CodeKitHubWhat Is This Tool?
A body fat calculator estimates what share of your body weight is fat tissue, as opposed to muscle, bone, organs and water. This one implements the U.S. Navy method, developed at the Naval Health Research Center: it uses the logarithms of a few circumference measurements (neck, waist, and for women also hip) together with height to predict body-fat percentage. The method was designed so that any service member could be assessed with just a tape measure, and it remains one of the most practical at-home approaches.
Compared with laboratory methods such as DEXA scans or hydrostatic weighing, the Navy formula is typically accurate within about 3–4 percentage points for most people. That is precise enough to tell an 'athletic' range from an 'obese' range and to track direction of change, but not precise enough to distinguish 18% from 20%. Measure at the same time of day, with the tape snug but not compressing the skin, and take the average of two or three measurements for the most consistent results.
Why Use It?
- Needs only a tape measure — no scales that pass current through you, no calipers, no lab appointment.
- Uses the published U.S. Navy formula, one of the most validated circumference methods available.
- Classifies your result on the ACE body-fat scale (essential fat / athletes / fitness / acceptable / obesity), with separate ranges for men and women.
- Supports centimeters and inches.
- Free and instant, with nothing uploaded — the calculation runs entirely in your browser.
How to Use
- Select your sex — the formula and category ranges differ for men and women.
- Enter your height, then measure and enter your neck circumference (just below the larynx) and waist circumference (at the navel for men, at the narrowest point for women).
- If you are female, also measure and enter your hip circumference at the widest point.
- Press the calculate button and read your estimated body fat percentage and its ACE category.
Example
Input
Male, height 178 cm, neck 38 cm, waist 85 cmOutput
Body fat ≈ 18% — AcceptableThe Navy formula for men uses log10(waist − neck) and log10(height). A result around 18% falls in the ACE 'acceptable' range for men (18–24%); 14–17% would be 'fitness' and 6–13% 'athletes'.
ACE body fat categories
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Obesity | 25% and above | 32% and above |
Tips for consistent measurements
- Measure at the same time of day — morning before eating is the most repeatable.
- Use a flexible, non-stretch tape and keep it parallel to the floor at every site.
- Take each measurement two or three times and use the average.
- Don't pull the tape tight enough to dent the skin — snug contact is the standard.
- Track the trend across weeks rather than reacting to any single reading; day-to-day water shifts alone can move the estimate.
- This calculator is an estimate for general reference only, not a medical assessment. If body composition matters for a health decision, ask a doctor about clinical methods such as DEXA.
Related health calculators
Body fat percentage tells you what your weight is made of; the BMI Calculator gives the faster weight-for-height screening number that clinics use, and the two together give a more complete picture than either alone.
If your goal is changing body composition, energy balance is the other half of the equation: the Calorie Calculator estimates how much you should eat per day, the BMR Calculator shows the resting metabolism underneath that figure, and the Water Intake Calculator covers daily hydration.
→ BMI Calculator · Calorie Calculator · BMR Calculator · Water Intake Calculator
Frequently Asked Questions
How does the U.S. Navy body fat formula work?
For men: body fat % = 495 / (1.0324 − 0.19077×log10(waist − neck) + 0.15456×log10(height)) − 450, with measurements in centimeters. For women: 495 / (1.29579 − 0.35004×log10(waist + hip − neck) + 0.22100×log10(height)) − 450. The formulas were fitted against hydrostatic weighing data by the Naval Health Research Center.
How accurate is the tape measure method?
Studies put the Navy method within roughly 3–4 percentage points of DEXA results for most adults, with larger errors at the very lean and very high ends. Measurement technique matters a lot: a tape pulled 1 cm tighter at the waist can shift the result by a full percentage point, so measure consistently and focus on the trend, not single readings.
What are healthy body fat ranges?
On the ACE scale, for men: 2–5% essential fat, 6–13% athletes, 14–17% fitness, 18–24% acceptable, 25%+ classified as obesity. For women: 10–13% essential, 14–20% athletes, 21–24% fitness, 25–31% acceptable, 32%+ obesity. Women naturally carry more essential fat than men, which is why every range sits higher.
Why is body fat percentage more informative than BMI?
BMI only compares weight with height, so it cannot tell muscle from fat — a muscular athlete and a sedentary person can share the same BMI with completely different body compositions. Body fat percentage measures composition directly, which is why fitness assessments often prefer it. The two are complementary: BMI is a fast screening number, body fat percentage describes what the weight consists of.
Where exactly should I place the tape measure?
Neck: just below the larynx (Adam's apple), tape sloping slightly downward to the front. Waist: for men at navel level, for women at the narrowest point of the torso. Hips (women): at the widest point around the buttocks. Keep the tape level and snug against the skin without compressing it, and breathe out normally before reading.
Is this a medical measurement, and is my data stored?
No on both counts. This is a statistical estimate for general fitness reference, not a medical diagnosis — body composition assessments for medical purposes should be done by professionals. The calculation runs entirely in your browser; your measurements are never sent to a server, stored or logged.