What this calculator does
The US Navy method estimates body fat from a few tape measurements rather than specialist equipment. It works because fat distribution follows reasonably predictable patterns: the ratio of waist to neck circumference, scaled by height, correlates well enough with body composition to be useful.
It is an estimate with a typical error of around three to four percentage points against laboratory methods. That is accurate enough to be genuinely informative and not accurate enough to agonise over a single reading. Its real strength is consistency: measured the same way each time, it tracks changes well.
The formula
The equations are logarithmic regressions fitted to hydrostatic weighing data. Men use the difference between waist and neck; women add hip circumference, because fat distribution differs. Height appears in both to scale for body size.
| Term | Meaning |
|---|---|
| Body fat % | The proportion of total body mass that is fat. |
| Fat mass | Body weight × body fat percentage. |
| Lean mass | Everything else: muscle, bone, organs, water. |
| Essential fat | The minimum needed for normal physiological function. |
The inputs explained
| Field | What to enter |
|---|---|
| Sex | Biological sex. The equations differ because fat distribution patterns differ. |
| Height (cm) | Height in centimetres. |
| Neck circumference (cm) | Neck circumference, measured just below the larynx with the tape sloping slightly downward at the front. |
| Waist circumference (cm) | Waist circumference. Men measure at the navel; women at the narrowest point. |
| Hip circumference (women) (cm) | Hip circumference at the widest point. Used in the equation for women only. |
| Weight (kg) | Body weight, used to convert the percentage into fat and lean mass in kilograms. |
When to use it
Tracking body composition during training
More informative than weight alone, because it distinguishes muscle gained from fat lost. Two people at the same weight can have very different compositions.
Understanding a misleading BMI
If BMI reads high but body fat reads athletic, the extra weight is likely muscle. This is the standard case where BMI on its own gives the wrong impression.
Measuring consistently
Measure at the same time of day, ideally in the morning before eating, with the tape snug but not compressing the skin. Consistency matters more than absolute precision, since the trend carries the information.
Setting realistic expectations
Essential fat is around 3 per cent for men and 12 per cent for women, and going near those levels is neither healthy nor sustainable for most people. The athletic and fitness bands are far more appropriate reference points.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Body fat at different waist measurements
A 175 cm man of 75 kg with a 38 cm neck, at a range of waist measurements.
| Waist | Body fat | Category | Fat mass | Lean mass |
|---|---|---|---|---|
| 76 cm | 9.31% | Athletic | 7.0 kg | 68.0 kg |
| 82 cm | 14.5% | Fit | 10.9 kg | 64.1 kg |
| 88 cm | 19.2% | Average | 14.4 kg | 60.6 kg |
| 94 cm | 23.4% | Average | 17.6 kg | 57.4 kg |
| 100 cm | 27.3% | Above average | 20.5 kg | 54.5 kg |
| 108 cm | 31.9% | Above average | 24.0 kg | 51.0 kg |
The effect of height at a fixed waist
The same measurements on frames of different heights.
| Height | Body fat | Category | Fat mass |
|---|---|---|---|
| 160 cm | 21.9% | Average | 16.4 kg |
| 170 cm | 20.1% | Average | 15.1 kg |
| 175 cm | 19.2% | Average | 14.4 kg |
| 185 cm | 17.6% | Fit | 13.2 kg |
| 195 cm | 16.0% | Fit | 12.0 kg |
Questions
How accurate is the Navy method?
Typically within three to four percentage points of hydrostatic weighing for most people. It is less reliable at the extremes of body composition and for people whose fat distribution is unusual. It is best used for tracking change rather than establishing an exact figure.
What is a healthy body fat percentage?
Commonly cited ranges are roughly 8 to 19 per cent for men and 21 to 32 per cent for women, with athletes often lower. These vary by age and by the source of the guidance, and there is no single correct target.
Where exactly should I measure my waist?
Men measure at the navel, women at the narrowest point of the torso. Keep the tape horizontal, snug against the skin without compressing it, and measure after breathing out normally.
Why does my result differ from a body composition scale?
Bioelectrical impedance scales infer composition from how current passes through the body, which is heavily affected by hydration. Neither method is a reference standard: expect several percentage points of disagreement and track one method consistently rather than comparing them.
Can body fat be too low?
Yes. Below essential levels, around 3 per cent for men and 12 per cent for women, normal physiological function is compromised, including hormonal and reproductive health. Sustained very low body fat carries real risks and warrants medical guidance.
For a simpler screening measure, see BMI or waist-to-hip ratio. For daily energy needs, use the TDEE calculator.