Calculate your lean body mass with the Boer, James, and Hume formulas — side by side, with their average — plus your FFMI (Fat-Free Mass Index) and how it compares to natural muscularity reference ranges.
Have a measured body fat percentage (DEXA, calipers, or the Navy tape method)? Enter it for the most accurate lean mass estimate.
Formula-based estimates are informational, not a medical measurement or medical advice.
No measured body fat yet? Estimate it with the body fat calculator, then set intake with the protein calculator and macro calculator. More free tools →
| Normalized FFMI | Interpretation |
|---|---|
| 16–17 | Below average |
| 18–19 | Average |
| 20–21 | Above average / trained |
| 22–23 | Excellent |
| 24–25 | Natural ceiling range |
| >25 | Rarely achievable naturally |
Ranges are the community convention built on the Kouri et al. (1995) cohort of male lifters — the oft-cited "~25 natural ceiling" comes from that study. They apply best to men; women's FFMI values run roughly 4–5 points lower across the board. Use the normalized (height-adjusted) value.
NutriMind tracks your weight trend, body measurements, and strength numbers together, and its AI coaching keeps protein where it needs to be — so the lean mass you build actually shows up in these formulas over time.
Which lean body mass formula should I trust — Boer, James, or Hume?
Boer is the most widely used today and validates best against DEXA in most modern comparisons, which is why it is the standard for medical dose calculations. James tends to overestimate lean mass in heavier people (its weight-squared term misbehaves at high BMI), and Hume is the oldest and most conservative. When the three disagree, the average shown here is a reasonable compromise — but a measured body fat percentage beats all three, because the formulas only know your height and weight, not your actual composition.
What is FFMI and what does it tell me?
Fat-Free Mass Index is lean mass divided by height squared (kg/m²) — essentially BMI computed on muscle and other lean tissue instead of total weight. It answers "how muscular am I for my height?" A normalized FFMI around 18–19 is average for untrained men, 20–21 reflects meaningful training, and 22–23 is an excellent natural physique. Because it ignores fat, it does not penalize lean people the way BMI does.
Is 25 really the natural FFMI ceiling?
Treat it as a strong guideline, not a law. The figure comes from Kouri et al. (1995), where self-reported steroid-free lifters essentially never exceeded a normalized FFMI of 25 while users commonly did. A few natural outliers with elite genetics and long training careers exceed it slightly, and measurement error (your BF% estimate) can move the number a point either way. What is safe to say: sustained values well above 25 are rare without pharmacological help, and most natural lifters plateau in the 22–24 range.
Why does the measured body-fat route give a different number than the formulas?
Boer, James, and Hume are population regressions from just height, weight, and sex — they assume average body composition for those inputs. If you carry more muscle or less fat than average (or the reverse), they will miss. Weight × (1 − BF%) uses your actual composition, so when the BF% comes from a decent measurement (DEXA, a consistent tape protocol, calipers by an experienced tester), it is the number to use.
Does lean body mass include water and bone?
Yes. Lean body mass is everything that is not fat: skeletal muscle, bone, organs, skin, and body water. Skeletal muscle is typically only about half of LBM. That is why day-to-day LBM changes are mostly water shifts — glycogen, sodium, creatine — and why a 2 kg jump after a carb-up is not new muscle.
How fast can I actually add lean mass?
Under a good program with adequate protein, newer lifters can gain roughly 0.5–1 kg (1–2 lb) of actual muscle per month in the first year; experienced lifters progress at a fraction of that. If your calculated LBM jumps faster, it is water, glycogen, or measurement noise. Track the trend over months — see the body recomposition calculator for realistic projections.