Lean Body Mass Calculator: Formulas, Accuracy, What's Normal

Enter your sex, height, and weight, and the calculator estimates lean body mass using three published formulas: Boer, James, and Hume. Add your body-fat percentage to see weight × (1 − body fat), a much better estimate for anyone who lifts. Results appear in pounds, kilograms, and as a share of your bodyweight.

How do you use the lean body mass calculator?

Enter your sex, height, and weight. If you have a body-fat estimate from a DXA scan, skinfolds, or a smart scale, add it; for lifters, trust that result over the formulas. You can find all our calculators in one place.

1. Enter your sex, height, and weight.
2. Add your body-fat percentage if you have an estimate.
3. Compare the results in pounds, kilograms, and as a percentage of bodyweight.

The body-fat calculation is lean mass = weight × (1 − body-fat percentage ÷ 100). For example, at 180 lb and 15% body fat, lean mass is 180 × 0.85 = 153 lb. The percentage output is 100% minus body fat, so that same estimate is 85% of bodyweight. Use the same body-fat method each time you track change, since estimates from different methods may not line up. A smart-scale estimate can be several points off, but it still gives a more useful read on changes in body composition than a height-and-weight formula.

Which lean body mass formulas does the calculator use?

The calculator uses three height-and-weight equations from clinical research: Boer (1984), James (1976), and Hume (1966). They use weight in kilograms and height in centimeters, with separate equations for men and women, and were developed for medical uses such as drug dosing, not tracking muscle.

Boer men:   0.407W + 0.267H − 19.2
Boer women: 0.252W + 0.473H − 48.3

James men:   1.10W − 128(W/H)²
James women: 1.07W − 148(W/H)²

Hume men:   0.32810W + 0.33929H − 29.5336
Hume women: 0.29569W + 0.41813H − 43.2933

Here, W is weight in kg and H is height in cm. The equations estimate lean body mass (LBM), which includes a small amount of essential fat. That makes it different from fat-free mass (FFM), by up to 3% in men and 5% in women (Wikipedia: Lean body mass).

• Hume developed his equations from 29 men and 27 women, measuring lean mass through total body water using antipyrine. The estimates correlated with the measurements at 0.96 for men and 0.83 for women (Hume, 1966).
• The original James equation uses 128 for men; 120 is a common misquote (DICOM CP-1612). James also has a high-weight limit: for a 5′11″ (180 cm) man, it peaks above about 306 lb (139 kg), then predicts less lean mass as weight rises. The calculator flags James above its peak.
• Boer was recommended for estimating lean weight in obese patients receiving contrast-enhanced CT, a clinical setting. That study involved 55 patients with a BMI of at least 35 (Caruso et al., 2018).

How accurate are lean body mass formulas for lifters?

Not very. These equations only know height, weight, and sex, so a muscular and an untrained person of the same size get the same estimate. For a lean 180 lb (81.6 kg) lifter at 15% body fat, they fall 7–12 kg (15–26 lb) below the body-fat result.

These examples are calculations from the listed equations, not measurements. In the first example, the body-fat result is 180 × 0.85 = 153 lb; the James estimate is 138 lb, a 15 lb gap. For the woman, 140 × 0.75 gives 105 lb, while James estimates 102 lb. How far apart the results are depends on the person’s build and body-fat estimate.

The body-fat method can also be off—smart scales can miss by several points—but it responds to changes in body composition while the formulas never will. Lean mass includes water and glycogen, so carbs, salt, and creatine can move the reading. Judge monthly trends, not single measurements. Use the FFMI calculator to compare lean mass across heights, since the same lean weight means something different at different heights.

What is a normal lean body mass?

In US national data, fat makes up about 24% of bodyweight in 25-year-old men and 38% in women, so typical young adults are roughly 76% and 62% lean, respectively (Kelly, Wilson & Heymsfield, 2009). A median young man carries about 60 kg (132 lb) of fat-free mass at 5′10″.

• Among adults ages 18–34, median fat-free mass index is 18.9 kg/m² for men and 15.4 kg/m² for women (Schutz, Kyle & Pichard, 2002).
• Those medians work out to about 60 kg (132 lb) at 5′10″ for men and 42 kg (93 lb) at 5′5″ for women.
• At 5′10″, 160 lb of lean mass gives an FFMI of about 23, well above the young-men median of 18.9. At 6′2″, it is about 20.5. The shorter person’s higher index reflects more lean mass relative to height.
• If 74% of your bodyweight is lean, the other 26% is fat: slightly above the typical 25-year-old man’s 24%, and well below the typical woman’s 38%.

How should lifters use lean body mass?

Use lean mass as a trend line, measured the same way every four weeks. On a bulk, rising lean mass with modest fat gain suggests the plan is working; on a cut, falling lean mass suggests your deficit, protein, or training may not be holding muscle.

• Bulk check: compare the change in lean mass with the change in total weight. If your weight rises but lean mass stays flat, the extra weight may mostly be fat or normal fluctuation.
• Cut check: aim to hold lean mass; the protein intake calculator can base fat-loss protein on lean mass.
• Don’t chase a formula number; only a body-composition measurement can show muscle gain.

What if your lean mass isn't going up?

Then you aren’t building muscle, whatever the scale says. If lean mass stays flat across two or three monthly checks on a bulk, look at training dose and effort, calories, recovery, and how you spread protein through the day.

• Your weekly sets may be too low, or your sets may be too far from failure.
• You may be eating at maintenance instead of in a surplus.
• Sleep or protein built into meals may need attention.
• If your physique looks skinny fat: how to fix it, or you’re asking why you're not gaining muscle, start with the part of the plan that has been least consistent.

If you’re not sure which issue is holding you back, find out what's stopping your muscle gain. A diagnosis can help rank the likely causes so you have a clear place to start.

FAQ

How do you calculate your lean mass?

Multiply your weight by (1 − body-fat percentage). At 180 lb and 15% body fat, that gives 153 lb. Without a body-fat estimate, Boer, James, and Hume formulas guess from height and weight, but they can’t distinguish muscle from fat and tend to underestimate lean lifters.

What is a normal lean mass?

In US national data, fat is about 24% of bodyweight in 25-year-old men and 38% in women, so typical lean mass is about 76% and 62%, respectively (Kelly, Wilson & Heymsfield, 2009). By height, a median young man has roughly 60 kg (132 lb) of fat-free mass at 5′10″ (Schutz, Kyle & Pichard, 2002).

Is 160 lbs of lean mass good?

It depends on height. At 5′10″, 160 lb gives an FFMI of about 23, above the young-men median of 18.9 and close to the average of college football players, 23.6 (Fields et al., 2024). At 6′2″, it is about 20.5, still above the median.

Is 74% lean body mass good?

That means 26% body fat. For a man, it is slightly above the typical 25-year-old’s 24% in US data; for a woman, it is much leaner than the typical 25-year-old’s 38% (Kelly, Wilson & Heymsfield, 2009).