How to Calculate Lean Body Mass: Boer, James and Hume Formulas, With Worked Examples

Lean body mass is everything in your body that isn't fat: muscle, bone, organs, skin, and the water inside all of them. The simplest way to calculate it is to multiply your weight by (100 minus your body fat percentage) and divide by 100. If you don't know your body fat, three formulas (Boer, James and Hume) estimate it from your weight, height and sex. For a 78 kg man who is 172 cm tall, they give roughly 54 to 60 kg, depending on which one you use.

That spread is worth noticing early. Lean body mass can't be measured directly in a living person, so every number you see is an estimate, and different estimates can differ by several kilograms. This guide shows each method, runs them on the same two people, explains what "lean" does and doesn't mean, and tells you how to use the number sensibly.

Lean body mass, lean mass, fat-free mass: are they the same?

Close enough for everyday use, but the terms aren't identical.

• Fat mass is all the fat in your body, including the essential fat in nerves, organs and bone marrow.
• Fat-free mass is body weight minus all fat. It includes muscle, bone, water, organs and connective tissue.
• Lean body mass is often used as a synonym for fat-free mass. Strictly, some definitions leave out the small amount of essential fat, so lean body mass can be slightly higher than fat-free mass. Online calculators and body composition scales rarely draw the distinction.
• Skeletal muscle mass is only the muscle attached to your bones. It's one part of lean mass, and we cover it in a separate guide.

Because lean mass includes bone and organs, which barely change, tracking it over time mostly tells you about muscle and water.

Why you might want to know it

• To work out how many calories you burn at rest. Lean tissue drives most of your resting metabolism.
• To set protein targets. Some dietitians base protein on lean mass rather than total weight, especially for people with a lot of body fat.
• To check whether weight loss is costing you muscle. If your weight drops 6 kg and your lean mass drops 3 kg, half of what you lost was not fat.
• To dose some medicines. Doctors use lean mass or "ideal body weight" for certain drugs, particularly in people with obesity.
• To follow strength training progress, where scale weight misleads.

Method 1: from body fat percentage

If you've measured your body fat (tape method, smart scale or DEXA), this is the easiest route.

Lean body mass = body weight × (1 − body fat % ÷ 100)

Example: a man

Weight 78 kg, body fat 23.9 percent (the tape-method result for the man in our body fat guide). Fat mass = 78 × 0.239 = 18.6 kg Lean body mass = 78 − 18.6 = 59.4 kg

Example: a woman

Weight 62 kg, body fat 34.3 percent. Fat mass = 62 × 0.343 = 21.3 kg Lean body mass = 62 − 21.3 = 40.7 kg

This method is only as good as the body fat number you feed into it. A 3 to 4 point error in body fat becomes a 2 to 3 kg error in lean mass.

Method 2: the Boer formula (1984)

Boer's equations were derived from measurements in adults and are widely used in clinical settings.

• Men: LBM = (0.407 × weight in kg) + (0.267 × height in cm) − 19.2
• Women: LBM = (0.252 × weight in kg) + (0.473 × height in cm) − 48.3

Worked example: a man, 78 kg, 172 cm

(0.407 × 78) = 31.75 (0.267 × 172) = 45.92 31.75 + 45.92 − 19.2 = 58.5 kg

Worked example: a woman, 62 kg, 158 cm

(0.252 × 62) = 15.62 (0.473 × 158) = 74.73 15.62 + 74.73 − 48.3 = 42.1 kg

Method 3: the James formula (1976)

James's formula is a quadratic and behaves differently at higher weights.

• Men: LBM = (1.10 × weight) − 128 × (weight ÷ height in cm)²
• Women: LBM = (1.07 × weight) − 148 × (weight ÷ height in cm)²

Worked example: a man, 78 kg, 172 cm

weight ÷ height = 78 ÷ 172 = 0.4535; squared = 0.2057 (1.10 × 78) = 85.8 128 × 0.2057 = 26.33 85.8 − 26.33 = 59.5 kg

Worked example: a woman, 62 kg, 158 cm

62 ÷ 158 = 0.3924; squared = 0.1540 (1.07 × 62) = 66.34 148 × 0.1540 = 22.79 66.34 − 22.79 = 43.6 kg

A known limitation of the James formula is that, in very heavy people, it can predict a falling lean mass as weight rises, which is not realistic. It's best used within normal and moderately overweight ranges.

Method 4: the Hume formula (1966)

• Men: LBM = (0.32810 × weight) + (0.33929 × height in cm) − 29.5336
• Women: LBM = (0.29569 × weight) + (0.41813 × height in cm) − 43.2933

Worked example: a man, 78 kg, 172 cm

(0.32810 × 78) = 25.59 (0.33929 × 172) = 58.36 25.59 + 58.36 − 29.53 = 54.4 kg

Worked example: a woman, 62 kg, 158 cm

(0.29569 × 62) = 18.33 (0.41813 × 158) = 66.07 18.33 + 66.07 − 43.29 = 41.1 kg

Putting the methods side by side

| Method | Man, 78 kg, 172 cm | Woman, 62 kg, 158 cm | |---|---|---| | From body fat % (tape method) | 59.4 kg | 40.7 kg | | Boer | 58.5 kg | 42.1 kg | | James | 59.5 kg | 43.6 kg | | Hume | 54.4 kg | 41.1 kg |

For the man, Hume sits five kilograms below the others. For the woman, the four methods span almost three kilograms. This is normal. These formulas were fitted to different groups of people, and none of them knows whether you're muscular or not.

