Body Composition
15 calculators
Body composition calculators split a body weight into parts, or index it against height, so a single number on the scale becomes something with more structure behind it. The tools here cover four separate questions: how heavy someone is relative to their height, how much of their mass is fat, how much energy they burn at rest, and where their circumferences sit relative to their frame.
Every method on this page is an equation fitted to a reference population, not a measurement of the person in front of it. The arithmetic is exact; the fit to any individual is not. Each tool states the equation it uses and the population it was derived from.
- BMI Calculator Calculate Body Mass Index from weight and height using the standard kg/m² formula.
- BMR Calculator (Mifflin-St Jeor) Estimate daily resting energy expenditure using the Mifflin-St Jeor equation for males and females.
- Body Fat Percentage (Navy Method) Estimate body fat percentage using tape measurements via the U.S. Navy circumference method.
- TDEE Calculator Estimate total daily energy expenditure by scaling BMR with an activity factor.
- BMR Calculator (Harris-Benedict) Estimate basal metabolic rate using the 1984 revised Harris-Benedict equation.
- BMR Calculator (Katch-McArdle) Estimate basal metabolic rate using lean body mass instead of age and sex.
- Body Fat Percentage (3-Site Caliper) Estimate body fat percentage from three skinfold sites using the Jackson-Pollock female-protocol equation.
- Body Fat Percentage (7-Site Caliper) Jackson-Pollock 7-site skinfold body fat percentage from caliper measurements.
- FFMI Calculator Calculate Fat-Free Mass Index normalized for height using weight, height, and body-fat percentage.
- Lean Body Mass Calculator Calculate lean body mass by subtracting fat mass from total weight.
- Muscular Potential Calculator Estimate maximum drug-free lean body mass from height, wrist, and ankle using Casey Butt's equations.
- Reference Weight Formulas Compare the Devine, Robinson, Miller, and Hamwi height-based weight formulas side by side.
- Relative Fat Mass (RFM) Calculator Estimate body-fat percentage from height and waist circumference using the RFM equation.
- Waist-to-Height Ratio Calculator Calculate waist-to-height ratio (WHtR) by dividing waist circumference by height.
- Waist-to-Hip Ratio Calculator Calculate waist-to-hip ratio (WHR) from two tape measurements using the WHO measurement protocol.
What the different methods actually measure
BMI divides weight in kilograms by height in metres squared. It says nothing about what the mass is made of, which is why a lean 95 kg athlete and a sedentary 95 kg person of the same height return the same figure. Waist-to-height ratio and waist-to-hip ratio add a distribution term that BMI has no way to see, using tape measurements taken to the WHO protocol.
The body-fat estimates work from different inputs entirely. The Navy method uses neck, waist and (for women) hip circumference against height in a log-based equation. The Jackson-Pollock skinfold protocols use caliper readings at three or seven sites, converted first to body density and then to a fat percentage through the Siri equation. Relative Fat Mass needs only height and waist, trading precision for two tape readings.
The three BMR equations answer a fourth question: resting energy expenditure. Mifflin-St Jeor and the revised Harris-Benedict both work from weight, height, age and sex; Katch-McArdle takes lean body mass instead, which makes it dependent on a body-fat figure and therefore on whichever method produced that figure.
Where these estimates hold and where they drift
Circumference and skinfold equations were fitted to specific samples, and they carry the character of those samples. Published standard errors for skinfold methods sit in the region of 3 to 5 percentage points of body fat against hydrostatic weighing, and the Navy equation is generally reported as wider still. A figure of 21.5% is more honestly read as a band than a point.
Technique moves the answer as much as the equation does. A caliper site measured a centimetre off, or a tape pulled tight enough to compress tissue, shifts the result more than the choice between two published formulas. Taking measurements the same way each time makes a series comparable to itself even where the absolute figure carries error.
The BMR equations agree closely for people near the middle of their reference populations and separate at the edges. Comparing two of them on the same inputs shows the size of that disagreement directly, which is more informative than any single figure.
Common questions
Which body fat method is the most accurate?
Against hydrostatic weighing, the seven-site skinfold protocol generally reports the tightest standard error of the field methods here, followed by three-site, with circumference-based methods such as Navy and RFM wider again. That ordering assumes competent caliper technique; a poorly taken skinfold can easily be less accurate than a carefully taken tape measurement. All of these are estimates, and each tool states the equation and error range it carries.
Why do two calculators give me different body fat percentages?
Because they are different equations fitted to different samples using different inputs. The Navy method reads circumferences, Jackson-Pollock reads skinfolds, and RFM reads height and waist alone. Each was validated against a reference method with its own error band, so a spread of several percentage points between them is the expected outcome rather than a sign that one is broken. Tracking one method over time gives a more consistent series than switching between them.
What does BMI miss?
BMI is weight over height squared, so it has no term for what the weight is composed of or where it sits. Muscle and fat count identically, which is why heavily trained people often index as overweight on it. It also carries no distribution information: two people at the same BMI can differ substantially in waist circumference. Waist-to-height ratio and the body-fat estimates on this page address those two gaps separately.