Estimate your body fat percentage using the US Navy circumference method — supports metric (cm/kg) and imperial (in/lbs). Get fat mass, lean mass, age-adjusted fitness categories, and track your progress over time.
📏 For US Navy method accuracy, measure waist at navel level after a normal exhale — not the narrowest point or the midpoint used in WHO/NHS clinical guidelines. Using a different reference point will skew your result.
Enter your measurements to estimate body fat
This body fat calculator estimates your body fat percentage using the U.S. Navy circumference method — a tape-measure formula the U.S. military developed to assess recruits' fitness without lab equipment. As a free body fat percentage calculator that fully supports both metric (cm/kg) and imperial (in/lbs) units, it works equally well whether you think in centimeters or inches. Beyond a single percentage, it also breaks your total weight into fat mass and lean mass, applies age-adjusted body fat categories, and includes a built-in progress tracker so you can watch your body composition change over weeks and months — not just watch a number move on the bathroom scale.
The US Navy body fat method estimates body fat percentage from a handful of circumference measurements — waist and neck for men, plus hip for women — combined with height in a logarithmic regression formula. It doesn't measure fat tissue directly the way a DEXA scan or hydrostatic weighing does; instead it's an empirical formula that was calibrated against those more precise reference methods, trading some accuracy for the convenience of needing nothing more than a flexible tape measure.
This body fat calculator is useful for anyone who wants a more informative alternative to BMI: dieters tracking fat loss versus muscle loss, strength trainees monitoring body recomposition, athletes and personal trainers doing periodic client check-ins, and anyone curious how their fat mass and lean mass compare across gender- and age-adjusted fitness categories. It's built for adults — the age input accepts 18 to 80 years — and is not intended for children or teenagers, whose body composition needs different reference standards.
Weight and BMI can't tell the difference between muscle and fat, so two people at the same height and weight can have very different body compositions. Knowing your body fat percentage — broken into fat mass and lean mass — helps you set realistic fat-loss or muscle-gain targets, judge whether a diet or training program is changing your body composition rather than just moving the scale, and have a more informed starting point for conversations with a coach or healthcare provider. Because body fat naturally rises with age even in people who stay active, the calculator's age-adjusted categories give a fairer read on progress for adults over 40.
Exactly how this body fat calculator converts your measurements into a body fat percentage
The result is floored at a minimum of 2% (the formula can otherwise return unrealistically low values for very small waist-to-neck differences) and rounded to one decimal place. Fat mass is then calculated as your weight × (body fat % ÷ 100), and lean mass is your total weight minus fat mass.
The formula uses base-10 logarithms of your circumference measurements because body fat doesn't scale linearly with waist size — a logarithmic relationship fits the underlying body-density data more closely across a wide range of body sizes.
Fat Mass (kg) = Weight × (Body Fat % ÷ 100). Lean Mass (kg) = Weight − Fat Mass. Together these turn a single percentage into two concrete numbers you can track over time.
The percentage itself doesn't change with age, but the calculator's fitness-category cutoffs shift upward for older age brackets (under 40, 40–59, 60+), reflecting that some additional body fat is typical and expected as people age.
From entering your measurements to reading your body composition results
Gender determines which US Navy formula is used (men's formula uses waist and neck; women's formula adds hip), and age is used to apply age-adjusted body fat categories.
Pick Metric (cm/kg) or Imperial (in/lbs) depending on which units your tape measure and scale use. You can switch at any time — values convert automatically.
Type your height and current body weight into the corresponding fields, measured barefoot and standing straight for the most accurate reading.
Measure waist at navel level after a normal exhale, and neck at the widest point below the larynx. Women also enter a hip measurement at the widest point of the hips and buttocks.
See your body fat percentage, age-adjusted category, fat mass, lean mass, and two charts. Optionally click "Save Progress" to log the entry to your on-device history.
Using the calculator's own default scenario — a 30-year-old man, 170 cm tall, 75 kg
A 30-year-old man is 170 cm tall, weighs 75 kg, and measures 85 cm at the waist and 38 cm at the neck.
Explanation: A body fat percentage of 17.8% places this example just inside the "Fitness" band for a man under 40, meaning it's leaner than the "Average" range (18–24%) but not quite in the "Athletes" range (6–13%). Because the age entered is 30 (under 40), no age-adjustment note is shown — the standard under-40 thresholds apply directly. A 55-year-old with the exact same measurements would see the same 17.8% figure, since the formula itself doesn't use age, but would be compared against the wider 40–59 age-adjusted category bands instead.
