Body Fat Calculator
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Estimate your body fat percentage from tape measurements, in inches or centimeters, with your fat mass, lean mass and fitness category.
Quick answer: This body fat calculator uses the US Navy circumference method to estimate body fat from waist, neck and (for women) hip measurements plus height. A man with a 34-inch waist, 15-inch neck and 70-inch height comes out at about 17.4% body fat, and a woman with a 30-inch waist, 40-inch hips, 13-inch neck and 65-inch height at about 30.8%. It is a field estimate, typically within 3 to 4 percentage points of a DEXA scan, so use it to track your trend.
Estimated Body Fat
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Uses the US Navy circumference method. Measure at navel level for waist.
📐 Formula
All measurements in centimeters. Male: BF% = 495 ÷ [1.0324 − 0.19077×log₁₀(waist − neck) + 0.15456×log₁₀(height)] − 450. Female: BF% = 495 ÷ [1.29579 − 0.35004×log₁₀(waist + hip − neck) + 0.22100×log₁₀(height)] − 450. Inches × 2.54 = centimeters.
How to Use the Body Fat Calculator
Choose sex and units
Pick Male or Female, because the equation differs, then choose Imperial (inches and pounds) or Metric (centimeters and kilograms).
Measure your waist, neck and height
Measure the waist at the navel (belly button) level, not the narrowest point. Measure the neck just below the larynx. Stand relaxed, exhale normally, keep the tape level and do not pull it tight.
Measure your hips (women)
Women also measure the hips at the widest point. The hip box appears when Female is selected.
Enter your measurements
Type the measurements in the boxes. Use the same unit for all of them. The result updates as you type.
Add your weight for fat and lean mass
Optional. Enter your body weight to see your fat mass and lean mass.
Track the trend, not the number
A tape-measure estimate can be a few percentage points out. Measure the same way every few weeks and watch the direction of change.
How the US Navy body fat formula works
The US Navy circumference method was developed as a field alternative to hydrostatic weighing for military fitness assessments. It uses tape measurements to estimate body density with the Hodgdon & Beckett (1984) regression, then converts density to a body fat percentage with the Siri equation. The equations take measurements in centimeters. For men: BF% = 495 ÷ [1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)] − 450. For women: BF% = 495 ÷ [1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)] − 450. If you enter inches, the calculator multiplies by 2.54 first.
Validation studies show accuracy within 3 to 4 percentage points of DEXA (dual-energy X-ray absorptiometry) for most individuals. The method is less accurate for very lean or very obese individuals, and for people with atypical fat distribution.
Body fat percentage categories
For men: Essential fat (2 to 5%), the minimum for organ function; Athletic (6 to 13%); Fit (14 to 17%); Average (18 to 24%); Above average (25% and over). For women: Essential fat (10 to 13%); Athletic (14 to 20%); Fit (21 to 24%); Average (25 to 31%); Above average (32% and over). Women's essential fat is higher because of sex-specific fat (breasts, uterus, hips) needed for hormonal and reproductive function. This is normal, not excess fat. These categories describe adults in general and do not change with age on this page.
Fat mass and lean mass: what your percentage means in weight
A percentage is easier to act on when you can see it as pounds or kilograms. Fat mass is your weight times your body fat percentage, and lean mass is everything else. Take a 180 lb man at 17.4% body fat: fat mass is 180 × 0.174 = about 31 lb, and lean mass is about 149 lb. If he gains 5 lb of muscle and loses 5 lb of fat, his weight stays at 180 lb while his body fat drops to about 14.6% (26.3 ÷ 180). Weight alone hides that change, which is why the BMI calculator can read the same before and after. To turn your lean mass into a daily calorie figure, use the BMR calculator.
That is why the scale alone can mislead during a training block. Lean mass is not the same as muscle mass, because it also includes bone, organs and the water in your tissues. Use it to follow the direction of change, and read it together with your tape measurements, not as a lab number.
How to measure for the Navy method
Most of the error in this method comes from the tape, not the math. A one-centimeter slip at the waist moves the result by roughly half a point to three-quarters of a point, so how you measure matters.
- Waist: at the level of the navel, tape level all the way round, after a normal exhale. Do not pull your stomach in.
- Neck: just below the larynx (Adam's apple), tape sloping slightly down toward the front. Do not flare the neck muscles.
- Hips (women): at the widest point of the buttocks, feet together.
- Height: barefoot against a wall, looking straight ahead.
Pull the tape snug but not tight enough to dent the skin. Take each measurement twice and use the average. Measure at the same time of day, because your waist can change by a centimeter or more between morning and evening.
Tape, calipers, scales and DEXA compared
| Method | What you need | Good for | Watch out for |
|---|---|---|---|
| Navy tape method (this calculator) | Tape measure | Free, at-home trend tracking | Tape position; less accurate at the extremes |
| Skinfold calipers | Calipers and a trained person | Cheap repeat measurements | Technique varies a lot between people |
| Smart scales and watches (bioelectrical impedance) | The device | Daily convenience | Results move with hydration; use for trend only |
| DEXA scan | An imaging center visit | Fat, lean and bone mass measured separately | Cost and access |
Why body fat % tells more than BMI
BMI cannot distinguish between muscle and fat: a muscular athlete and an untrained person of the same height and weight get the same BMI with very different health profiles. Body fat percentage looks at the fat-to-lean ratio directly. A person can have a "normal" BMI but carry a high body fat percentage (sometimes called "normal-weight obesity" or "skinny fat"), with low muscle mass and elevated visceral fat. Athletes often register as "overweight" by BMI despite being very lean.
