How much of your body weight is not fat? That share is your lean body mass. It covers muscle, bone, organs, and water. The Lean Body Mass Calculator estimates it from your sex, weight, and height.
It suits lifters, runners, and anyone planning protein or calorie targets. The tool runs three well-known formulas side by side. It then uses their average to estimate body fat percentage.
Table of Contents
- What the Lean Body Mass Calculator Does
- How to Use It, Step by Step
- The Three Formulas
- Worked Examples
- How to Read the Results
- Why the Formulas Differ
- Limitations and Assumptions
- FAQs About the Lean Body Mass Calculator
What the Lean Body Mass Calculator Does
You choose a sex and a unit system, then enter weight and height. Results update as you type. Four result cards appear.
- Boer Formula: lean mass from the Boer equation.
- James Formula: lean mass from the James equation.
- Hume Formula: lean mass from the Hume equation.
- Est. Body Fat % (from average lean mass): your weight left over after the average lean mass is removed, as a share.
Lean mass shows in lbs for Imperial and kg for Metric. The tool has no copy or export button.
How to Use It, Step by Step
- Choose Sex: Male or Female. The equations use different constants for each.
- Choose Unit System: Metric (kg / cm) or Imperial (lbs / in). Imperial is the default.
- Enter Weight. It is labeled lbs or kg to match the unit. The sample is 175 lbs.
- Enter Height, labeled in or cm. The sample is 70 inches.
- Read the four cards. The Calculate Lean Mass button also works, but you do not need it.
Weight must be 30 to 300 kg (66 to 660 lbs). Height must be 100 to 250 cm (40 to 98 in). Changing the unit system converts what you typed to one decimal and updates the cards at once.
The Three Formulas
The tool converts pounds to kilograms and inches to centimeters first. Then W is weight in kilograms and H is height in centimeters. Each formula returns lean mass in kilograms.
Boer + James + Hume÷ Valid Count =Average Lean Mass→Est. Body Fat %
| Formula | Male | Female |
|---|
| Boer | 0.407 W + 0.267 H - 19.2 | 0.252 W + 0.473 H - 48.3 |
| James | 1.1 W - 128 (W / H)^2 | 1.07 W - 148 (W / H)^2 |
| Hume | 0.3281 W + 0.33929 H - 29.5336 | 0.29569 W + 0.41813 H - 43.2933 |
Body fat estimate = (W - average lean mass) ÷ W × 100. The average uses only the formulas that give a possible value. Imperial results divide kilograms by 0.45359237, the exact kilograms per pound.
Worked Examples
Carlos is male, weighs 175 lb, and stands 70 in tall. That equals 79.38 kg and 177.8 cm. These are also the sample values.
| Result | Calculation | Output |
|---|
| Boer | 0.407 × 79.38 + 0.267 × 177.8 - 19.2 | 60.58 kg, shown as 133.6 lbs |
| James | 1.1 × 79.38 - 128 × (79.38 ÷ 177.8)^2 | 61.80 kg, shown as 136.3 lbs |
| Hume | 0.3281 × 79.38 + 0.33929 × 177.8 - 29.5336 | 56.84 kg, shown as 125.3 lbs |
| Body Fat | (79.38 - 59.74) ÷ 79.38 | 24.7% |
Aisha is female, weighs 140 lb, and is 65 in tall. The tool shows Boer 101.0 lbs, James 101.5 lbs, and Hume 98.1 lbs. Her estimated body fat is 28.4%.
How to Read the Results
Each lean mass card answers the same question with a different equation. The numbers usually land within a few pounds of each other. The spread tells you how uncertain the estimate is.
The body fat card is a derived figure. It uses the average of the valid formulas, not any single card. It is not a measurement and can differ from skin-fold, scan, or scale readings.
Use lean mass to size up protein needs with the Protein Intake Calculator. For a body fat check from tape measures, try the Body Fat Percentage Calculator. The two estimates will rarely match exactly.
Why the Formulas Differ
Each equation was fitted to a different group of people. So each one leans a little higher or lower for a given body size. Hume often gives the lowest value in these examples.
The James formula has a known weak spot. It subtracts a term that grows with the square of your weight to height ratio. For very heavy bodies, lean mass can fall as weight rises, and can even turn negative.
The tool guards against this. A formula that gives zero, a negative value, or more than your body weight shows N/A and is left out. Take a female at 160 cm and 200 kg. James shows N/A, and the average uses Boer (77.8 kg) and Hume (82.7 kg). A message says the average uses 2 of 3 formulas.
Treat the average as a rough guide. Track the trend over weeks with the same unit and the same inputs. Pair it with the BMR and TDEE Calculator and the Macro Calculator when planning meals.
The BMI Calculator gives a different view of the same height and weight. For more tools on body numbers, visit the Health and Fitness tools hub.
Limitations and Assumptions
- The tool offers only Male and Female profiles. It has no age, activity, or body type input.
- The formulas were built for typical adult bodies. Results for children, pregnancy, and very large or very small frames are less reliable.
- A blank, zero, or out-of-range entry turns every card to N/A and shows a message with the allowed range.
- Body fat shows N/A when it falls outside 0 to 100 percent or when no formula is valid.
- The formulas do not account for muscle mass, age, or training history.
These figures are general estimates, not a diagnosis. Do not use them to set an extreme diet. People with an eating disorder, a medical condition, or pregnancy need professional advice. Talk with a doctor or dietitian before major changes.
FAQs About the Lean Body Mass Calculator
What is lean body mass?
It is your total weight minus fat mass. That includes muscle, bone, organs, skin, and body water. It is larger than muscle mass alone.
Which formula is the most accurate?
None is proven best for every person. They are all estimates from height, weight, and sex. Comparing the three shows a likely range.
Why are my results in lbs instead of kg?
Imperial is the default unit system. Switch the Unit System menu to Metric and the cards change to kg.
Can this tool replace a body scan?
No. A DEXA scan or similar test measures body composition directly. This tool only applies equations to two body measurements.
Why does my James result show N/A?
The James equation can fail at very high weight for the given height. In that case, rely on the Boer and Hume cards, or ask a professional.
How often should I recalculate?
Every few weeks is enough for most people. Daily changes in body water can move the scale without changing real lean tissue.
This calculator gives general estimates only and is not medical advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.