Body Fat Calculator
Estimate your body fat percentage, fat mass, and lean mass using the U.S. Navy circumference method — a documented, transparent formula, not a direct body-composition measurement.
Estimate Your Body Fat Percentage
Method: U.S. Navy circumference formula. Requires height and neck and waist circumference (plus hip circumference for women).
Measure just below the larynx (Adam's apple), tape level, not compressed.
Men: measure at the navel. Women: measure at the narrowest point.
Measure at the widest point of the hips.
Estimated Results
Method: U.S. Navy circumference formula. This is an estimate — not a direct body-composition measurement. Fat mass and lean mass require the optional weight field.
What Is Body Fat Percentage?
Body fat percentage is the estimated proportion of your total body weight that consists of fat. This calculator uses the U.S. Navy circumference method — a validated prediction equation based on neck, waist (and hip, for women) circumference and height — to estimate body fat. The result is an estimate, not a direct measurement.
Key Facts
The Formula: U.S. Navy Circumference Method
This calculator uses the U.S. Navy circumference method, a widely referenced prediction equation. It estimates body fat percentage from height and circumference measurements — it does not measure fat tissue directly.
Men (measurements in cm, height in cm):
%BF = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
Women (measurements in cm, height in cm):
%BF = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450
All circumferences are measured with a flexible, non-stretch tape held level and snug without compressing the skin. The calculator converts US customary inputs to centimeters internally before applying the formula.
Once body fat percentage is estimated, fat mass and lean mass (if you entered your weight) are derived as:
Fat Mass = Body Weight × Estimated Body-Fat Percentage
Estimated Lean Mass = Body Weight − Estimated Fat Mass
"Lean mass" here means body weight minus estimated fat mass — it includes muscle, but also water, organs, bone and other non-fat tissue. It is not the same as muscle mass alone.
Worked Examples
Example 1 — Metric, male
Height 180 cm, neck 38 cm, waist 85 cm.
waist − neck = 47 cm. %BF = 495 ÷ (1.0324 − 0.19077×log₁₀(47) + 0.15456×log₁₀(180)) − 450 ≈ 16.1%
With a body weight of 80 kg: fat mass ≈ 80 × 0.161 = 12.9 kg; estimated lean mass ≈ 80 − 12.9 = 67.1 kg.
Example 2 — US customary, female
Height 65 in, neck 13 in, waist 30 in, hip 38 in.
waist + hip − neck = 55 in. %BF ≈ 28.6%
With a body weight of 150 lb: fat mass ≈ 150 × 0.286 = 42.9 lb; estimated lean mass ≈ 150 − 42.9 = 107.1 lb.
Try the calculator above with your own measurements — every figure it displays is calculated live using the formulas shown, so you can verify the math yourself.
How Is Body Fat Measured?
Several methods exist for estimating or measuring body composition, and they differ in accuracy, reproducibility, cost and accessibility. This calculator uses only the circumference method described above — it does not mix or silently switch between methods.
Bioelectrical Impedance Analysis (BIA)
Smart scales and handheld devices send a small electrical current through the body. Estimates depend on hydration, device, and the prediction equation used, and can vary between readings.
Skinfold Calipers
Measures subcutaneous fat thickness at specific sites. Accuracy depends heavily on technique, operator skill, and the equation used.
DXA (Dual-Energy X-ray Absorptiometry)
A scan-based method that can provide detailed regional and total body-composition data. This calculator's estimate is not equivalent to a DXA result.
Hydrostatic Weighing / Air Displacement
Reference methods based on body density, typically available only in specialized labs or clinics.
Circumference Equations (used here)
Estimates body fat from tape-measure circumferences and height using a published prediction formula.
Imaging Methods
MRI and CT can assess body composition in research or clinical settings but are not typically used for routine body-fat estimation.
How Accurate Is a Body Fat Calculator?
