AMA #33: Hydrationelectrolytes, supplements, sports drinks, performance effects, and more

Muscle is far higher water content than fat. So, fat tissue is about 10% water by weight ... Muscle is probably 75% water.

The water inside of cells is called, not surprisingly, intracellular fluid; and that accounts for two-thirds of your water weight. So, if we're talking about 60% of your weight is water, then two-thirds of that 60% is within the cells themselves.

About 67% of your total body water is inside cells, about 20% is in the interstitium—this area that exists between everything... about 8% is in the vascular system, and about 5% or thereabouts is basically everything else, including CSF and things like that.