Lean Body Mass Calculator
Estimate your lean body mass (LBM) from weight, height and sex using the Boer, James, Hume and Peters formulas, or get an exact value from your body fat percentage.
Estimate Your Lean Body Mass from Weight and Height
Lean Body Mass Formula
Where:
- \(W\) — total body weight in kilograms
- \(H\) — height in centimetres
- \(BF\) — body fat percentage (0–100)
- \(LBM\) — lean body mass in kilograms
Each adult formula estimates lean body mass from weight and height with different regression coefficients. When no body fat percentage is entered, the calculator averages the applicable formulas; when one is entered, the direct body-fat equation takes priority because it reflects your actual composition.
Lean Body Mass – Calculation Example
Example: a 35-year-old man weighing 80 kg at a height of 180 cm, with no measured body fat percentage.
\[ LBM_{Boer} = 0.407 \times 80 + 0.267 \times 180 - 19.2 = 61.4 \text{ kg} \]
\[ LBM_{James} = 1.1 \times 80 - 128 \times (80/180)^2 = 62.7 \text{ kg} \]
\[ LBM_{Hume} = 0.32810 \times 80 + 0.33929 \times 180 - 29.5336 = 57.8 \text{ kg} \]
\[ LBM_{avg} = \frac{61.4 + 62.7 + 57.8}{3} \approx 60.6 \text{ kg} \]
Result: average lean body mass ≈ 60.6 kg, fat mass ≈ 19.4 kg, implied body fat ≈ 24.2%.
How to Use the Calculator
- Select your Sex — male or female. The adult formulas use different coefficients for each.
- Enter your Age in years (between 2 and 110). At age 13 or younger, the calculator automatically switches to the Peters pediatric formula.
- Enter your Weight (W) and choose kg or lb as the unit.
- Enter your Height (H) and choose cm or in as the unit.
- If you know it, enter your Body fat % (optional). This overrides the formula estimates and gives a direct calculation.
- Read the results: each formula's LBM, the average LBM, the implied body fat percentage, and your fat mass in kilograms.
What Is Lean Body Mass?
Lean body mass (LBM) is everything in your body that is not fat: skeletal muscle, bones, organs, skin, and body water. It is not the same as muscle mass alone — muscle is the largest single component of LBM, but water alone makes up roughly 60–70% of it. Because fat tissue contributes relatively little to metabolism compared with lean tissue, LBM is a better basis than total weight for many calculations in medicine, nutrition and sport science.
Doctors use LBM to size drug dosages (some anesthetics and chemotherapy agents are dosed per kg of lean mass), to assess malnutrition and muscle wasting, and to interpret kidney function. In fitness, LBM is central to tracking progress: if your weight stays the same but your LBM rises, you have replaced fat with muscle. It is also the reference for protein targets — a common guideline is 1.6–2.2 g of protein per kg of lean mass, which you can put into practice with our Macros Calculator.
The three adult equations used here come from clinical studies. Boer's formula (1984) was derived from body water measurements and is widely used in pharmacokinetics. James' formula (1976) underlies many hospital nutrition standards, and Hume's formula (1966) was developed from total body potassium and water studies. They typically agree within 10–15% for average adults, which is why the calculator shows all of them plus their average — the spread gives you a sense of the estimate's uncertainty.
To interpret your result: a typical healthy adult woman has an LBM of roughly 35–45 kg and a man roughly 50–65 kg, but values outside these ranges can be perfectly healthy. What matters more is the trend over time and the ratio of lean to total mass. If you want to quantify your build more precisely, combine your LBM with height in our Fat-Free Mass Index (FFMI) Calculator.
Assumptions and Limitations
- Weight and height cannot reveal composition. All four formulas estimate LBM statistically from a reference population. Two people with the same inputs get the same answer, even if one is muscular and the other is not.
- A body fat measurement is always better. If you know your body fat percentage, enter it — the result is then a direct calculation, not an estimate. You can find yours with our Body Fat Calculator.