Accuracy depends on the method, the equation, the inputs, and the individual. This calculator does not have a single universal accuracy percentage — a prediction equation is not a direct measurement, and the same person may get different results from a BIA scale, this circumference calculator, skinfold calipers, or a DXA scan. A different number from a different method does not necessarily mean one of them is "wrong."
Limitations of this calculator:
- It estimates rather than directly measures body composition.
- Measurement technique (tape tightness, placement, posture) affects the result.
- The equation was developed and validated on specific study populations and may not generalize perfectly to every individual.
- It cannot assess health status, fitness, or disease risk on its own.
- It does not replace clinical body-composition assessment where that's needed.
Body Fat Percentage vs BMI
BMI measures weight relative to height; it does not directly measure body fat. Two people can have the same BMI but different estimated body-fat percentages — for example, a very muscular person can have a higher BMI without correspondingly high body fat, while someone with lower muscle mass can have a "normal" BMI alongside a higher estimated body-fat percentage.
| BMI | Body Fat Percentage | |
|---|---|---|
| What it measures | Weight relative to height | Estimated proportion of weight that is fat |
| Inputs | Height, weight | Circumferences, height (this method) |
| Can indicate | A general weight-for-height reference category | An estimated body-composition breakdown |
| Cannot determine | Body composition or where weight comes from | Health status or disease risk on its own |
Calculate your BMI to see how it compares with your estimated body fat here.
Body Fat vs Lean Body Mass
Total Body Weight = Estimated Fat Mass + Estimated Lean/Fat-Free Mass. Body fat percentage is a proportion; fat mass is an absolute amount; lean body mass is what's left once fat mass is subtracted — it is not the same thing as muscle mass, since it also includes water, organs and bone.
Calculate your lean body mass for a dedicated breakdown of this figure.
Body Fat Percentage vs Ideal Weight
An ideal-weight calculator estimates a reference weight using a selected formula or methodology; it doesn't account for body composition. This Body Fat Calculator answers a different question — what proportion of your current weight is estimated to be fat.
Explore our Ideal Weight Calculator to see how these formulas differ from an estimated body-fat percentage.
Body Fat Percentage vs Healthy Weight
A healthy-weight calculator focuses on a weight range using a documented methodology, while this calculator focuses on estimated body composition. They answer related but different questions.
Calculate a healthy weight range to compare with your estimated body composition here.
Body Fat Percentage vs Body Shape
Body shape classifications generally describe body proportions and where circumference measurements are largest relative to each other, rather than directly measuring body-fat percentage. A body-shape category cannot determine your estimated body fat.
Find your body shape as a separate, complementary measurement.
Body Fat Percentage vs Body Surface Area
Body surface area (BSA) is an estimate of total skin surface area, typically used in specific clinical and dosing contexts — it does not measure body composition. BMI, body fat percentage and BSA are three different calculations with three different purposes.
Calculate body surface area if that's the figure you need.
Body Fat vs Adjusted Body Weight
Adjusted body weight is a specialized calculation used in certain clinical and nutrition contexts and is not a body-fat measurement. This page does not provide medication-dosing guidance, and the Body Fat Calculator does not replace a clinical assessment for that purpose.
Calculate adjusted body weight if you need that specific, separate methodology.
Body Composition in Children and Teens
This calculator's formula and reference context are intended for adults. Pediatric body composition requires age-appropriate methodology, and adult body-fat equations and reference ranges should not be applied to children or teenagers.
For pediatric weight-for-height context, use the Child BMI Calculator, which uses BMI-for-age rather than adult BMI categories.
How to Track Body Fat Over Time
If you want to track estimated body fat over time, consistency matters more than any single reading:
- Use the same method and formula each time (don't compare this calculator's result to a different device's reading as if they should match).
- Take measurements under similar conditions — similar time of day, hydration, and clothing.
- Use the same measurer or technique where possible, since technique affects circumference readings.
- Look at the trend over several weeks rather than reacting to a single reading.