A practical approach: calculate with two or three methods, and treat the middle as your working estimate. Or, if accuracy matters, measure body fat with DEXA and calculate from that.

What's a "normal" lean body mass?

There's no single normal figure, because lean mass scales with height and frame. Two more useful ways to judge it:

Lean mass as a share of body weight

This is just 100 minus your body fat percentage. Using common fitness ranges, lean mass is roughly 75 to 86 percent of body weight in men and about 68 to 79 percent in women, though South Asians tend to sit toward the lower end of lean mass share at a given BMI.

Fat-free mass index (FFMI)

FFMI is lean mass divided by height in metres squared, like BMI but for lean tissue. It lets you compare people of different heights.

• Man: 59.4 ÷ (1.72)² = 59.4 ÷ 2.958 = 20.1
• Woman: 40.7 ÷ (1.58)² = 40.7 ÷ 2.496 = 16.3

Commonly cited ballpark figures are around 18 to 21 for average men and 15 to 18 for average women, with strength-trained people higher. These are descriptive numbers from studies, not diagnostic cut-offs, so use them as rough orientation.

Lean mass and Indians

Many Indians have less lean mass for their weight than people of other ethnic groups. This is part of the "thin-fat" pattern: a normal BMI, a relatively high body fat percentage, and limited muscle. A review of South Asian body composition (Misra and Shrivastava, Nutrients, 2013) notes that at the same BMI, South Asians carry more body fat, which by simple arithmetic means less lean tissue per kilogram of body weight.

The practical lesson is that for many Indian adults, the thing to improve isn't weight but composition: more muscle, less fat, often at the same weight.

Lean mass and age

Lean mass peaks in the twenties and thirties. After about 30 to 40, adults lose muscle gradually, on the order of a few percent per decade if they're inactive, and the loss speeds up after 60 or 70. This age-related loss of muscle is called sarcopenia, and it matters because it raises the risk of falls, fractures and loss of independence.

The good news: resistance training and enough protein slow or partly reverse it at any age. Older adults who lift weights regularly keep far more lean mass than those who don't.

Preserving lean mass while losing weight

When people diet hard, a meaningful share of the weight lost can be muscle. To protect it:

• Lose weight slowly, around 0.5 kg a week.
• Eat enough protein, commonly 1.2 to 1.6 g per kg of body weight a day for people who are exercising and eating less (check with your doctor if you have kidney disease).
• Lift weights or do bodyweight resistance exercises two to three times a week.
• Sleep seven to eight hours.
• Avoid very low calorie diets without medical supervision.

You can check your progress by recalculating lean mass every four weeks from your body fat estimate. If it falls faster than a kilogram a month, your plan may be too aggressive.

Gaining lean mass

To build muscle you need three things: a stimulus (progressive resistance training), enough protein and enough energy. Eating slightly above your maintenance calories, about 250 to 300 kcal a day, supports muscle gain without adding much fat. Realistic rates for beginners are roughly 0.25 to 0.5 kg of lean mass per month for men and half that for women. Faster claims usually involve water, glycogen or fat.

What lean mass isn't

• It isn't "muscle only." Roughly half or less is skeletal muscle in many people; the rest is organs, bone and water.
• It isn't constant day to day. A big meal, a salty dinner or a hard workout can shift water and therefore "lean" readings on a smart scale by a kilogram or more.
• It isn't a health score by itself. A high lean mass in someone with a large waist can still go along with metabolic risk.

Frequently asked questions

How to calculate lean body mass?

Multiply your body weight by (1 − body fat percentage ÷ 100). If you don't know your body fat, estimate lean mass with the Boer, James or Hume formulas using your weight and height.

What is the lean body mass formula?

The Boer formula for men is (0.407 × weight) + (0.267 × height in cm) − 19.2, and for women (0.252 × weight) + (0.473 × height in cm) − 48.3.

What is a good lean body mass in kg?

It depends on height and sex. An FFMI of roughly 18 to 21 for men and 15 to 18 for women is a common ballpark for the general population.

What is the difference between lean mass and fat mass?

Fat mass is the weight of the fat in your body. Lean mass is everything else: muscle, bone, organs and water.

How do I find my lean mass at home?

Measure body fat with the tape method or a smart scale, then use the weight formula above. Remember that the result inherits the error of the body fat number.

Is lean body mass the same as muscle mass?

No. Lean mass includes muscle, bone, organs and water. Muscle mass is one part of it.

How can I increase lean body mass?

Do resistance training two to four times a week, eat enough protein and calories, sleep well and be patient. Gains are slow.

When to see a doctor

If you're losing weight without trying, notice your strength or muscle size falling, or find daily tasks harder (climbing stairs, getting up from a chair), see a doctor. Loss of muscle can come from poor nutrition, thyroid disease, chronic illness, certain medicines and low testosterone, and it is worth investigating.

Care at Prakash Hospital Noida

Whether you live in Sector 18, Sector 62, Greater Noida West, or anywhere nearby, Prakash Hospital Noida is a trusted name for nutrition counselling and metabolic health in Noida. Our physicians can help you understand your body composition and plan for a healthier balance of muscle and fat.

To book a consultation, call the number.

Sources: Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Physiology, 1984. James WPT. Research on obesity, 1976. Hume R. Prediction of lean body mass from height and weight. Journal of Clinical Pathology, 1966. Misra A, Shrivastava U. Obesity and dyslipidemia in South Asians. Nutrients, 2013.

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