BMI & body composition
Standard BMI & Body Composition Calculator
Calculate your BMI, adult BMI category, estimated body-fat percentage, lean mass, fat mass, waist-to-height ratio and weight range for your height. Choose U.S. or metric units and see each step of the calculation.
Enter your measurements
Measurements are converted internally before the formulas are applied.
Define → Validate → Normalize → Calculate → Check → Present. U.S. measurements are converted to standard internal units before BMI and body-composition formulas are applied.
Formula & methodology
How BMI & Body Composition Are Calculated
The calculator first converts measurements into a consistent unit system. It then calculates BMI, applies adult BMI thresholds when age 20 or older, calculates optional circumference-based body-fat estimates, and uses body-fat percentage to estimate fat mass and lean body mass.
Governing Equations
BMI uses weight relative to the square of height. Optional body composition calculations use either a supplied body-fat percentage or the circumference estimate selected in the calculator.
BMI — metric formula
Weight is measured in kilograms and height in meters. BMI is normally reported in kg/m².
BMI — U.S. customary formula
This is the direct U.S. customary form. The calculator can also convert pounds and inches into kilograms and meters before applying the metric formula.
Adult BMI reference weight range
This gives the weights at the lower and upper edges of the adult Healthy Weight BMI range for a given height. It is a BMI reference range, not a personal target prescribed by the calculator.
Waist-to-height ratio
Both measurements must use the same unit. For example, inches divided by inches or centimeters divided by centimeters.
Circumference body-fat estimate — male equation
Waist, neck and height are normalized to inches before this equation is used.
Circumference body-fat estimate — female equation
Waist, hip, neck and height are normalized to inches before calculation.
Fat mass
Lean body mass
In this calculator, lean body mass means the portion of body weight not assigned to estimated fat mass. It is still an estimate when body-fat percentage is estimated.
Variable Definitions & Units
| Symbol / value | Meaning | Internal unit |
|---|---|---|
| BMI | Body mass index | kg/m² |
| Wkg | Body weight | kilograms |
| Wlb | Body weight | pounds |
| Hm | Height | meters |
| Hin | Height | inches |
| BF% | Estimated or supplied body-fat percentage | percent (%) |
| Waist | Waist circumference | inches for circumference formula |
| Neck | Neck circumference | inches for circumference formula |
| Hip | Hip circumference when required | inches for circumference formula |
| WHtR | Waist-to-height ratio | dimensionless |
| Fat mass | Estimated mass assigned to body fat | lb or kg |
| Lean body mass | Body weight minus estimated fat mass | lb or kg |
Calculation Methodology
Read age, height, weight and selected measurements.
Check for missing, impossible or nonnumeric values.
Convert measurements into the units required by each formula.
Calculate BMI, ratios and optional body composition.
Confirm values are finite and suitable for the selected method.
Round results for clear display and add age-appropriate context.
Measurement Normalization
Unit conversion happens before the main formulas are evaluated. This avoids mixing pounds with kilograms or inches with meters.
Pounds to kilograms
kg = lb × 0.45359237
Inches to meters
m = in × 0.0254
Centimeters to meters
m = cm ÷ 100
Centimeters to inches
in = cm ÷ 2.54
Feet and inches to total inches
total inches = feet × 12 + inches
Ratio normalization
Waist and height must use the same unit before waist-to-height ratio is calculated.
Adult BMI Classification
For adults age 20 and older, the calculator compares BMI with standard adult screening categories.
| BMI | Classification |
|---|---|
| < 18.5 | Underweight |
| 18.5 to < 25 | Healthy Weight |
| 25 to < 30 | Overweight |
| 30 to < 35 | Class 1 Obesity |
| 35 to < 40 | Class 2 Obesity |
| ≥ 40 | Class 3 Obesity |
Adult BMI categories are not used for ages 2–19. BMI for children and teens should be interpreted using sex-specific BMI-for-age percentiles because growth changes the meaning of the BMI number.
