Muscle Mass Estimator

Skeletal muscle mass (SMM) is the total weight of the muscles attached to your skeleton, the ones you move on purpose. It is not the same thing as lean mass, which also counts organs, bone, and body water, and it is a good deal smaller. A man carrying 58 kg of lean mass typically has around 31 kg of it as skeletal muscle.

The Lee estimation formula

This calculator uses the height-and-weight prediction equation published by Lee and colleagues in 2000, fitted against whole-body MRI (magnetic resonance imaging):

SMM (kg) = 0.244 × weight + 7.80 × height(m) − 0.098 × age + 6.6 × sex + race − 3.3

Sex is 1 for male and 0 for female. Height is in metres, weight in kilograms. The race term was −1.2 for Asian participants, +1.4 for African American participants, and 0 for white and Hispanic participants. This calculator uses 0.

The fit was good for a formula this simple (R² of 0.86), but its standard error is 2.8 kg. Two times out of three the true figure sits within 2.8 kg of the estimate, and the rest of the time further away. Lee also fitted it only on non-obese adults, with a body mass index (BMI) under 30, so above that the answer is outside the data it came from.

What to compare it with

The best reference is the MRI study the same group ran on 468 men and women aged 18 to 88 (Janssen and colleagues, 2000). Skeletal muscle averaged 33.0 kg in men and 21.0 kg in women, which is 38.4% and 30.6% of body weight. The share starts falling in the third decade, mostly because fat accumulates, while the absolute amount holds until the late forties and then falls, mostly from the legs.

Worked example

A man aged 35, 80 kg and 178 cm:

• SMM = 0.244 × 80 + 7.80 × 1.78 − 0.098 × 35 + 6.6 − 3.3
• = 19.52 + 13.88 − 3.43 + 6.6 − 3.3 = 33.3 kg, which is 41.6% of his weight
• The 2.8 kg error is 3.5% of his weight, so the honest reading is 38.1 to 45.1%
• Janssen’s average man is 38.4%, inside that range

So the estimate says his proportions are typical. It cannot say more, and that is true for most people who run it: the gap between you and the average is usually smaller than the equation’s own error.

Sarcopenia

From around age 30, adults lose roughly 3-8% of muscle mass per decade without deliberate effort to keep it. The loss accelerates after 60. That is sarcopenia, and it is a leading risk factor for falls and fractures, which are in turn a leading cause of losing independence. Doctors no longer diagnose it from muscle mass alone. The European definition (EWGSOP2, 2019) starts from low strength, a grip under 27 kg for men or 16 kg for women, because strength predicts falls and disability better than mass does. The grip strength calculator checks that one. The good news is that it responds to training at any age: Fiatarone’s 1990 trial in nursing home residents in their nineties produced large strength gains from progressive resistance work.

What moves the number

Resistance training is the effective lever, and nothing else comes close. On protein, Morton’s 2018 meta-analysis of 49 trials found gains stopped growing at about 1.6 g per kg of body weight a day, with the upper end of its confidence range at 2.2. Older adults need a bigger dose at each meal for the same response: about 0.4 g per kg against 0.24 in young men (Moore and colleagues, 2015). Sleep matters for recovery. Testosterone and other hormonal factors set part of the ceiling and are largely outside your control.

How much to trust this

Anthropometric predictions are reasonable population estimates, not measurements. They cannot match DXA (dual-energy X-ray absorptiometry), bioelectrical impedance, or MRI. Two people of identical height, weight, age and sex get identical answers here whatever their actual body composition, which is the limitation in a sentence: the equation sees your dimensions, not your training.

What this is genuinely good for is a ballpark, and a reminder of what the average is. Tracking change is harder than it looks, because the equation credits a quarter of any weight you gain as muscle whether you trained for it or not. Measure the muscle you care about directly instead: tape the arm and thigh, or keep a strength log.