MariposasBody Fat Calculator
By Zachary, builder of Mariposas · Reviewed against published exercise-science sources (references)
· Last updated June 2026
Body Metrics Using just a tape measure, the US Navy method estimates body-fat percentage from your height and circumference measurements. It is an estimate, useful for tracking change over time rather than a precise reading.
⚕️ A general-information estimate from population-level formulas, a starting point, not a precise measurement and not medical advice.
How it works
The US Navy circumference method uses a logarithm-based formula developed by Hodgdon and Beckett in the 1980s for the US military to estimate body-fat percentage from measurements most people can take at home. For men, the formula uses neck circumference, waist circumference (measured at the navel), and height. For women, it adds a hip measurement to account for the different fat distribution patterns typical in female physiology. The math converts those three or four tape measurements into a body-density estimate and then into a body-fat percentage using a standard logarithmic equation. Because the underlying research was done on a large military population, the formula tends to be reasonably accurate for adults with average build, though it can underestimate body fat in people who carry fat in unusual patterns and overestimate it in very muscular individuals with thick necks.
When to use it
This calculator is most useful for anyone who wants a repeatable, low-cost way to track changes in body composition over weeks or months without needing a DEXA scan or hydrostatic weighing. It is especially practical for people who have hit a plateau on the scale and want to see whether their ratio of fat to lean mass is shifting even when total weight stays flat. Athletes, gym-goers returning from a break, and people making dietary changes all use it as a rough compass rather than a final verdict.
Worked example
Say a 35-year-old man is 5 feet 10 inches tall (70 inches), measures his waist at 36 inches, and his neck at 16 inches. Plugging those into the Navy formula gives an estimated body-fat percentage somewhere around 20 to 21 percent, which puts him in the 'acceptable' range for his age group but above the 'fitness' threshold that typically sits closer to 14 to 17 percent for men. That number by itself is less important than what happens next: if he runs the same measurement protocol six weeks later after consistent training and diet changes and gets 18.5 percent, he has real evidence of progress that the scale alone might not show, because he may have added muscle while losing fat, keeping his weight nearly unchanged.
Tips for an accurate result
- Measure first thing in the morning before eating, drinking, or exercising. Hydration and food volume can shift waist measurements by an inch or more across a day, which meaningfully changes the output.
- Use a flexible but non-stretchy tape measure and keep it level and snug, not compressive. A tape that dips at the back or pinches the skin will give you a different number every time.
- For the waist, the Navy method specifically calls for measuring at the navel, not at the narrowest point. Using the narrower natural waist instead will underestimate your result.
- Take each measurement twice and average them. If two readings differ by more than half an inch, take a third and use the two closest numbers.
- Retest under the same conditions every four to six weeks rather than weekly. Short-interval retesting picks up noise from hydration and measurement error more than actual body composition change.
Formula & sources: methodology · references.
Now go hit the number Mariposas turns every workout, run and class into progress · collect a cute pet 🐾FAQ
- How accurate is the Navy method compared to DEXA or hydrostatic weighing?
- Research comparing the Navy method to laboratory techniques typically finds it accurate within roughly 3 to 4 percentage points for most adults. That margin of error matters if you are trying to nail down an exact number, but for tracking direction of change over time, the method is quite reliable as long as you measure consistently.
- Why does the formula use neck circumference?
- Neck circumference serves as a proxy for lean mass. People with more muscle generally have thicker necks, so including it helps the formula distinguish between someone who has a large waist due to fat versus someone who is simply big all over from muscle. Without it, the estimate would skew high for muscular people.
- Can this calculator be used for teenagers?
- The Hodgdon and Beckett formula was developed and validated on adult military personnel, so its accuracy for adolescents is less established. Body composition norms also differ significantly by age during development, so results for teenagers should be interpreted with extra caution and ideally discussed with a healthcare provider.
- My body fat result seems way off from what my gym's scale says. Which is right?
- Both are estimates using different assumptions. Bioelectrical impedance scales (the kind common in gyms) are heavily affected by hydration, and their accuracy varies a lot by model and population. Neither method is definitively 'right.' If the two give very different results, the more useful approach is to pick one method and track it consistently over time rather than comparing absolute numbers across methods.
- What body fat percentage is considered healthy?
- General reference ranges vary by source and by age, but a commonly cited framework places 'essential fat' for men around 2 to 5 percent and for women around 10 to 13 percent, with 'fitness' ranges roughly at 14 to 17 percent for men and 21 to 24 percent for women. These are population-level descriptors, not clinical cutoffs, and what is optimal for a given person depends on many factors beyond a single number.