Manual Calculation Methods
How to calculate BMI manually
- Record body weight and height.
- Convert weight to kilograms and height to meters, if needed.
- Square the height in meters.
- Divide weight in kilograms by height squared.
- If age is 20 or older, compare the BMI with the adult classification thresholds.
How to calculate the adult BMI reference weight range
- Convert height to meters.
- Square height.
- Multiply height² by 18.5 for the lower reference value.
- Multiply height² by 24.9 for the upper reference value.
- Convert kilograms back to pounds if U.S. units are selected.
How the circumference body-fat estimate is calculated
- Convert height and circumference measurements to inches.
- Select the equation required by the chosen sex-specific method.
-
For the male equation, calculate
waist − neck. -
For the female equation, calculate
waist + hip − neck. - Apply base-10 logarithms to the required measurements.
- Complete the selected equation without rounding the intermediate logarithms.
- Round the final estimated body-fat percentage only for display.
How fat mass and lean body mass are estimated
- Obtain the selected body-fat percentage.
- Divide that percentage by 100.
- Multiply body weight by the decimal body-fat fraction.
- This gives estimated fat mass.
- Subtract fat mass from body weight.
- This gives estimated lean body mass.
Calculation Breakdown
The default example uses age 30, height 5 ft 10 in, weight 165 lb, waist 34 in and neck 15 in.
BMI calculation
165 lb at 5 ft 10 in
70 × 0.0254 = 1.778 m
Circumference body-fat estimate
Default male-method example
Fat mass and lean body mass
Uses the unrounded body-fat estimate internally.
Waist-to-height ratio
Uses measurements in the same unit.
Precision, Rounding & Interpretation
Internal precision
Converted measurements and intermediate values are kept at full available numerical precision during calculation.
Display rounding
BMI and body-fat estimates may be shown to one decimal place, while ratios may be shown to three decimal places.
Estimated values
Circumference-based body-fat percentage, fat mass and lean mass are estimates. Extra decimal places do not make the method more exact.
Screening, not diagnosis
BMI is a screening measure. It does not directly measure body fat or determine a person’s overall health by itself.
If the estimated body-fat percentage is 17.513085…%, the calculator uses that unrounded value to estimate fat mass and lean body mass. It does not first replace it with 17.5%. Rounding happens when the final values are displayed.
Validation Rules
Height must be positive
BMI cannot be calculated when height is zero or missing because height² is the denominator.
Weight must be positive
A missing, zero or negative body weight is rejected as an invalid input.
Adult thresholds require age 20+
The calculator may still calculate the numerical BMI for a younger user, but it does not label that result with adult BMI categories.
Circumference equations require valid differences
For the male method,
waist − neck must be greater than zero.
For the female method,
waist + hip − neck must be greater than zero.
Body-fat percentage must be plausible
A calculated or supplied value outside the calculator’s usable range is rejected instead of being used to create misleading fat-mass estimates.
No NaN or Infinity
Nonfinite calculation results are treated as errors rather than displayed as valid health measurements.
Worked examples & analysis
BMI & Body Composition Examples
These examples show how height, weight and circumference measurements affect BMI and related estimates. BMI describes weight relative to height; it does not directly measure body fat, muscle mass or overall health.
Worked Example: Adult BMI & Body Composition
Consider a 30-year-old adult who is 5 ft 10 in tall, weighs 165 lb, has a 34 in waist and a 15 in neck. The male circumference equation is selected for the body-fat estimate.
Example measurements
Age 30 · 5 ft 10 in · 165 lb · 34 in waist · 15 in neck
Normalize the measurements
165 lb × 0.45359237 = 74.84274105 kg. Height is 70 in, which equals 1.778 m.
Calculate BMI
BMI = 74.84274105 ÷ 1.778² ≈ 23.6748
Apply the adult BMI category
At age 30, adult categories apply. A BMI of about 23.7 is within the 18.5 to <25 Healthy Weight category.
Estimate body fat
For the selected circumference method: waist − neck = 34 − 15 = 19 in. Applying the circumference equation gives an estimated body-fat percentage of about 17.5%.
