Total body water

All the water in your body, inside and outside the cells. Bioimpedance measures this most directly and derives much of the rest of the sheet from it.

Water is roughly half to two thirds of your weight, so it accounts for most of the day-to-day movement on the scale.

Because the other numbers are calculated from it, a badly hydrated measurement distorts the whole page. Measure in the same state every time: same hour, before eating, before training.

Worth mentioning a sharp change that holds across several measurements.