Body composition

What the body is made of, and what it costs to keep running. These change over months rather than days, and a scale measures most of them directly instead of inferring them from weight alone.

Weight

Lower is better

Total body weight.

Lower is the goal — a sustained downward trend is the desired direction.

BMI

Lower is better

Body Mass Index — weight relative to height² (kg/m²). A rough screen, not body composition.

Lower is generally the goal within a healthy range — a high BMI raises cardiometabolic risk.

Fat mass

Lower is better

Total mass of body fat, in kg.

Lower is the goal — reducing body fat lowers cardiometabolic risk.

Fat ratio

Lower is better

Body fat as a share of total weight.

Lower is the goal — a smaller body-fat share is the target.

Fat-free mass

Higher is better

Everything that isn't fat — muscle, bone, water and organs.

Higher reflects more lean mass — though it includes body water, so it swings with hydration.

Muscle mass

Higher is better

Estimated total skeletal muscle mass.

Higher is better — more skeletal muscle supports strength and metabolism.

Visceral fat

Lower is better

Fat stored around the abdominal organs (Withings index, not kg).

Lower is better — visceral (around-organ) fat is the most metabolically harmful.

Bone mass

Higher is better

Estimated bone mineral mass.

Higher is better — more bone mineral reflects stronger bones.

Hydration

Estimated total body water.

BMR

Estimated calories your body burns at rest (kcal/day).

Waist circumference

Waist measurement — a simple gauge of central (visceral) fat.

Height

Your height — used to compute BMI.

Which measurements you actually have depends on your devices and your lab results. Watches differ: an older model may never record heart-rate variability, while a newer one may not estimate VO₂max until it has recorded outdoor activity. A measurement you do not have is missing from your device, not missing from you.

Educational information — not medical advice or a diagnosis. Discuss your health with a physician.

← What we measure