What this actually measures
The scale gives you one number and refuses to say what it is made of. Two people at the same weight can carry completely different amounts of muscle — and when you are losing weight, which of the two you become is the whole question.
This calculator uses the US Navy circumference method: your height, neck and waist (plus hip for women) predict body fat percentage well enough to be useful, and it needs nothing but a tape measure. It is what the US military uses to assess personnel at scale, for exactly that reason.
Lean mass is the number to watch
Body fat percentage is the headline, but the figure that decides your next decade is lean mass — the pounds of muscle, organ and bone underneath. It is where you store glucose, most of what determines how much you burn at rest, and what lets you get off the floor unaided at seventy.
In any sustained calorie deficit, part of what comes off is lean tissue. The faster the loss, the larger that share tends to be. That is why the third number above — your daily protein floor — sits next to the other two: it is the lever that pushes back.
How to measure properly
- Neck — just below the larynx, tape sloping slightly downward at the front. Do not flex.
- Waist — at the navel for men; at the narrowest point for women. Relaxed, at the end of a normal breath out. Do not suck in.
- Hip — women only, at the widest point, feet together.
- Same conditions every time — morning, before eating, same tape. Consistency matters more than precision, because what you are tracking is the change.
What it can and cannot tell you
This is an estimate, not a diagnostic measurement. DEXA and hydrostatic weighing are more accurate and cost money. The circumference method's real strength is that it is repeatable and free — which makes it far more useful for watching a trend than a one-off scan you take once and never repeat.
Body composition is individual, and existing conditions affect what is appropriate for you. Bring these numbers to your clinician rather than treating them as a verdict.
What to do next
Write the three numbers down and measure again in four weeks under the same conditions. If body fat falls and lean mass holds, it is working. If both fall together, protein and resistance training are where to look first — and the protein floor calculator gives you the daily number in ninety seconds.