- Avoid interpreting small day-to-day changes as meaningful — normal fluctuation in hydration and other factors can shift the estimate by a percentage point or more.
Body weight can stay similar while body composition shifts, and estimated body fat can change with or without a change in weight — this calculator estimates composition, not muscle gain or fat loss directly.
Frequently Asked Questions
What is body fat percentage?
Body fat percentage is the estimated proportion of your total body weight that consists of fat, calculated as fat mass divided by total body mass, multiplied by 100.
How is body fat percentage calculated?
This calculator uses the U.S. Navy circumference method, a prediction equation based on neck and waist circumference (plus hip circumference for women) and height. It is a documented formula, shown in full above.
How accurate is a body-fat calculator?
Accuracy depends on the method, equation, and how carefully measurements are taken. A prediction equation is an estimate, not a direct measurement, and results can differ from other methods like BIA scales or skinfold calipers.
Does BMI measure body fat?
No. BMI measures weight relative to height and does not directly measure body composition or body fat percentage.
What is the difference between BMI and body fat?
BMI is a weight-for-height ratio; body fat percentage is an estimate of what proportion of your weight is fat. Two people can share the same BMI with different body-fat percentages.
What is fat mass?
Fat mass is the absolute amount of body weight estimated to be fat, calculated as body weight multiplied by estimated body-fat percentage.
What is lean body mass?
Lean body mass is body weight minus estimated fat mass. It includes muscle as well as water, organs, bone and other non-fat tissue.
Is lean body mass the same as muscle mass?
No. Lean body mass includes muscle but also water, organs, bone and other non-fat tissue — it is a broader category than muscle mass alone.
Why can body-fat readings vary?
Different methods (circumference formulas, BIA scales, skinfold calipers, DXA) use different assumptions and equations, and factors like hydration and measurement technique can shift any given reading.
How accurate are smart scales?
Smart scale (BIA) estimates depend on the device, its prediction equation, and factors like hydration and recent food or exercise — individual-level error can be substantial, so results can fluctuate meaningfully between readings.
How accurate are skinfold calipers?
Skinfold accuracy depends heavily on measurement sites, operator technique and skill, caliper quality, and the specific equation used, and can vary noticeably between measurers.
What is the most accurate way to measure body fat?
Reference methods like DXA, hydrostatic weighing, and air displacement plethysmography are generally considered more precise than field methods like circumference formulas, BIA scales, or skinfolds, but they require specialized equipment and are less accessible.
Can body fat percentage change without weight changing?
Yes. Body weight can stay the same while the proportion of fat versus lean mass shifts, which this calculator would reflect as a change in estimated body-fat percentage even without a change in weight.
Can two people have the same BMI but different body-fat percentages?
Yes. BMI does not account for body composition, so two people with identical BMI values can have quite different estimated body-fat percentages.
Does age affect body-fat percentage?
Body composition can change over the lifespan, though this calculator's formula does not use age as an input — it estimates based on your current measurements only.
Does sex affect body-fat percentage?
Yes. The Navy method uses different equations for men and women, reflecting differences in typical fat distribution and the circumferences used in each formula.
What body-fat percentage is healthy?
There is no single universal "ideal" body-fat percentage that applies to everyone — appropriate ranges can vary by individual circumstances. This calculator does not provide a health verdict based on your result.
Can children use a body-fat calculator?
This calculator's formula and reference context are designed for adults and should not be applied to children or teenagers, who require age-appropriate methodology such as BMI-for-age.
What is the difference between body fat and lean mass?
Body fat percentage and fat mass describe the fat portion of body weight; lean mass describes everything else — muscle, water, organs and bone combined.
How should I track body-fat changes?
Use the same method consistently, measure under similar conditions, and look at trends over several weeks rather than reacting to any single reading.
Why does my body-fat percentage change from day to day?
Hydration, food and fluid intake, recent exercise, and measurement technique can all shift circumference-based or BIA-based estimates from one reading to the next, even without a real change in body composition.