Estimate lean body mass from body fat percentage or compare the Boer, James, and Hume height-and-weight formulas. See lean mass, estimated fat mass, formulas, and a visual body-composition breakdown.
Choose a method below and press Calculate for an instant estimate.
Lean body mass is a body-composition concept used to describe the portion of body weight that is not storage body fat, although terminology varies across clinical and research settings.
In everyday fitness language, people often use "lean body mass," "lean mass," and "fat-free mass" as if they mean exactly the same thing. They are closely related, but technical definitions can differ.
An Obesity Medicine Association clinical practice statement explains that fat-free mass excludes body fat, while lean body mass can include small amounts of essential fat. DXA reports may also use the term "lean mass" differently by separating bone mineral content from lean soft tissue.
That distinction matters because a calculator cannot directly measure any of those body compartments.
What it can do is estimate them.
Lean body mass is a broader concept than skeletal muscle. It is a grouping of several non-fat tissues and fluids, not a single tissue type.
| Component | Role |
|---|---|
| Skeletal muscle | One part of lean body mass, not the whole of it |
| Bone / bone mineral | Structural tissue included in the lean portion |
| Organs | Non-fat tissue included in the lean portion |
| Body water | Fluid included in the lean portion |
| Connective tissue | Non-fat tissue included in the lean portion |
This calculator gives you two ways to estimate lean body mass. If you already know your body-fat percentage, it can subtract estimated fat mass from your total weight. If you do not know body fat, it can compare three classic height-and-weight prediction equations: Boer, James, and Hume.
If you have a body-fat percentage, the arithmetic is simple:
Suppose you weigh 80 kilograms and estimate that 18% of your body weight is fat.
Fat mass: 80 × 0.18 = 14.4 kg
Lean estimate: 80 − 14.4 = 65.6 kg
You can also calculate it directly: 80 × 0.82 = 65.6 kg
The direct method follows one path: start from total weight, apply the body-fat percentage, subtract the resulting fat mass, and what remains is the lean estimate.
| Step | What Happens |
|---|---|
| 1. Weight | Start with total body weight |
| 2. Fat % | Apply the known body-fat percentage |
| 3. Fat Mass | Weight × fat % = estimated fat mass |
| 4. Remaining Lean Estimate | Weight − fat mass = estimated lean mass |
This method uses more individualized information than a height-and-weight equation, but the result is only as accurate as the body-fat percentage used to calculate it.
A body-fat percentage from DXA, skinfolds, bioelectrical impedance, circumference measurements, or another method can produce different results in the same person.
That is why this page describes the result as an estimate rather than an exact measurement.
If you do not know your body-fat percentage, prediction equations can estimate lean body mass from easier measurements such as height and weight.
This calculator compares three well-known formulas.
Peter Boer published an equation in 1984 while studying the use of estimated lean body mass to normalize body-fluid volumes.
This calculator uses Boer as the primary height-and-weight result, but it should not be presented as a direct measurement.
The James equations use a different mathematical structure:
Because the weight-to-height term is squared, the equation can behave differently from the linear Boer and Hume equations, particularly at body-size extremes.
R. Hume published height-and-weight lean-body-mass prediction equations in 1966.
Hume's original study found that lean-body-mass estimates derived from body-water measurements could be predicted from height and weight in the study sample.
These formulas were developed from different populations, methods, and eras. They should not be expected to return identical answers.
Same example person, 80 kg and 180 cm, male. Three different equations, three slightly different estimates.
| Formula | Estimated Lean Body Mass |
|---|---|
| Boer | about 61.4 kg |
| James | about 62.7 kg |
| Hume | about 57.8 kg |
For an 80 kg, 180 cm man:
Boer: about 61.4 kg
James: about 62.7 kg
Hume: about 57.8 kg
That is a difference of nearly 5 kilograms from the lowest to highest prediction.
If the same person has a separately measured body-fat estimate of 18%, the direct arithmetic gives:
80 × 0.82 = 65.6 kg
That result is higher than all three height-and-weight formulas.
The lesson is not that one calculator is "broken."
The formulas are estimates based on limited inputs. They cannot see the person's actual muscularity or fat distribution.
Lean body mass is not another name for skeletal muscle.
Muscle is one part of the lean portion of the body.
Depending on the definition and measurement method, the non-fat or lean portion can also include:
A conceptual, illustrative breakdown of total body weight. Exact proportions vary by individual and measurement method; this is not a personalized result.
| Component | Category |
|---|---|
| Fat mass | Storage body fat |
| Muscle | Lean / non-fat component |
| Water | Lean / non-fat component |
| Bone | Lean / non-fat component |
| Organs | Lean / non-fat component |
| Connective tissue | Lean / non-fat component |
If a smart scale says your lean mass changed by several pounds overnight, you almost certainly did not build or lose several pounds of skeletal muscle overnight. Hydration and measurement variability can change body-composition estimates.
