Published by MetaSys Limited · Page updated · Editorial policy. Educational information; not a medical review.
What Lean Body Mass Actually Tells You
Lean body mass (LBM) is everything in your body that isn't fat: your muscle, skeleton, organs, connective tissue, and the water distributed through all of it. When people talk about "getting toned" or "recomping," what they're usually describing is a shift in the ratio between lean mass and fat mass — often without the scale number changing much at all. This distinction matters: total body weight on its own tells you very little about what's actually happening underneath it.
The Boer Formula, and Why We Use It
This calculator uses the Boer formula, published in 1984 and still one of the most widely cited equations for estimating lean body mass from simple measurements. For men, the formula is LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2. For women, it's LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3. Notice that height carries much more weight in the women's equation — this reflects real physiological differences in average frame size and body composition between sexes. The Boer formula was derived from body density data and has held up well against modern DEXA scan comparisons, typically landing within a few percentage points of gold-standard measurements for people in a normal weight range.
Once we have your LBM, the rest is simple subtraction. Fat mass is just your total weight minus your lean body mass. From there, we divide fat mass by total weight to get a derived body fat percentage. It's worth being upfront that this derived percentage is a secondary calculation, not a direct measurement — it's mathematically consistent with the Boer formula's assumptions, but it isn't the same as a skinfold test or a bioelectrical impedance reading, and it can diverge from those methods, sometimes by a noticeable margin.
How to Actually Use This Number
The most practical use of lean body mass is as a more honest baseline than total body weight — especially for nutrition planning. If you're trying to build muscle or preserve it during a cut, setting your protein target based on lean mass rather than total weight avoids overestimating how much protein you actually need, since fat tissue requires very little protein to maintain. A common guideline is 0.7 to 1 gram of protein per pound of lean body mass per day, adjusted based on training intensity and goals.
LBM is also a better number to track over time than weight alone. If you're strength training consistently and your total weight stays flat but your calculated lean mass creeps up while fat mass drops, that's a body recomposition happening in real time — something the scale by itself would never show you. I'd recommend recalculating every few weeks using consistent measurement conditions (similar time of day, similar hydration) so the trend, not any single number, is what you pay attention to.
Where This Estimate Falls Short
No formula-based estimate can account for individual variation the way a direct scan can. The Boer formula was built on average population data, so it can be less accurate for people with unusually high muscle mass (competitive bodybuilders or athletes), very low body fat, or body proportions that differ significantly from the reference population it was derived from. It also doesn't account for bone density differences, which can vary meaningfully with age and activity history. If you need precise numbers for medical or competitive purposes, a DEXA scan remains the most reliable option. For everyone else tracking general fitness progress, this calculator gives a solid, consistent estimate to work from.
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