What each category generally means for adults under 40 (see the age filter in the calculator for 40–59 and 60+ ranges)
| Category | Men | Women | General Consideration |
|---|---|---|---|
| Essential Fat | 2–5% | 10–13% | Minimum fat needed for normal physiological function; going meaningfully below this is a health risk, not a fitness goal |
| Athletes | 6–13% | 14–20% | Typical range for competitive athletes and very lean, highly trained individuals |
| Fitness | 14–17% | 21–24% | Generally associated with a lean, fit appearance and low average health risk |
| Average | 18–24% | 25–31% | Common range for generally healthy, non-athletic adults under 40 |
| Obese | 25%+ | 32%+ | Associated with increased average risk of weight-related health conditions |
If your result falls in the Athletes or Fitness range: this generally aligns with lower average health risk and a lean body composition — maintaining your current training and nutrition approach is typically reasonable, alongside periodic re-checks.
If your result falls in the Average or Obese range: the "Ideal Body Fat %" and "Fat to Lose" figures shown in your results give a concrete target window. Gradual, sustainable changes to nutrition and activity are generally favored over rapid or extreme approaches.
This result is for informational purposes only and is not a medical diagnosis. The US Navy method is a population-calibrated estimate, not an individual diagnostic measure — it does not know your bone density, medical history, or exact fat distribution.
This calculator provides educational estimates only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Results may vary depending on individual health conditions. Always consult a qualified healthcare professional regarding medical concerns.
Where a fast, tape-measure body composition estimate is genuinely useful
Watch fat mass and lean mass move independently during a cut or bulk, instead of judging progress by scale weight alone.
Use the "Fat to Lose" and "Ideal Body Fat %" figures to set an evidence-informed goal rather than guessing a target weight.
Get a clearer picture than BMI for muscular individuals, since this method estimates fat directly instead of penalizing extra muscle weight.
Trainers can use waist/neck/hip measurements as a quick, equipment-free baseline and progress check for clients.
Athletes and coaches can log a body composition baseline before a training block or competitive season.
The Navy method mirrors circumference-based standards used in some military and public-safety fitness assessments.
Bodybuilders and physique competitors can log measurements weekly with the built-in progress tracker during a prep cycle.
New parents can track body composition changes over time as an alternative to focusing solely on scale weight.
Bring an objective fat mass and lean mass starting point into a conversation with a dietitian or coach.
Track body composition changes from a home training program without needing gym-based equipment.
Students and educators can use the worked formula to illustrate circumference-based body fat estimation.
Get a periodic, equipment-free read on body composition trends as part of a broader wellness routine.
Older adults can use the age-adjusted categories and lean mass figure to keep an eye on sarcopenia-related changes.
What this body fat calculator does well, and where it can't replace professional assessment
How this calculator's category cutoffs shift across the three age groups you can select
| Age Group | Men — Obese Threshold | Women — Obese Threshold |
|---|---|---|
| Under 40 | 25% and above | 32% and above |
| 40–59 | 28% and above | 38% and above |
| 60 and over | 30% and above | 40% and above |
Summary: This free body fat calculator gives you an instant US Navy method estimate of body fat percentage, fat mass, and lean mass in metric or imperial units, along with age-adjusted categories and a progress tracker — a useful tracking tool, not a medical diagnosis. Pair it with related tools like the BMI Calculator and Ideal Weight Calculator for a fuller picture of your body composition and healthy weight range.
Common questions about body fat calculations
| Category | Body Fat % |
|---|
| Category | Body Fat % |
|---|
⚠️ Categories based on ACE (American Council on Exercise) standards, with age-group adjustments from ACSM clinical norms. Asian populations may have higher metabolic health risks at lower body fat percentages — similar to the well-known BMI Asian cutoff effect. Consult a healthcare provider for a personalised assessment.
Uses circumference measurements to estimate body density and derive body fat %. It's more accurate than BMI for assessing body composition, especially for muscular individuals. Accuracy is ±3–4%.
Measure waist at navel level after a normal exhale — this is the US Navy protocol, which differs from WHO/NHS guidelines that use the midpoint between the lowest rib and hip bone. Measure neck below the larynx. Measure hips (women) at the widest point. Take measurements twice and average them.
Essential fat is the minimum needed for normal physiological function. Going below this threshold — common in extreme cutting phases — causes hormonal disruption, immune suppression, and organ dysfunction. Never go below essential fat levels.
Official guidance to complement this calculator — not a substitute for licensed medical advice
Explore other health and body composition tools