Body Fat Calculator
Free body fat calculator using the US Navy circumference method. Estimate body fat %, fat mass, and lean mass from waist, neck, and hip measurements.
Body fat from tape measurements
This tool estimates body fat percentage with the US Navy circumference method, using waist and neck for men and adding hip for women. It also reports fat mass, lean body mass, and a broad category based on the height and weight you enter.
The US Navy formulas
All circumference and height inputs are in inches, weight in pounds. For men:
Body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
For women the coefficients change and the hip is included:
Body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
Once a percentage is found, fat mass is weight × body fat % and lean body mass is weight minus fat mass. A minimum of 2% is enforced so the output never goes negative.
Worked example with the default inputs
The tool opens as male, 70 in tall, 175 lb, waist 34 in, neck 15 in. Hip is ignored for men.
- waist − neck = 34 − 15 = 19 in, and log10(19) = 1.2788
- log10(70) = 1.8451
- Denominator = 1.0324 − (0.19077 × 1.2788) + (0.15456 × 1.8451) = 1.0324 − 0.2439 + 0.2852 = 1.0736
- 495 ÷ 1.0736 = 461.05, then − 450 = 11.1%
The output then reads fat mass = 175 × 0.111 = 19 lb, lean body mass = 156 lb, and the category Athletic.
Category reference
These are the calculator’s own thresholds, drawn from widely used adult ranges:
| Category | Men | Women |
|---|---|---|
| Essential fat | under 6% | under 14% |
| Athletic | 6–13.9% | 14–20.9% |
| Fitness | 14–17.9% | 21–24.9% |
| Average | 18–24.9% | 25–31.9% |
| Obese range | 25% and up | 32% and up |
Women carry more essential fat for hormonal and reproductive function, which is why the bands sit several points higher.
Measuring so the number is meaningful
Use a flexible tape, keep it flat against bare skin, and do not cinch it tight. Measure the waist at its narrowest point and the neck just below the larynx. For women, measure the hips at the widest point over the buttocks. Take each reading twice and average them. Consistency matters more than perfect technique, because a change of half an inch in the waist moves the result by roughly a percentage point.
Common mistakes
- Measuring over clothing. Even a thin shirt adds inches and inflates the result.
- Using the wrong landmark. The waist is the narrowest point, not the navel; the neck is below the larynx, not at the jaw.
- Comparing single readings. Hydration, posture, and tape tension can swing the number by 1–2 points, so track month to month.
- Judging a result as a diagnosis. The Navy method is a field estimate, not a clinical measurement.
How accurate is the Navy method compared with DEXA?
For most adults it lands within about 2–4 percentage points of a DEXA scan, which is strong for a tape measure. Very lean and very muscular people see larger gaps because the formula infers fat from circumference alone.
Do I need the hip measurement as a man?
No. The male formula uses only waist and neck. The hip field is ignored unless sex is set to female.
Why is my percentage different from my smart scale?
Bioelectrical impedance scales estimate fat from the body’s resistance to a small current, which shifts with hydration. The two methods often disagree by several points; pick one and track its trend rather than switching between them.
Why do athletes sometimes get an unrealistic result?
Very muscular people can read higher than their true fat percentage, because a thick neck and waist partly reflect muscle rather than fat. At the other extreme, very lean people can read lower than reality. If you fall at either end, treat the number as a trend indicator and consider a DEXA scan for a precise figure.
Does the method work at any age?
The Navy equations were developed on service members, so they are most reliable for adults roughly aged 18 to 50. Results for older adults and adolescents should be read more loosely, since fat distribution and body composition change with age.
Fat mass and lean body mass
Once you have a body-fat percentage, the rest is arithmetic. Fat mass is weight multiplied by the percentage; lean body mass is everything else — muscle, bone, organs, and water. The calculator reports both, so you do not need a separate step to find them.
Lean body mass is the more useful number for tracking training. Two people can weigh the same and carry very different amounts of muscle, and lean mass is what drives resting metabolism. Re-measuring every few weeks and watching lean mass hold steady while fat mass falls is a far better signal than the scale alone.
This is an estimate, not medical advice. For a fuller picture, pair it with the BMI calculator for weight relative to height — and note this tool already reports the fat-free side as lean body mass. Calorie targets built on that lean mass start with the TDEE calculator.