TAPE
Body fat from three tape measurements. Needs a tape measure and two minutes. Run it and BASELINE recalculates itself off lean mass, which is the sharper formula for a trained body.
A number the mirror can't give you
The US Navy circumference method — three measurements and two logarithms, standing in for equipment that costs a few hundred pounds a scan. It is the most useful body-composition estimate you can take in your own bathroom, which is a lower bar than it sounds and still worth clearing.
Neck — just below the larynx, tape sloping slightly down at the front.
Waist — men, across the navel; women, at the narrowest point.
Hips — women only, at the widest point.
Bare skin, tape snug but not compressing, breathe out normally and do not suck in. Same time of day every time — morning, before food. Measure three times and take the middle value.
Watch the direction, ignore the decimal. A single reading has a real error band; six weekly readings have a trend, and the trend is the only part that was ever going to be useful.
It also makes every other station sharper — with a body-fat figure on file, BASELINE switches to a lean-mass formula and QUOTA doses your protein off the tissue that actually needs it.
Expect ±3–4 percentage points against a DEXA scan, and worse at the extremes. The formula only knows girths, so it cannot tell muscle from fat — a thick waist on a heavily built man reads as fat when some of it is obliques.
It will also move on things that have nothing to do with your body composition: a salty dinner, where exactly you laid the tape, whether you measured before or after breakfast. That is precisely why we tell you to track the line and not the number.
Estimates, not diagnoses. These figures describe averages and your body is not one. Nothing here treats, cures or prevents anything — and if you are managing a health condition, taking medication, or working through a relationship with food, talk to a professional before acting on a number a website gave you.