly at unusually high or low body weights.

Worked Example: 80kg, 180cm Male

Here's how the three formulas compare for a male at 80 kg body weight and 180 cm height:

FormulaEstimated LBMLBM %Est. Fat Mass
Boer61.4 kg76.8%18.6 kg
Hume57.8 kg72.3%22.2 kg
James62.7 kg78.4%17.3 kg

Notice the ~5 kg spread between formulas — this is normal and expected. Use one formula consistently over time to track trends rather than comparing across formulas.

FFMI: Adjusting Lean Mass for Height

Fat-Free Mass Index (FFMI) adjusts your lean body mass for height, the same way BMI adjusts total weight for height. It's a useful way to compare muscularity between people of different heights.

FFMI = LBM(kg) / height(m)²
Normalized FFMI = FFMI + 6.1 × (1.8 − height in meters)
FFMI Range (Men)Interpretation
16–18Below average muscle mass
18–20Average, untrained to lightly trained
20–22Above average, consistently trained
22–25Very muscular, advanced natural lifter
25+Rare without performance-enhancing drugs

Typical Lean Body Mass Percentage by Category

LBM percentage is simply 100% minus your body fat percentage. Based on commonly cited body-composition categories:

CategoryMen — LBM %Women — LBM %
Essential fat only95–98%87–90%
Athletes87–94%80–86%
Fitness83–86%76–79%
Average76–82%69–75%
Above average body fat75% or below68% or below

How to Increase Lean Body Mass

  • Progressive resistance training — gradually increasing weight, reps, or volume over time is the primary driver of muscle growth.
  • Adequate protein intake — roughly 1.6–2.2 g per kg of total body weight per day for most trained individuals.
  • Sufficient total calories — a slight surplus supports muscle building; a moderate deficit with high protein can help preserve LBM while losing fat.
  • Recovery and sleep — muscle repair and growth happen primarily during rest, not during training itself.
  • Consistency over months — meaningful LBM changes typically take 8–12+ weeks to become measurable.

Factors That Affect Lean Body Mass

  • Age (LBM naturally declines after roughly age 30 without training)
  • Sex (men typically carry more baseline muscle mass than women)
  • Training history and current activity level
  • Protein and overall calorie intake
  • Hormonal factors (testosterone, growth hormone, thyroid function)
  • Hydration status, which affects body water — a component of LBM

Frequently Asked Questions

Lean body mass is your total body weight minus body fat. It includes muscle, bone, organs, water, and connective tissue — essentially everything in your body except stored fat. It is often used interchangeably with fat-free mass (FFM).
In practice the terms are used interchangeably. Technically, fat-free mass (FFM) excludes 100% of body fat, while lean body mass (LBM) includes a small amount of essential fat (roughly 2–3% in men, 10–13% in women) that's necessary for hormonal and organ function. The difference is small enough that most calculators, including this one, treat LBM and FFM as equivalent.
No formula is universally 'most accurate' — each was validated against different reference populations. The Boer formula is widely considered a solid general-purpose estimate for most adults. The Hume formula tends to run slightly lower and is often cited for women. The James formula can be less reliable at very high or very low body weights because it relies on a weight-to-height-squared ratio. For clinical precision, a DEXA scan or bioelectrical impedance analysis (BIA) is more accurate than any formula.
The Boer, Hume, and James formulas used in this calculator only require your weight, height, and gender — not a body fat percentage. This makes them practical when you don't have access to skinfold calipers, BIA scales, or a DEXA scan.
For men, an LBM of roughly 80–90% of total body weight is typical (corresponding to 10–20% body fat). For women, 70–80% LBM is typical (corresponding to 20–30% body fat), since women carry more essential fat. Trained athletes often sit at the higher end of these ranges.
Fat-Free Mass Index (FFMI) adjusts lean body mass for height, similar to how BMI adjusts weight for height. It's calculated as FFMI = LBM (kg) ÷ height (m)². A normalized version adds a small height correction: FFMI_normalized = FFMI + 6.1 × (1.8 − height in meters). Most natural male lifters fall between 18–22; values above 25 are rare without performance-enhancing drugs.
Lean body mass increases primarily through progressive resistance training, adequate protein intake (roughly 1.6–2.2 g per kg of total body weight for most trained individuals), a slight calorie surplus or maintenance intake, and sufficient sleep and recovery. Consistency over months, not weeks, drives measurable change.
Yes. Aggressive calorie deficits, low protein intake, and inadequate resistance training during a cut can cause you to lose muscle along with fat, lowering your lean body mass. Preserving LBM during a diet generally requires a moderate deficit, protein intake at the higher end of the typical range, and continued strength training.
A commonly cited range for building or preserving muscle is roughly 2.2–2.6 g of protein per kg of lean body mass per day for resistance-trained individuals, which is somewhat more precise than basing intake on total body weight alone, especially for people with higher body fat percentages.
Yes. A DEXA (dual-energy X-ray absorptiometry) scan directly measures bone, fat, and lean tissue and is considered a clinical gold standard. Formula-based calculators like Boer, Hume, and James estimate LBM from height, weight, and gender alone, so they're convenient for tracking trends but less precise than DEXA, BIA, or hydrostatic weighing for a single accurate reading.

This calculator uses established formulas from peer-reviewed medical and sports-science literature (Boer, Hume, and James). Results are estimates for general fitness and educational purposes only and are not a substitute for a DEXA scan, BIA measurement, or advice from a doctor, dietitian, or certified trainer. Last reviewed: August 2026.