Lean Body Mass, Fat Mass and FFMI Calculator

Scale weight answers one question: how many pounds of total matter are on the floor. Lean body mass answers the question that actually predicts health and performance: how much of that matter is metabolically active tissue. Two people can weigh 180 pounds and have radically different futures. The one with 150 pounds of lean mass is three times more insulin sensitive, has a resting metabolic rate 250 kcal higher, and will carry the weight loss better than the one at 120 pounds of lean mass. Scale weight loss is a metric. LBM-preserved fat loss is the actual goal.

This calculator estimates LBM four ways. First, if you know your body fat percentage (from a DEXA scan, Bod Pod, skinfold measurement, or smart scale), it computes LBM directly: total weight times (1 minus body fat). Second, it runs the Boer, James, and Hume equations, which predict LBM from height, weight, and sex alone. The direct method is more accurate when you have a reliable body fat number; the formulas are the right fallback when you don't.

What LBM actually is

Lean body mass is composed of four major components. Skeletal muscle makes up 40–50% of LBM in healthy adults and is the tissue you can directly train and grow. Bone makes up 15%, and while you can increase bone density with resistance training, total bone mass only shifts slightly across adulthood. Organs (brain, liver, kidneys, heart, lungs, digestive tract) make up another 15–20%. Water in these tissues makes up most of the rest — muscle is 75% water, which is why glycogen depletion shows up so dramatically on a scale.

The only component you can meaningfully change through diet and training is skeletal muscle. Adding 10 pounds of muscle requires 6–18 months of serious training depending on your training age, your protein intake, and whether you are in a caloric surplus. Losing 10 pounds of muscle, by contrast, can happen in 6 weeks of a crash diet without resistance training. LBM is easy to lose and slow to build — the asymmetry is the reason dieting strategy matters.

The Boer formula

Boer (1984) is the most widely cited LBM equation in clinical research. For men: LBM (kg) = 0.407 × weight (kg) + 0.267 × height (cm) - 19.2. For women: 0.252 × weight + 0.473 × height - 48.3. The equation was derived from more than 1,000 adults of both sexes and correlates with underwater weighing (the gold standard in 1984) within 4%. It tends to slightly underestimate LBM in very muscular individuals.

Boer works best for adults between 5 foot and 6 foot 4, between 100 and 250 pounds, and between 18 and 70 years old. Outside those ranges the formula loses accuracy quickly, and a direct measurement becomes more reliable than any prediction.

The James formula

James (1976) was developed by the World Health Organization for tracking adolescent and adult growth. For men: LBM = 1.1 × weight - 128 × (weight / height in cm)². The formula is quadratic in weight, which makes it more accurate for heavier individuals than the linear Boer equation. James systematically returns LBM values 1–3 kg higher than Boer for the same inputs.

The Hume formula

Hume (1966) is the oldest of the three and remains the standard for calculating drug doses based on lean mass, particularly in chemotherapy and anesthesia. The equation is more forgiving at extreme heights than Boer or James and slightly less accurate at the middle of the distribution.

Average the three formulas and you have an LBM estimate that rarely drifts more than 3 kg from a DEXA measurement for otherwise-healthy adults. If the three estimates disagree by more than 5 kg for your inputs, you are probably outside the population the formulas were fit on — get a real body composition measurement before making diet decisions on these numbers.

FFMI: putting LBM in context

FFMI — fat-free mass index — normalizes LBM to height. It answers the question 'How muscular are you for your frame?' rather than 'How much muscle do you have?'. A 5-foot-4 man with 120 pounds of LBM has the same FFMI as a 6-foot-2 man with 170 pounds of LBM, and both are advanced natural lifters for their respective heights.

The reference scale was developed by Harvard psychiatrist Harrison Pope and colleague Kouri in 1995. Natural FFMI distributions peak at 18–20, top out at 25 for exceptional genetic outliers, and rarely exceed 25 without performance-enhancing drugs. The Kouri 'normalized' FFMI adjusts for height (FFMI + 6.1 × (1.8 − height in meters)), which keeps the scale comparable for short and tall lifters. If your normalized FFMI is above 25, you have the genetics of a rare outlier — or you are taking something.

For women, subtract about 3 from the male scale: natural female FFMI distributions peak around 15–17, advanced is 18–19, and 24+ is rarely seen in natural training.

How to use LBM in a cut

The single most important number for protecting LBM during fat loss is protein. Eat 0.8–1 gram of protein per pound of lean body mass, not per pound of total weight — a 200-pound adult with 160 pounds of lean mass needs 128–160 grams of protein, not 160–200. Overweight adults generally need less protein per pound of scale weight than lean adults.

Combine high protein with resistance training three times per week and a moderate deficit (300–500 kcal below TDEE) and you can reasonably expect to lose 90%+ of cut weight as fat. Skip the resistance training and that number drops to 70%. Skip protein too and you will lose more than 30% of cut weight as lean mass — a terrible trade even if the scale moves.

When LBM actually increases during a cut

'Body recomposition' — fat loss and muscle gain simultaneously — is real but slow, and most dramatic in three groups: total beginners to lifting (the noob gain window), detrained returning lifters (muscle memory), and obese individuals with poor insulin sensitivity (who can partition surplus protein into muscle even at a caloric deficit).

For everyone else — adults with 1+ years of serious training and near-average body fat — a cut preserves LBM at best. Chasing simultaneous gain and loss during a cut leads to undereating protein or over-training. If recomp is your target, run the body recomposition calculator and plan the realistic pace.

Pair this with the right tools

LBM is a denominator, not a destination. Feed your LBM number into the BMR calculator (Katch-McArdle is the LBM-based BMR formula), then the TDEE calculator, then the protein calculator. The chain of LBM → BMR → TDEE → protein → deficit is the evidence-based path to a cut that keeps the muscle you worked for.

Track LBM every 6–8 weeks during a cut. If your scale weight drops 10 pounds but your LBM only drops 1 pound, you had a great cut. If your scale drops 10 and LBM drops 4, you cut too hard or too fast — pause at maintenance and recover.