More accurate methods for comparison
DEXA scanning (the most accessible accurate method, about $50 to $150 at imaging centers) measures bone density, lean mass and fat mass separately. Hydrostatic (underwater) weighing is a long-used laboratory reference method but rarely accessible. Air displacement plethysmography (Bod Pod) has comparable accuracy and is easier to find. Bioelectrical impedance (home scales, gym machines) is convenient but varies with hydration, so it is best for trend tracking. The Navy method here is more accurate than bioelectrical impedance for most people and needs only a measuring tape.
Worked example: the Navy method by hand
The method turns tape measurements into centimeters, then uses base-10 logarithms to estimate body density.
Men. A man with a 34-inch waist, 15-inch neck and 70-inch height: in centimeters that is 86.36, 38.1 and 177.8. waist − neck = 48.26 cm, so log₁₀(48.26) = 1.6836, and log₁₀(177.8) = 2.2499. Density = 1.0324 − (0.19077 × 1.6836) + (0.15456 × 2.2499) = 1.0324 − 0.3212 + 0.3477 = 1.0590. BF% = 495 ÷ 1.0590 − 450 ≈ 17.4%, in the "fit" category for men.
Women add a hip measurement. A woman with a 30-inch waist, 40-inch hips, 13-inch neck and 65-inch height is 76.2, 101.6, 33.02 and 165.1 cm. waist + hip − neck = 144.78 cm, so log₁₀(144.78) = 2.1607, and log₁₀(165.1) = 2.2177. Density = 1.29579 − (0.35004 × 2.1607) + (0.22100 × 2.2177) = 1.29579 − 0.7563 + 0.4901 = 1.0296. BF% = 495 ÷ 1.0296 − 450 ≈ 30.8%, in the "average" range for women.
Why the Navy method uses a different formula for women
Why does the female formula include a hip measurement?
Fat distribution differs between the sexes. Women typically carry more essential fat and store more of it around the hips, which a waist-and-neck equation would misrepresent. The hip term and the higher essential-fat range for women (roughly 10 to 13% against 2 to 5% for men) reflect a real physiological difference.
How accurate is a tape measurement compared with lab methods?
The Navy method is validated to within roughly 3 to 4 percentage points of methods such as DEXA or hydrostatic weighing for most body types. That makes it a reasonable field estimate and not a diagnostic measurement. It is less accurate at the extremes, such as very lean athletes and people with obesity, where the underlying regression was less densely sampled.
Why can body fat percentage change while BMI stays the same?
BMI uses only weight and height. Someone who loses fat and gains the same amount of muscle through resistance training keeps the same BMI while their body composition improves. Body fat percentage shows that change and BMI does not.
Frequently Asked Questions
For men: Athletic (6–13%), Fit (14–17%), Average (18–24%), Above average (25%+). For women: Athletic (14–20%), Fit (21–24%), Average (25–31%), Above average (32%+). Essential fat is 2–5% for men and 10–13% for women.
The US Navy method is accurate to within 3–4% for most people. It's a reliable field test but less precise than DEXA scanning or hydrostatic weighing. Consistent measurement technique is key.
Yes. Switch the units to Metric and enter waist, neck, hip and height in centimeters and weight in kilograms. The calculator uses the same equation either way and converts inches to centimeters for you (1 inch = 2.54 cm), so 34 inches and 86.4 cm give almost the same result, within a tenth of a percentage point.
Multiply your body weight by your body fat percentage to get fat mass, then subtract it from your weight for lean mass. At 180 lb and 17.4% body fat, fat mass is about 31 lb and lean mass is about 149 lb. Enter your weight in the optional box and the calculator does it for you. Lean mass includes muscle, bone, organs and water, not just muscle.
Yes, with a different equation. Women enter a hip measurement as well as waist, neck and height, and the result uses the female version of the equation. Women carry more essential fat than men, so the categories for women are shifted up by several points.
Measure at the navel (belly button) level, not the narrowest point. Keep the measuring tape horizontal, don't suck in your stomach, and measure after exhaling normally.
Different tools use different methods. Calculators based on BMI, skinfold calipers, bioelectrical impedance scales and DEXA scans can each give a different figure for the same person, sometimes by several points. Tape measurements also shift with a slightly different tape position. Stick with one method and one way of measuring, and judge change over weeks rather than a single reading.
It is an adult method. Children and teenagers are still growing, and their body fat is judged against age-specific charts, so a calculator built for adults can mislead. Ask a doctor or school nurse for guidance instead of relying on this result.
Every two to four weeks is enough. Body measurements change slowly, and daily readings mostly show noise from tape position, hydration and time of day. Measure at the same time of day, in the same place, with the same tape.
It's less accurate at the extremes. The regression it's built on was less densely sampled for very lean athletes and people with obesity, so DEXA scanning or hydrostatic weighing give more reliable results outside the typical range.
Yes. Someone who loses fat and gains an equal amount of muscle through resistance training will show an unchanged BMI despite a meaningfully improved body composition. Body fat percentage reflects that change, BMI does not.
Sources & Methodology
Calculations are based on the most current publicly available data from authoritative government and industry sources: