Skeletal muscle mass: how to calculate it with the Lee equation

By PounceFit Team · Updated on 2026-10-06

Technical content for health and fitness professionals. It reproduces formulas and protocols published in the scientific literature, with the sources at the end of the page. It is not a recommendation and does not replace the judgment of the professional who runs the assessment.

The Lee equation estimates skeletal muscle mass, in kilograms, from height, three girths (arm, thigh and calf) corrected by the skinfolds at the same sites, sex, age and ethnicity (Lee et al., 2000). It was developed against whole-body magnetic resonance imaging in 244 non-obese adults and explains 91% of the variance in measured muscle mass, with a standard error of 2.2 kg.

PounceFit, body assessment software, calculates muscle mass with this equation whenever the assessment has the required girths and skinfolds. For use in a clinic, see body assessment for dietitians.

The formula in full

First, correct each girth for the skinfold at the same site. The skinfold goes in centimeters (divide millimeters by 10):

• CAG (corrected arm girth) = arm girth - π × triceps skinfold
• CTG (corrected thigh girth) = thigh girth - π × thigh skinfold
• CCG (corrected calf girth) = calf girth - π × calf skinfold

Then apply the equation (Lee et al., 2000):

SMM (kg) = height × (0.00744 × CAG² + 0.00088 × CTG² + 0.00441 × CCG²) + 2.4 × sex - 0.048 × age + ethnicity + 7.8

Where:

• height in meters;
• corrected girths in centimeters;
• sex = 1 for men and 0 for women;
• age in years;
• ethnicity = -2.0 for Asians, +1.1 for African Americans and 0 for Whites and Hispanics.

PounceFit uses π = 3.1415, as in the original assessment method the app was built on. The difference from full π shows up in the fourth decimal place of the result.

Where to measure

Use the right side. In PounceFit, the sites are:

These are the sites used in the app. The Lee et al. (2000) abstract does not describe the measurement sites; to reproduce the study protocol exactly, check the methods section of the original paper and the erratum published in 2001.

Measure the girth and the skinfold at exactly the same level (see the technique in body circumferences). The correction only makes sense if the skinfold describes the fat in the ring the tape went around.

Worked example

A 35-year-old White man, 1.78 m (5 ft 10 in). Arm 34 cm, triceps 10 mm. Mid-thigh 55 cm, thigh skinfold 15 mm. Calf 38 cm, calf skinfold 8 mm.

1. CAG = 34 - 3.1415 × 1.0 = 30.86 cm. CAG² = 952.2.
2. CTG = 55 - 3.1415 × 1.5 = 50.29 cm. CTG² = 2,528.9.
3. CCG = 38 - 3.1415 × 0.8 = 35.49 cm. CCG² = 1,259.3.
4. Girth term: 0.00744 × 952.2 + 0.00088 × 2,528.9 + 0.00441 × 1,259.3 = 7.085 + 2.225 + 5.554 = 14.864.
5. Multiplied by height: 14.864 × 1.78 = 26.457.
6. Sex: + 2.4. Age: - 0.048 × 35 = - 1.68. Ethnicity: 0. Constant: + 7.8.
7. SMM = 26.457 + 2.4 - 1.68 + 0 + 7.8 = 34.98 kg.

The app reaches 34.98 kg with these numbers. If the client weighs 80 kg (176 lb), skeletal muscle mass is about 44% of body mass.

Look at the weight of each term. The arm has the largest coefficient, but the thigh, being much larger, still contributes. A 1 cm error in arm girth changes the result by about 0.8 kg; the same error at the thigh changes it by about 0.16 kg, and at the calf by about 0.56 kg.

How PounceFit shows the result

The value in kilograms appears on the "Muscle mass · Lee" card. Next to it, the app shows a proportionality Z-score (Phantom), which compares muscle mass with a height-adjusted reference. This score is a comparison of proportion, not a diagnosis. Lee muscle mass does not enter the body fat percentage calculation: the two results come from independent equations.

Limits of the method

• Original sample. The equation was developed and validated in adults with a BMI below 30 kg/m² (Lee et al., 2000). In people with obesity, frail older adults and very muscular athletes, the estimate may be less accurate.
• Ethnicity. The coefficients apply to the groups studied in the United States. There is no coefficient for Brazilian mixed-race (pardo) or Indigenous people; PounceFit uses 0 in these cases, which is an approximation.
• Standard error of 2.2 kg. For a client with 35 kg of muscle mass, the true value may be a few kilograms above or below. A 1 kg change between two assessments is not, on its own, proof of gain or loss.
• Technique. The result adds up the error of three girths and three skinfolds. ISAK standardization (Stewart et al., 2011) and the same assessor at every reassessment reduce that error.
• Correction model. A skinfold picks up a double layer of skin and fat, so the fat around the limb is taken as half the skinfold thickness. Subtracting π times the skinfold removes that ring, assuming a circular limb and a fat layer of even thickness all the way around. It is a geometric simplification, and what remains includes bone: corrected girth is not a direct measurement of muscle.

To track a client, keep the same sites, the same side and the same assessor. The trend in corrected girths alone already shows whether muscle is changing, without relying on the full equation.