NumberTally Body Fat Calculator
Estimate your body fat percentage with the U.S. Navy tape-measure method — no calipers or lab needed. See your fitness category, your fat vs. lean mass, and how the result compares to what BMI alone would say.
Advanced options
Unlocks fat mass vs. lean mass and a BMI cross-check. Enter in lb (inches mode) or kg (cm mode).
Needs weight too — shows how much fat mass to lose to reach it.
How the Navy tape method works
The U.S. Navy needed a fast, equipment-free way to estimate body fat across thousands of sailors, so researchers fitted equations to tape measurements. For men the key signal is waist minus neck (belly fat relative to frame); for women it’s waist plus hip minus neck. Height adjusts for overall size. The result tracks lab methods (like DEXA) within a few percentage points for most people.
Measure honestly: tape snug but not compressing skin, waist at the navel, neck just below the Adam’s apple, and don’t suck in. A half-inch of wishful thinking moves the result more than you’d expect.
Why waist-minus-neck predicts body fat
Two people can share the same waist size but carry very different frames — the neck measurement corrects for that. A 34-inch waist on a 14-inch neck means something different than the same waist on an 18-inch neck. The logarithm in the formula captures a real pattern: each extra inch of waist matters less, proportionally, as waists get larger.
Estimates only — not medical advice. Talk to a healthcare professional about your health.
Frequently Asked Questions
How accurate is the US Navy body fat method?
Typically within 3–4 percentage points of lab methods like DEXA for average builds. It’s less reliable at extremes — very muscular or very lean people — because it was fitted to a general military population.
Why does the calculator complain when my waist is smaller than my neck?
The men’s formula takes log₁₀(waist − neck), which needs a positive number. A waist at or below the neck usually means a measuring mix-up — double-check that the waist is at the navel and the neck is below the larynx.
Do I need to weigh myself for this calculator?
No — the Navy formula uses only tape measurements and height. Weight is optional here and only adds fat-mass vs. lean-mass numbers plus a BMI cross-check.
How often should I re-measure?
Every 2–4 weeks, under the same conditions (morning, before eating). Tape measurements are noisy day to day — trends over a month tell the real story.
Can I track my body fat over time here?
Yes — hit “Log today’s measurements” after calculating and the page draws your trend line, showing change in percentage points since your first entry. Entries live only in this browser’s storage.
Is a lower body fat percentage always better?
No. Essential fat (roughly 2–5% for men, 10–13% for women) protects organs and hormones; dropping near or below it can disrupt both. “Fitness” and “athlete” ranges are lean without crossing that line.