Why a tape measure instead of a fancier method
DEXA scans and hydrostatic weighing are more accurate, but neither is something you can do at home. The Navy circumference method (Hodgdon & Beckett, 1984) was developed and validated specifically as a field-usable alternative — it converts a few tape measurements into an estimated body density, then applies the Siri equation to convert that density into a body fat percentage. It won’t match a lab, but it’s the most validated method that only needs a tape measure.
Getting a consistent measurement
Measure on bare skin with a flexible, non-elastic tape, snug but not compressing the skin. Take each measurement two or three times and use the average. Measuring at the same time of day — ideally in the morning, before eating — removes most of the day-to-day noise from hydration and food volume.
Using this with the calorie calculator
Once you know your body fat percentage, the Calorie & Macro Calculator can use it to estimate your lean body mass directly, which produces a tighter calorie estimate than formulas that only use height, weight, and age.
Frequently asked questions
How accurate is this?
Published validation studies put the Navy circumference method within about ±3-4 percentage points of a DEXA scan for most people — worse than a lab, far better than a mirror. It tends to be less reliable at the very lean end (under roughly 8% for men, 15% for women) and at the very high end, where the underlying formula was less well-fit to the original validation data.
Why does the calculator want a different measurement for women?
The Navy method uses separate formulas for men and women because fat distribution differs by sex — women's formula adds a hip measurement to account for typically greater fat storage in the hip and thigh region, which a neck-and-waist-only formula would miss.
Why do I need to enter my body weight if it's a percentage?
The percentage itself only needs your circumference measurements. Body weight is used purely to translate that percentage into fat mass and lean mass in real units, which is more useful for tracking progress and feeds directly into more accurate calorie calculations if you use it there.
Is this better or worse than a body fat scale (BIA)?
Neither method is highly precise, but they fail differently. Bioelectrical impedance (BIA) scales are sensitive to hydration status and can swing several points based on when you last ate, drank, or exercised. The tape method is more stable day to day since it doesn't depend on hydration, but depends heavily on consistent, correct measurement technique.
How should I use this number?
As a tracking tool, not a verdict. Measure the same way, at the same time of day, and watch the trend over weeks — the trend is far more informative than any single reading, given the method's error margin.