The FastDay Forum

Total Body Water in kg = 0.3625 × BW + 0.2239 × BH − 0.1387 × Y − 14.47, where Y is age in years, BW is body weight in kilograms, and BH is body height in centimeters.

For me it comes out at 48 kg or 60% of my body weight.

So I'm 80 kg in total, 48 kg of water, 22 kg of fat

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So that leaves 10% for muscle, bones and brain!!!

Interesting! About 50% for me. Maybe I need to drink more.