Estimate fat and lean mass
Using the unrounded body-fat estimate gives about 28.9 lb of fat mass and 136.1 lb of lean body mass.
Calculate waist-to-height ratio
34 ÷ 70 = 0.485714..., displayed as
0.486.
BMI Calculation Examples
The table holds height constant at 5 ft 10 in to show how changing body weight changes BMI. Adult classifications are shown for example purposes.
| Weight | Height | BMI calculation | BMI | Adult category |
|---|---|---|---|---|
| 120 lb | 5 ft 10 in | (120 ÷ 70²) × 703 | 17.2 | Underweight |
| 130 lb | 5 ft 10 in | (130 ÷ 70²) × 703 | 18.7 | Healthy Weight |
| 165 lb | 5 ft 10 in | (165 ÷ 70²) × 703 | 23.7 | Healthy Weight |
| 180 lb | 5 ft 10 in | (180 ÷ 70²) × 703 | 25.8 | Overweight |
| 210 lb | 5 ft 10 in | (210 ÷ 70²) × 703 | 30.1 | Class 1 Obesity |
| 250 lb | 5 ft 10 in | (250 ÷ 70²) × 703 | 35.9 | Class 2 Obesity |
| 280 lb | 5 ft 10 in | (280 ÷ 70²) × 703 | 40.2 | Class 3 Obesity |
Scenario Comparison: Why Height Changes BMI
These three adults all weigh 165 lb. Because BMI divides weight by height squared, the same body weight produces a different BMI at different heights.
5 ft 4 in and 165 lb
BMI 28.3
Adult category: Overweight
5 ft 10 in and 165 lb
BMI 23.7
Adult category: Healthy Weight
6 ft 2 in and 165 lb
BMI 21.2
Adult category: Healthy Weight
Explore How Weight Changes BMI
Keep height fixed and compare a starting weight with another weight. This makes it easier to see how a weight change affects BMI without re-entering the full body-composition calculator.
BMI Scenario Comparison
Enter an adult height and two body weights. The tool calculates both BMI values, the change in BMI and the adult BMI category for each scenario.
At 5 ft 10 in, changing from 165 lb to 180 lb changes BMI from about 23.7 to 25.8.
Calculation
Understanding the result
How to Interpret BMI & Body Composition Results
BMI and circumference-based body-fat estimates can provide useful screening information, but they do not give a complete picture of health or body composition. The number should be read together with age, measurements, method limits and the reason the calculation is being used.
Understanding the Result
BMI compares body weight with height
BMI is calculated from weight divided by height squared. A BMI of 23.7 means the person’s weight-to-height relationship produces a BMI value of 23.7 kg/m².
This may meanFor an adult age 20 or older, 23.7 falls within the standard Healthy Weight BMI category. It does not prove that the person has a particular body-fat level or health status.
BMI category is a screening classification
Adult BMI categories place a calculated BMI into a defined range such as Underweight, Healthy Weight, Overweight or an obesity class.
This may meanThe category can help identify whether further health context may be useful, but it is not a diagnosis and does not account for every difference in body composition.
Body-fat percentage is an estimate
When the circumference method is selected, the calculator estimates body-fat percentage from tape measurements and a sex-specific equation.
This may meanThe result can provide rough body-composition context, but measurement technique and individual body shape can affect the estimate.
Lean mass is not the same as muscle mass
Lean body mass is calculated as total body weight minus estimated fat mass.
This may meanLean mass includes more than skeletal muscle. It also includes water, bone, organs and other non-fat tissue.
Waist-to-height ratio compares two measurements
The ratio is calculated by dividing waist circumference by height using the same measurement unit.
This may meanIt provides additional context about waist size relative to height. It should be treated as a screening indicator rather than a diagnosis.
A reference weight range is derived from BMI
For adults, the calculator can estimate the body weights that correspond to BMI values of 18.5 through 24.9 at the entered height.