This is why trends collected under similar conditions are often more informative than one isolated reading.
These terms are often used interchangeably in consumer fitness discussions, but technical sources may distinguish them.
The Obesity Medicine Association describes fat-free mass as body mass excluding fat, while lean body mass can include a small amount of essential fat.
Some DXA reports use "lean mass" to describe lean soft tissue while reporting bone mineral separately.
That means numbers from different machines or equations should not automatically be treated as identical measurements.
When comparing progress, try to compare the same method with itself.
BMI and lean body mass answer different questions.
BMI uses height and weight to create a screening measure. It does not directly distinguish fat from muscle.
Lean-body-mass estimation tries to separate body weight into composition components, but a formula-based estimate still has limitations.
A very muscular athlete and a sedentary person could have the same height and weight. A height-and-weight lean-mass equation receives identical inputs for both, even though their body composition may be very different.
The CalculateThisWay BMI Calculator is useful when you want BMI and healthy-weight context.
The Body Fat Calculator is more relevant when you want an estimated body-fat percentage.
The Lean Body Mass Calculator then helps translate body weight and body-fat information into a lean-weight estimate.
A scale cannot tell you what kind of tissue changed.
Suppose your body weight falls by 10 pounds.
That change could include some combination of:
If the goal is fat loss while maintaining strength and muscle, total scale weight is therefore only one piece of information.
Resistance training, adequate nutrition, and sufficient protein are commonly used to support lean tissue during weight management.
If protein planning is your main question, use the CalculateThisWay Protein Calculator for a dedicated estimate instead of using lean mass as a substitute for individualized nutrition advice.
Lean tissues generally have different metabolic demands than adipose tissue, which is one reason body composition influences resting energy expenditure.
However, this does not mean each pound of estimated lean mass lets a calculator predict your metabolism perfectly.
BMR formulas remain population estimates.
Use the CalculateThisWay BMR Calculator if you want an estimate of energy used at rest, and the TDEE Calculator when you want to include daily activity.
No.
A lean-body-mass estimate should not be turned into an arbitrary "ideal" body weight.
Healthy body composition differs with age, sex, genetics, training history, health status, and other factors.
CalculateThisWay has intentionally retired its stand-alone Ideal Weight and Healthy Weight calculators and consolidated healthy-weight context into the BMI experience rather than pretending that one equation defines the perfect body weight.
A formula-based lean mass calculator is an estimate.
Its limitations include:
Height-and-weight formulas may perform poorly in people whose body composition differs substantially from the populations used to create them.
The direct body-fat method can be more individualized, but a body-fat measurement can also be wrong.
Bioelectrical impedance changes with hydration. Tape methods depend on measurement technique. Skinfold measurements depend on the tester and equation. DXA is more sophisticated, but even DXA terminology and outputs should be interpreted correctly.
Use the result as context, not as a diagnosis.
Some calculators choose one equation and hide the fact that other formulas would give a different number.
CalculateThisWay should show the comparison.
If Boer says 61.4 kg, James says 62.7 kg, and Hume says 57.8 kg, that range teaches something important: body composition cannot be perfectly reconstructed from height and weight alone.
This is the same type of comparison the calculator above draws for you, here shown for the 80 kg, 180 cm male example. Calculate your own numbers above to see your personal version of this chart.
| Formula | Estimated Lean Body Mass |
|---|---|
| Boer | about 61.4 kg |
| James | about 62.7 kg |
| Hume | about 57.8 kg |
The goal is not false precision.
The goal is to understand what the formulas can and cannot tell you.
A lean body mass calculator can help answer:
How much of my weight is estimated to be lean?
How do I calculate lean body mass from body-fat percentage?
What do the Boer, James, and Hume formulas predict?
Why do different lean-mass calculators give different answers?
If you know body-fat percentage, use the direct body-fat method and remember that its accuracy depends on the measurement behind it.
If you do not know body-fat percentage, compare the classic equations rather than treating one prediction as a laboratory measurement.
Lean-body estimates are most useful when they help you understand body composition as more than one number on a scale.
The examples below use the exact same calculation engine as the calculator above. Values are computed live in JavaScript rather than typed in as fixed text, so they will always match what the calculator itself would return for these inputs.
Weight 80 kg. Body fat 18%.
Male. Weight 80 kg. Height 180 cm.
Female. Weight 65 kg. Height 165 cm.
Male. Weight 70 kg. Height 175 cm.
| Method | Inputs | Role on This Page |
|---|---|---|
| Direct body-fat method | Weight, body-fat percentage | Most individualized, but only as accurate as the body-fat value entered |
| Boer | Height, weight, sex | Primary height-and-weight estimate |
| James | Height, weight, sex | Comparison estimate with a squared weight-to-height term |
| Hume | Height, weight, sex | Comparison estimate developed from body-water research |
No formula here is presented as perfect, and none is averaged together into a single "more accurate" number. Each is a population-based estimate, and the differences between them are normal rather than a sign that something is wrong.
