Lean Mass Calculator: Find Your Muscle Mass
A lean mass calculator estimates your lean body mass: everything in your body that is not storage fat, including muscle, bone, water, organs and essential fat. Enter your height, weight and sex to compare three published estimation formulas, Boer, James and Hume, side by side, or enter a body fat percentage you already trust for a direct calculation, then read the calorie and macro targets the tool builds from the result for your fitness goal.
Lean Mass Calculator
Boer, James and Hume side by side, plus calorie and macro targets for your goal
| Formula | Lean mass | Implied body fat |
|---|---|---|
| Boer (1984) | ||
| James (1976) | ||
| Hume (1966) |
Fitness estimates only, not medical advice. These formulas estimate lean body mass from population data and cannot replace direct measurement or professional assessment. Consult a qualified trainer, dietitian, or clinician before starting a new training or nutrition program, especially if you have a health condition.
Quick Answer
The short answer: for a man weighing 80 kg at 180 cm, the three formulas give Boer 61.4 kg, James 62.7 kg and Hume 57.8 kg, averaging 60.6 kg (133.7 lbs) of lean mass. That implies 19.4 kg of fat mass (24.2% body fat), a BMI of 24.7, and a maintenance target of 2,604 kcal with 114 g of protein. Enter a body fat percentage and the headline switches to the direct figure: weight × (1 − body fat % ÷ 100).
- Quick Answer
- How to use the lean mass calculator
- The three formulas, in full
- Why the formulas disagree, and how far apart they run
- Lean body mass versus fat-free mass
- What the tool does with your lean mass
- Worked example, checked against the tool
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use the lean mass calculator
Pick your sex and unit system first, then enter height and weight, and press Calculate. Sex is not optional, because all three formulas use sex-specific coefficients. Height must land between 100 and 250 cm and weight between 25 and 400 kg after conversion; values outside those bands return a units warning rather than a made-up result, which catches the common mistake of typing pounds into the kilogram field.
The body fat field is the one optional input that changes the headline. Leave it blank and the headline is the average of the three formulas. Fill it in and the headline becomes the direct calculation, while the formula rows still print underneath. The fitness goal never changes lean mass; it only changes the calorie and macro targets built from it.
The three formulas, in full
All three formulas take weight in kilograms (W) and height in centimetres (H) and return lean mass in kilograms. Boer and Hume are straight lines in weight and height. James is the odd one out: it subtracts a term built on the squared weight-to-height ratio, which is why it drifts furthest from the other two at unusual body sizes.
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407 × W + 0.267 × H − 19.2 | 0.252 × W + 0.473 × H − 48.3 |
| James (1976) | 1.1 × 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 |
| Direct, from body fat % | W × (1 − body fat % ÷ 100), used for the headline only when you enter a body fat percentage | |
Boer published his equation in 1984 to normalise body fluid volumes, and it is still the one most used in clinical work such as drug dosing. Hume’s 1966 equation is the oldest of the three; the James equation comes from 1976 obesity research. None of the three measures you. Each predicts the average lean mass of study participants who shared your height, weight and sex, which is why the honest output is the spread across all three. For the height-scaled version, the fat-free mass index calculator divides lean mass by height squared.
Why the formulas disagree, and how far apart they run
The formulas disagree because they were fitted to different people with different methods, and the gap is bigger than most pages admit. In the worked example below, a completely ordinary 80 kg man at 180 cm, the spread is 4.9 kg: James says 62.7 kg, Hume says 57.8 kg. Any source telling you the three agree within about 2 kg for typical adults is understating it; the tool itself prints a note to that effect, and you should trust the three rows on your own result over that note. At a BMI of 35 or higher the tool swaps the note for a specific warning about James, because the squared term starts to dominate there. In the parity testing behind this page, a 130 kg man at 170 cm got 79.1 kg from Boer, 68.1 kg from James and 70.8 kg from Hume, a spread of 11 kg.