This may meanThe range is a BMI reference range. It is not automatically an ideal, required or medically appropriate personal target.
Assumptions
Small input errors can change BMI and body-composition estimates.
Waist, neck and hip circumference should be measured at the correct locations and without pulling the tape too tightly.
The calculator does not apply adult BMI categories to users under age 20.
U.S. and metric inputs are converted into the units required by each formula before arithmetic is performed.
Different body-composition methods can produce different results for the same person.
Extra decimal places would not make a tape-measure body-fat estimate more exact.
Limitations
BMI limitations
- BMI does not directly measure body fat.
- BMI does not separate fat mass from muscle, bone or water.
- People with similar BMI values can have different body compositions.
- BMI does not show where body fat is distributed.
- Adult BMI categories are not appropriate for children and teens.
Body-composition estimate limitations
- Tape-measure formulas are indirect estimates.
- Results depend on measurement technique.
- Sex-specific equations use simplified population models.
- Fat mass and lean mass inherit any error in the body-fat estimate.
- A calculator cannot replace a clinical or laboratory body composition assessment.
Common Calculation & Measurement Errors
Circumference and height values must be normalized before a formula is applied. Mixing units can produce a meaningless result.
BMI divides weight by height squared, not by height alone.
The numerical BMI can be calculated, but users under age 20 need age- and sex-specific BMI interpretation instead of adult thresholds.
Moving the tape to a different location between measurements can change the circumference estimate even when body composition has not meaningfully changed.
Compressing the skin can reduce a circumference measurement and affect the estimated body-fat percentage.
Lean body mass includes all non-fat body mass, not just skeletal muscle.
Fat mass and lean mass should be calculated from the unrounded internal body-fat value, then rounded for display.
A small change that moves BMI across a category threshold changes the classification label, but the body itself has not suddenly changed at that exact decimal point.
Adult BMI Classification Reference
These categories are used by the calculator only for adults age 20 or older.
| BMI | Category | Calculator interpretation |
|---|---|---|
| < 18.5 | Underweight | Below the standard adult Healthy Weight BMI range. |
| 18.5 to <25 | Healthy Weight | Within the standard adult Healthy Weight BMI range. |
| 25 to <30 | Overweight | Above the standard adult Healthy Weight BMI range. |
| 30 to <35 | Class 1 Obesity | BMI is within the first adult obesity class. |
| 35 to <40 | Class 2 Obesity | BMI is within the second adult obesity class. |
| ≥ 40 | Class 3 Obesity | BMI is within the third adult obesity class. |
Output Reference
| Output | What it measures | What it does not establish |
|---|---|---|
| BMI | Weight relative to height squared. | Direct body-fat percentage or overall health. |
| BMI category | Adult screening classification when age 20+. | A medical diagnosis. |
| Estimated body fat | Body-fat estimate from the selected method. | Direct laboratory measurement. |
| Fat mass | Body weight multiplied by estimated body-fat fraction. | Exact measured fat tissue mass. |
| Lean body mass | Body weight minus estimated fat mass. | Muscle mass alone. |
| Waist-to-height ratio | Waist circumference relative to height. | A diagnosis of metabolic or cardiovascular disease. |
| BMI reference weight range | Weights corresponding to adult BMI 18.5–24.9 at the entered height. | A personalized ideal or required weight. |
Age & BMI Interpretation
| Age | BMI calculation | Interpretation |
|---|---|---|
| 2–19 years | The numerical BMI can still be calculated. | Use age- and sex-specific BMI-for-age interpretation, not adult categories. |
| 20+ years | Standard adult BMI formula. | Adult BMI categories may be applied as screening classifications. |
Why BMI Uses Weight Relative to Height
BMI was developed as a simple way to relate body weight to height across groups of people. Squaring height helps adjust weight for differences in body size more effectively than comparing body weight alone.
Its main advantage is simplicity: height and weight are easy to measure and the calculation is quick. That same simplicity is also its main limitation because it does not directly measure fat, muscle or fat distribution.