MariposasLean Body Mass Calculator
By Zachary, builder of Mariposas · Reviewed against published exercise-science sources (references)
· Last updated June 2026
Body Metrics Lean body mass is everything that is not fat, muscle, bone, organs and water. This estimates it from your weight and height using the Boer formula, which is handy for setting protein and calorie targets.
⚕️ A general-information estimate from population-level formulas, a starting point, not a precise measurement and not medical advice.
How it works
The Boer formula estimates lean body mass (LBM) using only height and weight, applying sex-specific constants that reflect the average difference in bone density and organ mass between males and females. For men the formula is LBM = (0.407 × weight in kg) + (0.267 × height in cm) − 19.2, and for women it is LBM = (0.252 × weight in kg) + (0.473 × height in cm) − 48.3. Those coefficients were derived from cadaver and densitometry studies, which is why Boer tends to outperform simpler two-variable formulas like Devine or Robinson on people who fall outside a narrow height range. The output is expressed in kilograms (or pounds) and represents everything your body contains except stored fat: skeletal muscle, bone mineral, organs, connective tissue, and total body water. Because fat mass is the only component excluded, LBM gives you a meaningful denominator for calculating protein needs per unit of metabolically active tissue rather than total body weight.
When to use it
This calculator is most useful when total body weight is a misleading number, for example in someone who has a high body fat percentage or who has just added significant muscle mass and wants a more honest baseline for nutrition planning. Coaches and athletes often use LBM to anchor daily protein targets (commonly discussed in the range of 1.6 to 2.2 g per kg of LBM in sports nutrition research) or to set calorie numbers that scale to actual tissue mass rather than scale weight. It also gives a consistent reference point to track over months: if your weight stays flat but your LBM estimate rises, that pattern suggests a body composition shift worth investigating with a more direct method like a DEXA scan.
Worked example
Take a 35-year-old woman who weighs 72 kg and is 168 cm tall. Plugging those numbers into the Boer formula: (0.252 × 72) + (0.473 × 168) − 48.3 = 18.14 + 79.46 − 48.3 = 49.3 kg of lean body mass. That means roughly 22.7 kg of her total weight is estimated fat mass, implying a body fat percentage around 31.5%. For her nutrition setup, a coach might anchor a protein discussion around that 49 kg LBM figure rather than her 72 kg total weight, which would otherwise inflate the gram target as though all 72 kg were active tissue.
Tips for an accurate result
- Weigh yourself first thing in the morning after using the bathroom and before eating or drinking. Total body water fluctuates by 1 to 2 kg across the day, and since water is counted inside LBM, afternoon weigh-ins will inflate the estimate.
- Use centimeters and kilograms directly if you can; converting from feet/inches introduces small rounding errors that compound in the formula constants.
- Treat the output as a baseline, not a verdict. Formula-based LBM has a margin of error versus lab methods like DEXA or hydrostatic weighing, often cited in research as plus or minus 2 to 3 kg on typical body compositions.
- Re-run the calculator at consistent intervals (every 4 to 8 weeks) rather than daily. Short-term weight swings from glycogen loading or hydration will move the number without any real change in muscle or bone mass.
- If you have an unusually high or low body fat percentage, formula accuracy drops. Athletes at very low body fat and people at higher body fat percentages both sit outside the population averages the Boer constants were built from, so a skinfold or DEXA measurement adds useful context in those cases.
Formula & sources: methodology · references.
Now go hit the number Mariposas turns every workout, run and class into progress · collect a cute pet 🐾FAQ
- What is the difference between lean body mass and muscle mass?
- Lean body mass includes everything except fat: muscle, yes, but also bone, organs, skin, connective tissue, and all body water. Skeletal muscle alone typically accounts for roughly 40 to 50 percent of LBM in a healthy adult. So LBM is always a larger number than your actual muscle mass, and it should not be used interchangeably with 'muscle mass' when tracking hypertrophy specifically.
- Why does the Boer formula use different constants for men and women?
- Men and women differ systematically in bone density, organ size relative to body weight, and fat distribution patterns. The sex-specific coefficients in Boer reflect those average physiological differences across the populations used to derive the formula. Using the wrong set of constants would meaningfully skew the output, particularly the height coefficient, because pelvic structure and limb proportions differ between sexes in ways that affect how height predicts tissue mass.
- Can I use LBM to figure out my body fat percentage?
- Yes, with a simple subtraction. Body fat mass equals total weight minus LBM. Dividing that fat mass by total weight and multiplying by 100 gives you an estimated body fat percentage. Keep in mind this is still a formula estimate, not a measured one, so the percentage carries the same margin of error as the LBM figure itself.
- Does gaining muscle change my LBM estimate if my total weight stays the same?
- In theory yes, because muscle is denser than fat and genuine recomposition would shift the weight/height ratio, but in practice the Boer formula only has weight and height as inputs. A true recomposition where weight is unchanged produces no change in the formula output even if your actual composition improved. That is one reason formula-based calculators are better suited to tracking larger-scale changes and why a DEXA scan provides clearer recomposition data.
- How accurate is this compared to a DEXA scan?
- Population-level validation studies generally show Boer-estimated LBM correlating well with criterion methods, but individual error can still reach 2 to 4 kg in either direction depending on where a person's body composition sits relative to the study population. DEXA, which measures bone mineral, lean soft tissue, and fat mass separately via X-ray attenuation, is considered a reference standard in clinical and research settings. For day-to-day tracking and nutrition planning, the formula is practical and free; for clinical decisions or precise body composition assessment, a direct measurement method is worth the added cost.