What is lean body mass?
Lean body mass (LBM) is everything in your body that is not fat: muscle, bone, organs, connective tissue and body water. Because muscle is the part of LBM that responds most to training, people often use “lean mass” and “muscle mass” as if they meant the same thing. They do not. Your skeletal muscle weighs less than your lean body mass, often by a wide margin.
Tracking LBM alongside body weight tells you more than the scale alone. If your weight stays flat while your lean mass rises and your fat mass falls, you are changing body composition even though the scale does not move.
How lean body mass is calculated
This calculator offers three published equations and one direct method. All weights are in kilograms and all heights in centimetres inside the equations. If you choose imperial units, the calculator converts for you.
| Method | Men | Women |
|---|---|---|
| Boer (1984) | 0.407 × W + 0.267 × H − 19.2 | 0.252 × W + 0.473 × H − 48.3 |
| James (1976) | 1.10 × W − 128 × (W ÷ H)² | 1.07 × W − 148 × (W ÷ H)² |
| Hume (1966) | 0.32810 × W + 0.33929 × H − 29.5336 | 0.29569 × W + 0.41813 × H − 43.2933 |
| Body fat % | W × (1 − body fat ÷ 100) | |
W = weight in kg, H = height in cm.
Worked example
A man who weighs 70 kg and is 175 cm tall, using the Boer formula: 0.407 × 70 + 0.267 × 175 − 19.2 = 56.0 kg of lean body mass. His fat mass is 70 − 56.0 = 14.0 kg, which is about 20% body fat.
Which method should I use?
- Boer or James when you only know your weight and height. Compare both to see the spread.
- Body fat percentage when you have a measurement you trust, such as a DEXA scan. It reflects your actual build rather than an average one.
- Be cautious with the formulas at high body fat or very high muscle mass. They assume an average build. The James equation stops being valid at high body weights, and the calculator tells you when that happens.
Lean body mass vs. muscle mass
Lean body mass includes skeletal muscle, but also bone, organs, skin, connective tissue and the water held in all of them. In a typical adult, skeletal muscle is only part of the total. A calculator that reports “muscle mass” and one that reports LBM are therefore not measuring the same thing, and their numbers should not be compared directly.
What is FFMI?
Fat-Free Mass Index relates lean mass to height, so you can compare people of different sizes:
FFMI = lean body mass (kg) ÷ height² (m²)
Normalised FFMI = FFMI + 6.1 × (1.8 − height in m)
The normalised version was proposed by Kouri and colleagues in 1995 to correct for the fact that taller people tend to have a higher raw FFMI. In that study of athletes, those not using anabolic steroids rarely exceeded a normalised FFMI of roughly 25. That is one sample, not a rule, and your FFMI is only as accurate as the lean mass estimate behind it.
Protein based on your size
The calculator shows 1.6 to 2.2 grams of protein per kilogram of body weight per day. This comes from a 2018 meta-analysis by Morton and colleagues, which found that extra protein stopped adding muscle gain in resistance-trained adults at around 1.6 g/kg, with the upper end of the confidence interval near 2.2 g/kg. It is a general range for healthy adults who train, not a prescription. If you have kidney disease or another medical condition, or are pregnant, ask your doctor before changing your protein intake.
Accuracy and limits
Every number here is an estimate. The equations were fitted to groups of people decades ago and use only sex, height and weight. Two people with the same measurements but very different builds will get the same answer. For a direct measurement, ask about a DEXA scan, or use the same method consistently over time so you can track the change rather than the absolute number.
Frequently asked questions
What is lean body mass?
Lean body mass (LBM) is your total body weight minus your body fat. It includes muscle, bone, organs, connective tissue and body water.
How do you calculate lean body mass?
You can estimate it with an equation based on weight, height and sex (Boer, James or Hume), or subtract fat mass from total weight if you know your body fat percentage. Fat mass is body weight multiplied by body fat percentage.
Is lean body mass the same as muscle mass?
No. Muscle is the largest changeable part of lean body mass, but LBM also includes bone, organs, skin, connective tissue and water. Your skeletal muscle mass is always lower than your LBM.
Which formula is the most accurate?
None of them measures anything directly. Boer and James are among the most widely cited, and Hume is an older alternative. They often differ by a few kilograms, which is why this calculator shows all three side by side. If you have a reliable body fat measurement, such as a DEXA scan, the body fat method will usually be closer to the truth.
What is FFMI?
Fat-Free Mass Index (FFMI) is lean mass divided by height squared (kg/m²). It lets you compare lean mass between people of different heights. A normalised version adjusts for height further.
How accurate is a lean body mass calculator?
These are estimates, not measurements. Formulas only know your weight, height and sex, so they cannot tell a muscular person from a person of the same size with more body fat. DEXA scans, and to a lesser extent skinfold and bioimpedance methods, measure body composition more directly.
How much protein do I need?
A common evidence-based range for people doing resistance training is 1.6 to 2.2 grams of protein per kilogram of body weight per day. Your own needs depend on your goals, training, age and health, so check with a doctor or registered dietitian if you have a medical condition.
Does this calculator save my data?
No. All calculations run in your browser. Your measurements are not sent to any server and are not stored.
References
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984;247(4):F632–F636.
- James WPT. Research on Obesity. London: Her Majesty’s Stationery Office; 1976.
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol. 1966;19(4):389–391.
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med. 1995;5(4):223–228.
- Morton RW, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376–384.
Important information
This calculator gives mathematical estimates for education and general information. It is not a medical device and does not diagnose, treat or prevent any condition. Talk to a doctor or registered dietitian before making decisions about your health, diet or training.