Lean Body Mass Calculator - Boer, James & Hume Formulas

Lean body mass in one sentence

Lean body mass (LBM) is everything you weigh that isn't fat: muscle, bone, organs, blood, skin, and water. If you weigh 180 lb and carry 36 lb of fat, your lean mass is 144 lb.

The clean way to get it is to measure body fat (DEXA, hydrostatic weighing, or at least a tape-measure method) and subtract. When you don't have a body fat number, clinicians estimate LBM from height, weight, and sex. That's what the Boer, James, and Hume equations do, and it's why they show up in anesthesia dosing, contrast dye dosing for CT scans, and kidney drug calculations.

The three equations

All use weight (W) in kilograms and height (H) in centimeters, and give LBM in kilograms.

Boer (1984). Men: 0.407W + 0.267H - 19.2. Women: 0.252W + 0.473H - 48.3. Derived from measured body composition and the most common modern clinical reference; radiology studies on contrast dosing in obese patients often prefer it.

James (1976). Men: 1.1W - 128(W/H)². Women: 1.07W - 148(W/H)². Published in a UK government nutrition report and widely used in pharmacology. It has a known flaw: because of the squared term, it peaks and then declines as weight rises. For men that turn happens around a BMI of 43, for women around 36. Past that point it says adding weight reduces lean mass, which is obviously wrong.

Hume (1966). Men: 0.32810W + 0.33929H - 29.5336. Women: 0.29569W + 0.41813H - 43.2933. Built from total body water measurements, and the oldest of the three.

If you do know your body fat percentage, enter it. The calculator then shows the direct answer, weight x (1 - body fat %), above the formula estimates.

Worked example

A man, 5'10" and 180 lb. Convert: 70 in = 177.8 cm; 180 lb = 81.6 kg.

Boer: 0.407 x 81.6 + 0.267 x 177.8 - 19.2 = 33.2 + 47.5 - 19.2 = 61.5 kg (135.6 lb). James: 1.1 x 81.6 - 128 x (81.6 / 177.8)² = 89.8 - 27.0 = 62.8 kg (138.5 lb). Hume: 0.3281 x 81.6 + 0.33929 x 177.8 - 29.5336 = 57.6 kg (126.9 lb).

The formulas agree within about 12 lb, implying roughly 23-30% body fat. If he's actually a lifter at 12% fat, his true lean mass is about 158 lb, far above all three. That's the core limit: these equations assume an average build.

Now a woman, 5'4" and 260 lb (BMI 44.6). Boer gives 128.6 lb and Hume 131.3 lb, but James drops to 106.5 lb, less than it would give her at a lower weight. This is the James breakdown in action, and the calculator flags it.

What lean body mass is good for

Protein targets. Setting protein per pound of lean mass instead of total weight avoids overshooting for people with higher body fat. Our protein calculator can take it from there.

Tracking a cut or a bulk. If weight drops 10 lb and your measured body fat drops too, you can see how much of the loss was fat versus lean tissue. Use the same measurement method each time.

Understanding medical dosing. If a care team mentions "lean body weight" for a drug, this shows roughly what they're working with. Dosing decisions stay with them.

Common mistakes

Confusing lean mass with muscle. Muscle is only about half of lean mass. The rest is bone, organs, and water, which is why LBM swings a few pounds with hydration and glycogen.

Using James at high BMI. Above a BMI of about 35, lean on Boer or Hume.

Expecting formulas to see muscle. A bodybuilder and a sedentary office worker of the same height and weight get identical formula results. Measure body fat if you train seriously.

Mixing units. Every equation here needs kilograms and centimeters. Plugging in pounds and inches gives nonsense; the calculator converts for you in US mode.