Lean Body Mass Calculator

The formulas (weight in kg, height in cm)

Boer is shown as the headline because it performs reasonably across a range of body sizes. These equations were developed largely for medical uses such as drug dosing and body-fluid estimates.

Why lean mass matters

Lean mass drives your resting metabolism, which is why the Katch-McArdle equation in our BMR Calculator uses it. Clinicians use lean mass to dose some medicines, and athletes track it to see whether weight changes come from fat or muscle.

Formula estimate vs. measured body fat

These formulas only know your height, weight and sex, so they assume average body composition. They will underestimate lean mass for muscular people and may overestimate it for people with low muscle. If you have a body fat estimate, lean mass = weight × (1 − body fat %) is usually more personal — try our Body Fat Calculator.

The James formula at high weights

Because the James formula subtracts a term that grows with the square of weight, it starts to decrease as weight rises beyond a certain point, giving unrealistic results for people with a high BMI. The calculator warns you when BMI is above 35.

What lean body mass is made of

Lean body mass includes skeletal muscle, organs, bone, connective tissue, skin and body water. Water alone makes up roughly 70% of lean tissue, which is why lean mass readings can change with hydration. Skeletal muscle is often around 40–50% of lean mass in adults, depending on training and age.

Lean body mass vs. fat-free mass

The terms are often used interchangeably. Strictly, fat-free mass excludes all fat, while lean body mass includes the small amount of essential fat in organs and bone marrow. For practical purposes, the difference is only a few percent.

How lean mass changes over life

Lean mass usually peaks in early adulthood and declines gradually from middle age, a process that accelerates after about 60 and is linked with loss of strength and mobility (sarcopenia). Regular resistance training, adequate protein and staying active can slow this decline substantially. Rapid weight loss can also reduce lean mass, which is one reason moderate deficits are recommended.

Using lean mass in other calculations

Your lean mass in kilograms can be entered indirectly in the BMR Calculator via body fat percentage (Katch-McArdle), and some coaches set protein targets per kilogram of lean mass rather than total weight — roughly 1.6–2.2 g/kg of lean mass is a common sports range.

Improving lean mass

Two to three resistance-training sessions a week covering the major muscle groups, progressive increases in load or reps, enough protein spread across meals, and adequate sleep are the foundations. Expect slow change: beginners might gain around 0.5–1 kg of lean mass per month at first, with progress slowing over time.

Lean mass and weight loss

When you lose weight, aim for most of it to come from fat. Checking lean mass alongside weight every month or two — with the same method each time — helps confirm that strength training and protein are protecting muscle during a diet.

Worked examples

Example 1: Man, 80 kg, 180 cm

Boer = 32.6 + 48.1 − 19.2 = 61.4 kg; James ≈ 62.7 kg; Hume ≈ 57.8 kg. Implied body fat ≈ 23% (Boer).

Example 2: Woman, 60 kg, 165 cm

Boer = 15.1 + 78.0 − 48.3 = 44.9 kg; James ≈ 44.6 kg; Hume ≈ 43.4 kg.

Limitations

• Assumes average body composition for your size.
• Developed in specific adult populations decades ago.
• James is unreliable at high BMI.

Who should not rely on this calculator

Not for children, pregnancy, or for medication dosing decisions — clinicians should use their own protocols. Very muscular people and those with fluid retention will get misleading results.