BMI vs Body Fat Calculator: Compare BMI and Body Fat %

Calculate BMI and estimated body fat percentage together, see why the measures may differ, and learn which result is more informative for your situation. The quick estimate uses BMI, age, and sex; optional tape measurements add an independent US Navy comparison.

By Starlight Tools Editorial Team · Last reviewed: 18 July 2026 · No medical reviewer is claimed

BMI and body fat calculator

Quick comparison: Height, weight, age, and sex calculate BMI plus the Deurenberg body-fat estimate. Open the optional tape section to add a separately derived Navy estimate and a stronger method-to-method comparison.

Unit system

Metric
(kg, cm)

Imperial
(lb, ft/in)

Sex used by the equations

Male

Female

The published equations use sex-specific constants and do not provide another coefficient.

Age (years) Required

Adults 18–120 only.

Weight (kg) Required

Enter 25–350 kg.

Height (cm) Required

Enter 120–230 cm, measured barefoot.

Add tape measurements for the Navy estimate (optional)

Complete every visible tape field to calculate the Navy estimate. Leave all of them blank for the quick Deurenberg-only comparison.

Men’s waistMeasure horizontally at the navel after a normal exhalation.

NeckMeasure just below the larynx, with shoulders relaxed and tape slightly downward at the front.

HeightStand barefoot, upright, heels together, looking straight ahead.

Wear thin clothing or measure against skin. Keep a flexible tape level and snug without compressing skin. Take each circumference two or three times after a normal exhalation; if readings differ, repeat and use the average. Round only after averaging.

Waist at navel (cm) Optional set

Men: 40–250 cm.

Neck (cm) Optional set

Enter 20–70 cm.

Your BMI vs body fat results

Your comparison

Calculated BMI, body fat, mass, category, and reference results
Result	Estimate	Category or range	Actionable detail
BMI	—	—	—
Deurenberg body fat	—	—	—
US Navy body fat	—	—	—

BMI and body-fat percentage use different units and scales. Do not compare their raw marker positions or bar lengths; interpret each result only against its own labeled range.

Estimate, not diagnosis: Use these figures mainly to track trends under consistent conditions. Results can be less representative for highly muscular adults, older adults with low muscle mass, people at body-composition extremes, and some ethnic groups. This adult calculator is not for pregnancy or anyone under 18.

BMI vs body fat: quick answer and comparison

BMI is the faster screening measure; body-fat percentage is the more composition-specific estimate. If BMI and a carefully repeated Navy estimate disagree, the tape estimate often adds more useful composition context, but neither number alone measures health or makes a diagnosis.

Measure	What it measures	Inputs	Strengths	Limitations	Best use
BMI	Weight relative to height	Height, weight	Fast, standardized screening	Cannot distinguish fat from muscle	Population screening and broad weight context
Deurenberg body fat	Predicted body-fat percentage from BMI	BMI, age, sex	No tape needed; quick context	Not independent of BMI; original adult equation reported a 4.1 percentage-point standard error of estimate	Rough first estimate and trends when tape data is unavailable
US Navy body fat	Predicted body-fat percentage from body circumferences	Height, neck, waist; plus hip for women	Independent comparison to BMI; inexpensive and repeatable	Sensitive to tape position, tension, body shape, and source population	Consistent circumference-based trend tracking

When results agree or disagree

If BMI is above the healthy-weight band while a tape estimate is in an athlete, fitness, or general acceptable band, additional lean mass may be raising weight without the same rise in estimated fat. If BMI is in the healthy-weight band while body fat is above its reference band, lower muscle mass or different fat distribution may help explain the pattern. A large gap between Deurenberg and Navy can also come from the first method inheriting BMI's assumptions while the second is sensitive to circumference technique.

How each calculation works

BMI: weight in kilograms ÷ height in metres². Imperial inputs are converted before calculation.
Deurenberg: BF% = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for male and 0 for female in the published adult equation.
Navy, men: 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76.
Navy, women: 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387. Navy formula circumferences and height use inches.

Measurement instructions

Stand upright in thin clothing or measure against skin. Keep feet together and shoulders relaxed.
Measure men at the navel; measure women at the narrowest natural waist. Take the reading after a normal exhalation without pulling the abdomen in.
Measure the neck just below the larynx. For women, also measure the widest point of the hips and buttocks.
Keep the tape horizontal, snug, and flat without compressing skin. Repeat two or three times; repeat again if readings differ, then average before rounding.

Accuracy and other body-composition methods

Both body-fat outputs are predictions and individual error can be larger than a study average. The original Deurenberg adult equation reported a standard error of estimate of 4.1 body-fat percentage points. The Navy equations were developed in military samples and do not have one universal error figure that applies to every person or measuring technique. Use repeat measurements and trends rather than treating a decimal place as certainty.

