A walkthrough, end to end.
- 1
Enter height, weight, and sex. The Boer formula uses sex to account for typical body composition differences (men carry more lean mass at the same body size).
- 2
Read your lean body mass — the total weight of everything that isn't fat. Muscle, bone, organs, water, glycogen — all of it.
- 3
The split bar shows your estimated lean-to-fat ratio. Lean mass is shown in kg or lb depending on your unit choice; fat mass is the rest of your body weight.
Boer formula (1984)
The Boer formula estimates lean body mass from height, weight, and sex. It's one of several published formulas (others include Hume, James, and the Janmahasatian fat-free mass equation). Boer is straightforward and well-validated for adults of normal body composition.
What you can do with this.
Lean body mass for muscle gain
Track LBM alongside body weight when bulking. If your weight is going up but LBM is flat, you're mostly adding fat — slow the surplus. If LBM rises with weight, you're building muscle effectively.
Lean body mass for cutting / weight loss
During a calorie deficit, the goal is to lose fat while preserving LBM. Track LBM every 2–4 weeks; if it drops more than 1–2% per month, your protein is too low or your deficit is too aggressive.
LBM for medication dosing
Some medications dose by lean body mass rather than total body weight (especially in obese patients) because fat tissue clears them slowly. Your clinician will use the formula their pharmacy standardizes on.
LBM-based BMR (Katch–McArdle)
If you have a reliable LBM number, the Katch–McArdle BMR formula (370 + 21.6·LBM kg) tends to be more accurate than Mifflin–St Jeor for very lean or very muscular people, since it's indexed to lean tissue rather than total body weight.
Lean body mass for women
The Boer formula has a sex-specific constant for women (0.252·W + 0.473·H − 48.3) that accounts for typical higher essential body fat. Women's LBM is usually 65–75% of body weight; men's is 75–85%.
Body recomposition tracking
‘Recomp’ means losing fat and gaining muscle simultaneously — possible especially for beginners and people returning to training. The scale won't move much; LBM going up is the signal that recomp is working.
Lean body mass calculator 2026 — what's current
The Boer formula remains a solid 2026 default for height/weight/sex-based LBM estimation. What's shifted is wider availability of consumer DEXA scans (now under $50 in many cities) and improved smart-scale bioimpedance. The Boer estimate is still the best no-equipment baseline; treat any direct measurement as more accurate when you have access to one.
Frequently asked.
Boer is accurate within ~5% for adults of average body composition. It overestimates LBM for very obese people and underestimates for very lean / muscular people. For precise numbers, DEXA, hydrostatic weighing, or BodPod are the gold standards.
Casually, the same. Strictly, lean body mass includes a small amount of essential fat (around organs); fat-free mass excludes all fat. The difference is typically 1–3 kg and most calculators (including this one) use them interchangeably.
A body fat calculator measures the fat percentage directly (typically using waist/neck/hip measurements via the Navy method). This LBM calculator works backwards from height/weight/sex using a population formula — it's a rougher estimate but doesn't require tape measurements.
Functionally no — more lean mass is generally better for metabolic health and longevity. Bodybuilders push LBM as high as their genetics allow without health concerns. The exception is severe edema or fluid retention being misread as muscle.
No — LBM doesn't change week-to-week meaningfully. Track it monthly during goal phases (bulk or cut) and quarterly during maintenance. Day-to-day water-weight changes can swing the formula's output by ±1 kg.
No. Every calculation runs in your browser. There's no server processing, no analytics on inputs, no cookies storing your numbers.