Body Fat Calculator
Estimate body fat percentage from tape measurements using the US Navy circumference method, and separate your weight into fat mass and lean mass.
Body Fat Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: The US Navy method estimates body fat from circumference measurements and height. For men it uses waist and neck; for women it adds hip circumference. It is accurate to roughly ±3–4% against hydrostatic weighing, making it the most reliable method requiring only a tape measure.
Formula
Women use %BF = 495 / (1.29579 − 0.35004 log(waist + hip − neck) + 0.22100 log(height)) − 450. All measurements in centimetres, logarithms base 10.
Worked Example
- Find the circumference difference. Waist 92 − neck 39 = 53 cm.
- Take base-10 logs. log(53) = 1.72428, log(178) = 2.25042.
- Build the denominator. 1.0324 − (0.19077 × 1.72428) + (0.15456 × 2.25042) = 1.05174.
- Divide and subtract. 495 ÷ 1.05174 − 450 = 20.9% body fat.
- Split the mass. 82 kg × 20.9% = 17.1 kg fat, leaving 64.9 kg lean mass.
Clinical note. At 20.9% this result sits in the average range for men, even though the same person's BMI of 25.9 reads as overweight. This divergence is exactly why circumference methods add real information over BMI alone.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Tracking recomposition, not just weight
Scale weight can stay flat while fat falls and muscle rises. Measuring circumferences monthly reveals that change where the scale hides it. Pair with the Lean Body Mass Calculator.
Setting a realistic target
Essential fat is roughly 3% for men and 12% for women; going near those floors is neither necessary nor safe for most people. A target in the fitness range is more sustainable than an athletic one.
Improving on BMI for muscular builds
Where BMI misclassifies a muscular person as overweight, body fat percentage and the fat-free mass index give a far more accurate picture of actual health status.
Methodology & Editorial Standards
This engine implements the US Navy circumference method developed by Hodgdon and Beckett, using base-10 logarithms and metric measurements internally, with imperial input converted at exact factors. The method carries a standard error of roughly 3 to 4 percentage points against hydrostatic weighing, and is less reliable at the extremes of body composition. Measure at the narrowest point of the waist for women, at the navel for men, and keep the tape level and snug without compressing the skin. This engine implements the published formula exactly as specified in the source literature, with inputs validated for physiological plausibility before evaluation. Values outside the domain of the formula return an em-dash rather than a misleading figure.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Body Fat Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is a healthy body fat percentage?
For men, roughly 14 to 24% is considered acceptable, with 6 to 13% athletic. For women the equivalent bands are about 21 to 31% and 14 to 20%. Women require more essential fat for hormonal function, which is why every threshold is higher.
How accurate is the US Navy method?
It carries a standard error of about 3 to 4 percentage points compared with hydrostatic weighing. That is meaningfully better than BMI as a proxy for adiposity, though less precise than DEXA scanning or air displacement plethysmography.
How should I take the measurements?
Use a flexible tape, keep it level and snug without compressing skin, and measure on bare skin after exhaling normally. Men measure the waist at the navel; women at the narrowest point. Neck is measured just below the larynx. Take each measurement three times and average them.
Why does the formula need my height?
Height acts as a proxy for frame size. The same waist circumference implies very different adiposity on a person of 160 cm than on one of 190 cm, and the logarithmic height term corrects for that.
What is essential body fat?
The minimum required for normal physiological function — roughly 3% for men and 12% for women. It is structural fat in organs, bone marrow and the central nervous system. Dropping below these levels carries serious health risk, particularly for hormonal and reproductive function.
What is FFMI?
Fat-Free Mass Index is lean mass divided by height in metres squared. It measures muscularity independently of fat, and is often used to assess whether a physique is achievable naturally. Values around 25 and above are considered exceptional in drug-free athletes.
Why is my body fat different from my smart scale?
Bioelectrical impedance scales estimate body fat by passing a small current through the body, and are highly sensitive to hydration, recent food, exercise and skin temperature. Readings can swing several percent within a single day. Circumference methods are more stable, if less precise in principle.
Can I lose fat without losing weight?
Yes — this is body recomposition, gaining muscle while losing fat. It is most achievable for beginners, those returning after a break, and anyone with substantial fat to lose. Adequate protein and resistance training are the prerequisites.
Does body fat distribution matter?
Considerably. Visceral fat around the organs is far more strongly associated with cardiometabolic risk than subcutaneous fat on the hips and thighs. This is why waist measurement carries independent predictive value beyond total body fat.
How fast can I safely lose body fat?
Roughly 0.5 to 1% of body weight per week is a common guideline. Leaner individuals should aim at the slower end, since aggressive deficits at low body fat disproportionately cost lean tissue.
Does body fat percentage change with age?
Yes. Body fat tends to rise and lean mass to fall with age even at constant weight, a pattern sometimes called sarcopenic obesity. Resistance training and sufficient protein are the most effective countermeasures.
Why is the hip measurement only used for women?
The Navy equations were derived separately by sex. Female fat distribution is more gynoid — concentrated around the hips and thighs — so hip circumference adds predictive accuracy that it does not contribute in the male equation.
Is this body fat calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.