Two quirks at high body weight are worth knowing. First, the fitness category is a body fat category wearing a fitness label: the top band prints as “Above average”, meaning above-average body fat, not fitness. Second, the typical range is a fixed share of your weight (75 to 90% for men, 65 to 85% for women), so at very high weights the formula average can fall well below that range. Both are printed exactly as the tool produces them. The CDC BMI categories behind the BMI label are a population screen, not a diagnosis.
Lean body mass versus fat-free mass
Fat-free mass excludes every gram of fat. Lean body mass keeps the essential fat inside organs, bone marrow and cell membranes, roughly 2 to 5% of body weight for men and 10 to 13% for women. These equations estimate lean body mass in the practical sense, total weight minus storage fat, so a formula result sits slightly above a DEXA fat-free mass figure for the same person. For protein targets the difference rarely changes a decision. For medication dosing it can, which is why clinical use pairs the estimate with judgement.
What the tool does with your lean mass
The headline lean mass feeds three further calculations. First, a Katch-McArdle basal metabolic rate: BMR = 370 + 21.6 × lean mass in kg, keying resting burn to the tissue that burns it rather than to total weight. Second, the BMR is multiplied by a fixed moderate activity factor of 1.55. That factor is baked in and never changes with your goal, so very sedentary or very active readers should adjust the calorie target, not the lean mass. Third, your goal applies its own calorie adjustment and macro split:
| Goal | Calories | Protein | Carbs / fat share |
|---|---|---|---|
| Maintain current mass | Maintenance (BMR × 1.55) | 0.85 g per lb of lean mass | 45% carbs, 30% fat |
| Build lean muscle | Maintenance + 10% | 1.0 g per lb | 45% carbs, 25% fat |
| Lose fat, keep muscle | Maintenance − 15% | 1.2 g per lb | 35% carbs, 25% fat |
| Body recomposition | Maintenance | 1.1 g per lb | 40% carbs, 25% fat |
Fat grams come from the fat share of calories at 9 kcal per gram, protein from the per-pound target at 4 kcal per gram, and carbs take the calories that remain. That is why carb grams do not always match the carb share exactly: protein is fixed first, by lean mass, not by calories. The multipliers sit inside the evidence range in the International Society of Sports Nutrition position stand on protein and exercise, and the baseline line in every result always uses 0.85 g per lb, whatever goal you picked. For a gaining phase, the clean bulk calculator and the bulking calories calculator approach the surplus from total weight and activity instead.
Worked example, checked against the tool
A man of 80 kg at 180 cm, no body fat entered, goal set to maintain, produces the following. Boer: 0.407 × 80 + 0.267 × 180 − 19.2 = 61.42 kg, printed as 61.4 kg (135.4 lbs), implying 18.6 kg of fat (23.2%). James: 1.1 × 80 − 128 × (80 ÷ 180)² = 62.72 kg, printed as 62.7 kg (138.3 lbs), implying 17.3 kg of fat (21.6%). Hume: 0.32810 × 80 + 0.33929 × 180 − 29.5336 = 57.79 kg, printed as 57.8 kg (127.4 lbs), implying 22.2 kg of fat (27.8%). The headline average is 60.6 kg (133.7 lbs): 75.8% lean mass, 19.4 kg of fat (24.2%), BMI 24.7 (Normal), fitness category Average.
The follow-on figures check the same way. BMR = 370 + 21.6 × 60.64 = 1,679.9 kcal, and 1,679.9 × 1.55 rounds to the printed 2,604 kcal target. Protein is 133.7 lb × 0.85 = 114 g. Fat takes 30% of 2,604 kcal, 781 kcal or 87 g. Carbs take the rest: (2,604 − 456 − 781) ÷ 4 = 342 g. The typical range line prints 60 kg (132.3 lbs) to 72 kg (158.7 lbs), exactly 75% and 90% of 80 kg. A second check: a 65 kg woman at 165 cm entering 25% body fat gets a direct headline of 48.8 kg (107.5 lbs), while her formula rows print Boer 46.1 kg, James 46.6 kg and Hume 44.9 kg underneath.
