The short answer
The US Navy method estimates body fat from your height and the circumference of your neck and waist, plus hips for women. It typically lands within 3 to 4 percentage points of a DEXA scan: not exact, but accurate enough to track a direction, and considerably more informative than BMI.
How the method works
The US Navy developed this method because it needed something that could be done anywhere, in a minute, with equipment that costs nothing. It works on a simple observation: fat distributes in predictable places, so the ratio between where fat accumulates and where it does not carries information about the total.
For men that ratio is waist against neck. For women it is waist plus hips against neck. Height scales the result, because the same waist measurement means something different on a person of 160 cm than on one of 190 cm.
The equations are logarithmic:
Metric (cm)
Men: %BF = 495 / (1.0324 − 0.19077·log10(waist − neck) + 0.15456·log10(height)) − 450
Women: %BF = 495 / (1.29579 − 0.35004·log10(waist + hips − neck) + 0.22100·log10(height)) − 450
Imperial (inches)
Men: %BF = 86.010·log10(waist − neck) − 70.041·log10(height) + 36.76
Women: %BF = 163.205·log10(waist + hips − neck) − 97.684·log10(height) − 78.387
These are the same regression written twice, once in each unit system, and they agree with each other to within about a third of a percentage point. This calculator computes in metric and converts your inputs, so you get the same answer whichever unit you choose.
How to measure
Measurement error is the main source of error in the result, and it is larger than the difference between formulas. A centimetre at the waist moves the answer by roughly a full percentage point.
- Neck: just below the larynx, tape sloping slightly downward toward the front. Do not flex.
- Waist: at the navel for men, at the narrowest point for women. Stand relaxed and exhale normally. Do not pull your stomach in, and do not hold your breath.
- Hips (women): at the widest point, feet together.
Keep the tape snug against skin without compressing it. Measure at the same time of day, ideally in the morning before eating, and take each measurement twice. If the two readings differ by more than half a centimetre, measure again.
Reading the result
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 to 5% | 10 to 13% |
| Athletic | 6 to 13% | 14 to 20% |
| Fitness | 14 to 17% | 21 to 24% |
| Acceptable | 18 to 24% | 25 to 31% |
| Obese | 25% and above | 32% and above |
These are the American Council on Exercise categories. Note that every band sits higher for women: that is physiology, not a different standard. Women carry substantially more essential fat, the minimum required for hormonal and reproductive function, and dropping toward male essential levels causes real harm.
What this method gets wrong
It is an estimate from proxies. The tape does not see fat, it sees circumference, and it infers the rest. Two people with identical measurements and different fat distribution get the same answer despite different bodies.
It degrades at the extremes. Accuracy is best in the middle of the range. For very lean people and for people with high body fat, the error widens, and at the far end the equation can return values that are not physiologically plausible at all, which is why this calculator rejects measurements outside a sensible range rather than showing a confident wrong number.
It cannot tell you where the fat is. Visceral fat around the organs carries more metabolic risk than the same amount on hips and thighs. The waist measurement captures some of this, but the percentage alone does not.
Use the trend, not the number
Here is the practical takeaway that makes this method genuinely useful despite everything above.
The absolute figure carries an error of a few percentage points. The change in the figure, measured the same way by the same person, carries much less, because most of the error is systematic and cancels out when you compare against yourself. Someone who measures at 22 percent and then 19 percent three months later has almost certainly lost fat, even if neither number is exactly right.
So measure monthly, keep the method identical, and read the direction. Chasing decimal places on a single reading is the one thing this tool cannot support.
What actually moves it
Body fat percentage changes through a sustained gap between energy in and energy out, held over months, with enough protein and enough resistance training that the loss comes from fat rather than muscle. There is no shortcut and no exception.
The hard part was never the principle. It is that knowing what you actually ate, day after day, takes more effort than most people can keep up, and a food log with gaps in it stops being evidence of anything.
CalAlly removes that friction. Describe the meal out loud, the AI breaks it into ingredients, calories and macros, one tap logs it, about 3 seconds. The AI coach reads the real data, so when a plateau is a genuine plateau rather than a drifting log, it can tell you which one you are looking at.