DXA, air-displacement plethysmography, hydrostatic weighing, bioelectrical impedance analysis (BIA), and skinfold testing are different methods with different assumptions and sources of error. Their results are not interchangeable with each other or with these equations; compare trends using the same method and conditions.

Worked BMI and body-fat comparisons

Example 1: muscular build, higher BMI

Inputs: man, age 30, 180 cm, 88 kg, 82 cm waist, 40 cm neck.

BMI = 88 ÷ 1.80² = 27.2 (overweight screening category).
Deurenberg = 1.20 × 27.2 + 0.23 × 30 − 10.8 − 5.4 = 23.3% (acceptable fitness reference band).
Navy = 86.010 × log10(32.28 − 15.75) − 70.041 × log10(70.87) + 36.76 = 11.9% (athlete reference band).

Interpretation: BMI and the Navy estimate disagree. A muscular build could raise BMI, while Deurenberg partly repeats BMI's limitation. Consistently repeated circumference data adds more composition context, though the 11.4-point method gap is a reason to remeasure rather than declare one exact value.

Example 2: normal BMI, elevated estimates

Inputs: woman, age 55, 168 cm, 60 kg, 88 cm natural waist, 32 cm neck, 105 cm hip.

BMI = 60 ÷ 1.68² = 21.3 (healthy-weight screening category).
Deurenberg = 1.20 × 21.3 + 0.23 × 55 − 5.4 = 32.8% (above the general reference band).
Navy = 163.205 × log10(34.65 + 41.34 − 12.60) − 97.684 × log10(66.14) − 78.387 = 37.9% (above the general reference band).

Interpretation: The body-fat methods agree with each other but not with BMI. BMI may miss composition differences when lean mass is lower. The circumference estimate provides more specific context, but a clinician or a directly assessed method is more appropriate for health decisions.

The “Try muscular-build example” button loads Example 1 into the calculator so you can inspect its personalized result and gauges.

Frequently asked questions

Is BMI or body fat percentage more accurate?

They answer different questions. BMI is a useful weight-for-height screening measure, while body fat percentage estimates composition. Neither calculator estimate is a diagnosis; a consistently measured body-fat method may be more informative when muscle mass makes BMI misleading.

Why do my BMI and body fat results disagree?

BMI cannot separate fat from muscle. Deurenberg also inherits BMI's limits, while the Navy method depends on circumference distribution and tape technique. High muscle mass, low muscle mass, age, body shape, and measurement error can therefore produce different categories.

Can BMI calculate body fat?

BMI cannot measure body fat directly. The Deurenberg equation uses BMI, age, and sex to predict body fat percentage for adults, so its output is an estimate derived from BMI rather than an independent measurement.

What is a healthy body-fat percentage?

There is no single diagnostic healthy percentage for every adult. This calculator shows sex-specific fitness reference bands: 18–25% is the general acceptable band for men and 25–31% for women, with separate essential, athlete, and fitness bands. Age, health, and athletic context also matter.

How accurate is the Navy method?

The Navy method is a field estimate, not a direct body-composition measurement. Its accuracy depends on the population and especially on consistent tape placement and tension. Repeat each circumference and use the same technique when tracking trends.

Where should women and men measure the waist?

For this Navy equation, men measure horizontally at the navel. Women measure the natural waist at the narrowest point. Stand relaxed, measure after a normal exhalation, and keep the tape level and snug without compressing skin.

Why is hip circumference required for women?

The Hodgdon–Beckett equation for women was developed with height, neck, natural-waist, and hip circumferences. Hip circumference is therefore part of that specific prediction formula and cannot be omitted or substituted without changing the method.

Can children or pregnant people use this calculator?

No. This calculator and its adult reference bands are not designed for children, teenagers, or pregnancy. Children require age- and sex-specific growth assessment, and pregnancy changes weight and circumferences in ways these equations do not account for.

Methodology and sources

Calculations run locally in your browser. Starlight Tools has not independently clinically validated this calculator, and no medical or exercise-science reviewer is named because no real reviewer was available for this review.

Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas (1991).
Hodgdon JA, Beckett MB. Prediction of Percent Body Fat for U.S. Navy Men from Body Circumference and Height, Report 84-11 (1984).
Hodgdon JA, Beckett MB. Prediction of Percent Body Fat for U.S. Navy Women from Body Circumference and Height, Report 84-29 (1984).
Centers for Disease Control and Prevention. Adult BMI Categories (BMI is a screening measure).
U.S. Navy ROTC. Guide 4: Body Composition Assessment (measurement and circumference procedure).
Garmin manual, reproducing American Council on Exercise bands. Adult body-fat percentage status reference. These are fitness reference categories, not diagnoses.

Medical disclaimer: This educational adult calculator does not diagnose body fatness, obesity, nutritional status, or disease. Seek qualified clinical advice for symptoms, pregnancy, eating-disorder concerns, major weight changes, or decisions about treatment.