What competing calculators miss
Most lean mass calculators show one formula and hide the disagreement. CalculatePilot and CalcMacro print all three, the right instinct, but neither builds calorie or macro targets from the result, and neither warns how wide the spread gets at high body weight. Pearson flags a formula returning more lean mass than total weight, a known regression failure at extreme ratios, yet stops at the flag. Several simpler tools skip the direct body fat route entirely, even though a trusted reading beats every regression here. This page closes those gaps: implied fat per formula row, a direct headline from body fat, a stated activity factor, auditable protein multipliers, and an honest 4.9 kg spread in its own worked example. No calculator measures you, though. DEXA, hydrostatic weighing and careful calipers measure; formulas estimate.
Frequently asked questions
Lean body mass is everything in your body that is not storage fat: muscle, bone, water, organs, and essential fat. Clinicians use it to dose some medications, and trainers use it to set protein and calorie targets.
None is universally best, because all three are population estimates rather than measurements. Boer (1984) is the most widely used clinically. For the worked example in this article the three answers span 57.8 to 62.7 kg, so read the result as a range, not a single exact figure. A DEXA scan is the direct measurement alternative.
Multiply your weight by one minus your body fat fraction: lean mass = weight × (1 − body fat % ÷ 100). At 80 kg and 18% body fat, lean mass = 80 × 0.82 = 65.6 kg. The tool does this automatically when you enter a body fat percentage, and your entered figure becomes the headline result.
They were fitted to different populations with different math. Boer and Hume are linear in weight and height, while James adds a squared weight-to-height term that behaves differently at extreme body sizes. Disagreements of several kilograms are normal, and the gap widens at very high body weight.
It varies by sex and training. The tool compares you with a typical range of 75 to 90% of body weight for men and 65 to 85% for women. Falling below that band usually reflects higher body fat rather than low muscle alone, so read it alongside the body fat percentage the tool reports.
The targets built into this tool are 0.85 g per lb of lean mass for maintenance, 1.0 g for building muscle, 1.1 g for recomposition, and 1.2 g when cutting to preserve muscle. At the worked example of 133.7 lb of lean mass, maintenance protein is 114 g per day.
Yes. Switch the unit selector to imperial and enter feet, inches, and pounds. The tool converts internally at 2.54 cm per inch and 0.453592 kg per pound, and it shows every result in both kg and lbs.
No. The Boer, James, and Hume formulas were developed and validated in adults, and the tool rejects heights below 100 cm. Pediatric lean mass uses different equations, which this tool does not implement.
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References
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. American Journal of Kidney Diseases, 1984. Accessed October 22, 2026.
- Hume R. Prediction of lean body mass from height and weight. Journal of Clinical Pathology, 1966. Accessed October 22, 2026.
- Centers for Disease Control and Prevention. Adult BMI Categories. Accessed October 22, 2026.
- Centers for Disease Control and Prevention. Assessing Your Weight. Accessed October 22, 2026.
- National Institute of Diabetes and Digestive and Kidney Diseases. Understanding Adult Overweight and Obesity. Accessed October 22, 2026.
- Jäger R, et al. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition, 2017. Accessed October 22, 2026.
- 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. British Journal of Sports Medicine, 2018. Accessed October 22, 2026.
- Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Medicine and Science in Sports and Exercise, 2016. Accessed October 22, 2026.
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine, 1995. Accessed October 22, 2026.
- National Library of Medicine, StatPearls. Physiology, Body Mass Index. Accessed October 22, 2026.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services, RealFood.gov. Dietary Guidelines for Americans, 2025-2030: Protein, Dairy, and Healthy Fats. Protein target of 1.2-1.6 grams per kilogram of body weight per day; MyPlate was retired in January 2026 and its guidance succeeded by this site. Accessed October 22, 2026.