Lean body mass: the number that ages you well
Your bodyweight is two numbers pretending to be one: fat mass and lean body mass. Lean body mass — your muscle, bone, organs, and water — is the part that makes you strong, keeps your metabolism humming, and protects you against the frailty that creeps in with age. For a man over 35, protecting and building lean mass is arguably the most important physical goal there is.
This calculator estimates how much of you is lean versus fat, and rates your muscularity with FFMI — a height-fair alternative to BMI that actually rewards muscle instead of penalising it.
How lean body mass is calculated
There are two routes, and this calculator uses the best one available to you:
From body fat (most accurate)
If you know your body-fat percentage, lean mass is simply weight × (1 − body fat%). A 80 kg man at 18% body fat has about 65.6 kg of lean mass. Get your body fat from the Body Fat Calculator for the truest result.
From formulas (no measurements needed)
Without body fat, validated equations — Boer, James, and Hume — estimate lean mass from your height, weight, and sex. They agree closely for average builds and diverge for outliers, which is why we show all three so you can see the range.
See your lean mass and FFMI
Enter your details above — add body fat for the most accurate result.
Use the calculatorUnderstanding FFMI (your muscle score)
BMI famously punishes muscle. FFMI fixes that by measuring only your lean mass relative to height. We report the adjusted FFMI, normalised to a 1.8 m reference so it is fair across heights. A rough guide for men:
- ~18–19: average, untrained.
- 20–22: clearly fit and trained.
- 22–23: advanced — years of consistent lifting.
- 23–25: excellent, approaching the natural drug-free ceiling.
- 25+: rare naturally.
Most men will never need to chase the top of that scale. Moving from 19 to 21 over a couple of years is a transformation you will see and feel.
Why lean mass matters more with age
From your 30s onward, muscle is lost by default — a process called sarcopenia — unless you actively fight it. That loss quietly lowers your metabolism, weakens you, and raises injury and disease risk. The good news: resistance training and enough protein don’t just slow the decline, they reverse it. Tracking lean mass, not just scale weight, keeps your eye on the thing that matters.
How to use your lean mass number
- Feed the muscle with protein. Base your target on lean mass with the Protein Calculator.
- Set calories to build or preserve. Use the TDEE Calculator — a small surplus to grow, maintenance-plus-protein to recomp.
- Lift consistently. Two or three focused strength sessions a week is enough to build and protect lean mass.
- Re-measure every 8–12 weeks. Lean mass moves slowly; give it time to show.
What to expect
Building lean mass is a patient game — a beginner might add a few kilos of muscle in the first year, an experienced lifter far less. But every kilo compounds: more strength, a higher metabolism, and a body that stays capable for decades. That long-game, minimal-time approach is exactly what our free 3-day plan is built for.