Female. Height 5 ft 5 in. Weight 143.3 lb.
Which three basic pieces of information do Boer, James, and Hume all use?
Boer, James, and Hume all use: Height, Weight, and Sex. None of the three use age.
These are older regression equations shown with their original publication dates rather than presented as new research.
I want a circumference-based estimate of body-fat percentage first.
I want a height-and-weight screening result and healthy-weight context.
I want to estimate how many calories my body uses at rest.
I want my total daily calorie use or a calorie target.
I want a dedicated daily protein estimate.
I want to split a calorie target into protein, carbs, and fat.
Lean body mass is a body-composition concept describing the portion of body weight that is not storage body fat. Related terms such as fat-free mass and DXA lean mass are closely related but are not always defined identically.
You can calculate lean body mass by subtracting an estimated fat mass from total body weight if you already know your body-fat percentage, or by using a height-and-weight prediction equation such as Boer, James, or Hume if you do not. This calculator supports both approaches.
There is no single lean body mass formula. The direct method is weight multiplied by one minus body fat percentage. The Boer, James, and Hume equations instead estimate lean body mass from height, weight, and sex-specific coefficients.
Multiply body weight by one minus your body-fat percentage expressed as a decimal. For example, 80 kilograms at 18 percent body fat gives 80 times 0.82, or 65.6 kilograms of estimated lean mass.
No. Lean body mass includes skeletal muscle, but it also includes bone, organs, water, connective tissue, and other non-fat tissue, so it is always a larger figure than muscle mass alone.
Not necessarily. A modern Obesity Medicine Association clinical statement notes that fat-free mass excludes body fat entirely, while lean body mass can include a small amount of essential fat, so the two terms are related but not always interchangeable.
The Boer formula is a 1984 equation published by Peter Boer that estimates lean body mass from height and weight using sex-specific coefficients. This calculator uses it as the primary height-and-weight result.
The James formula estimates lean body mass using weight combined with a squared weight-to-height ratio, which gives it a different mathematical structure than the linear Boer and Hume equations.
The Hume formula is a 1966 equation published by R. Hume that predicts lean body mass from height and weight using sex-specific linear coefficients, developed by comparing predictions against body-water measurements.
No single formula is universally most accurate. This calculator uses Boer as the primary estimate and shows James and Hume for comparison rather than claiming that one equation is scientifically superior.
Boer, James, and Hume were developed from different research populations, methods, and eras, and James uses a squared term instead of a simple linear one, so some disagreement between the formulas is expected rather than an error.
Yes. Depending on the definition and measurement method used, bone or bone mineral is generally considered part of the non-fat, lean portion of the body.
Yes. Body water is typically included in the lean or fat-free portion of the body, which is one reason hydration can influence body-composition readings.
Yes. Organs are non-fat tissue and are generally included within lean body mass along with muscle, bone, water, and connective tissue.
A smart scale estimates body composition using bioelectrical impedance, which is sensitive to hydration and other factors, so its lean-mass reading is an estimate rather than a direct measurement.
No. BMI only uses height and weight to produce a screening number and cannot distinguish fat from lean tissue, so two people with the same BMI can have very different body composition.
Yes. If you already have a body-fat percentage from any method, this calculator's direct method can subtract the implied fat mass from your total weight to estimate lean mass.
Not necessarily. DXA reports may separate bone mineral content from lean soft tissue, and this calculator's height-and-weight or direct body-fat estimates are not a substitute for an actual DEXA scan.
Readings can shift from day to day because of hydration, recent food intake, and measurement variability, even though actual muscle tissue does not change that quickly.
Yes. Because water is part of the lean portion of the body, dehydration or overhydration can shift body-composition estimates from methods such as bioelectrical impedance.
Athletes can use this calculator for a general estimate, but height-and-weight formulas were not built specifically for very muscular or atypical body types, so results should be treated as a reference point rather than a precise measurement.
An estimated lean body mass can be one input some people consider when thinking about protein needs, but for a dedicated estimate, use the CalculateThisWay Protein Calculator.
Lean body mass is one input some BMR equations use, such as Katch-McArdle, but this calculator is not a BMR calculator. Use the CalculateThisWay BMR Calculator for a resting-energy estimate.
No. The Boer, James, and Hume equations were developed using adult populations and are not appropriate for the growth and development patterns of children or adolescents.
No. These equations were not developed for the body-composition changes that occur during pregnancy, so results during pregnancy should not be relied upon.
The Boer, James, and Hume equations were built using only height, weight, and sex, so age is not part of their mathematical structure and is not requested here.
Boer, James, and Hume each use different coefficients for men and women because the original research samples found different relationships between height, weight, and lean mass by sex.
There is no universal rule, but comparing results collected under similar conditions, such as similar hydration and time of day, using the same method each time, is generally more informative than comparing isolated single readings.