- Accuracy decreases at body extremes. The Boer, James and Hume equations were fitted on people near average weight. In very muscular individuals they underestimate LBM, and in people with obesity James' formula in particular can fail; some drug-dosing guidelines cap the weight used in such equations.
- Children use a different equation. Under age 14, the adult formulas do not apply; the calculator uses the Peters formula, which was derived from total body water studies in pediatric patients.
- Don't confuse LBM with fat-free mass. They differ slightly: fat-free mass excludes all fat, while lean body mass includes a small amount of essential lipid (about 2–3% of body weight). For everyday purposes the two are used interchangeably.
- Fluid status matters. Because water is the largest part of LBM, dehydration or oedema shifts the result. Weigh yourself under consistent conditions, ideally in the morning, for comparable numbers over time.
Frequently Asked Questions
What is the difference between lean body mass, fat-free mass and muscle mass?
They are not identical. Lean body mass includes muscle, bone, organs, skin, water and about 2–3% essential fat. Fat-free mass strip out all fat, including that essential lipid, so it is slightly lower. Muscle mass is only the skeletal muscle portion — roughly half of your LBM. None of the formulas on this page measure muscle alone, so don't use your LBM number as your "muscle weight."
Which of the four formulas is the most accurate?
For children under 14, only the Peters formula is valid. For adults, none is universally best: Boer was fitted on normal-weight hospital patients and is the standard in drug dosing, James tends to run slightly higher, and Hume was derived from potassium and body-water studies, so it often reads lower. They usually agree within 10–15%; a spread wider than that on your inputs suggests you sit outside the reference population. A measured body fat percentage beats all of them.
How do I calculate lean body mass by hand?
Use the Boer equation as the simplest option: for men \( LBM = 0.407 \times W + 0.267 \times H - 19.2 \) and for women \( LBM = 0.252 \times W + 0.473 \times H - 48.3 \), with weight \(W\) in kg and height \(H\) in cm. If you weigh 70 kg: first multiply the weight term, then the height term, then add both and subtract the constant. If you only know pounds and inches, convert first: 1 lb = 0.4536 kg and 1 in = 2.54 cm.
What is a good lean body mass for my height and weight?
Rather than an absolute number, look at the percentage of total weight: an LBM around 75–85% of body weight is typical for lean men, and 65–75% for lean women. Athletes can be even higher. More useful still is the trend: a stable or rising LBM while total weight falls means you are losing fat, not muscle. Combine your LBM with height in an FFMI calculation to see whether your build is typical, muscular or below average for your frame.
How do I convert my LBM result to pounds?
Multiply the kilogram value by 2.20462. For example, an LBM of 60.6 kg equals \( 60.6 \times 2.20462 \approx 133.6 \) lb. To go the other way, divide pounds by 2.20462 or multiply by 0.4536. Be careful not to apply conversions twice: if you entered your weight in lb or height in inches, the calculator converts them internally, so the result it shows is already correct and needs no further adjustment.
Why did the calculator show an error or a strange result for my inputs?
The most common cause is a unit mix-up — entering pounds as kilograms or inches as centimetres. The formulas also break down at body extremes: in people with obesity, James' equation can produce nonsensical values (sometimes even negative ones), and drug-dosing guidelines cap the weight plugged into such equations. If any formula returns an LBM below zero or above your total weight, the inputs lie outside the range the equation was fitted for. In that case, a body-fat-based calculation is the better route.
Why do I need lean body mass for protein and calorie targets?
Fat tissue burns far fewer calories than lean tissue, so basing intake on LBM instead of total weight is more accurate for people with both high and low body fat. Protein guidelines of 1.6–2.2 g per kg of lean mass daily are typical for building or preserving muscle, and some medication doses are also scaled to lean mass. Note these are general rules, not medical advice — if you have kidney disease or are on weight-sensitive medication, ask your doctor or dietitian before applying them.