72 Hour Fasting Calculator
Most glycogen—and the water stored with it—is depleted within about 72 hours of water-only fasting. That is why the scale often drops quickly in the first three days.
This calculator uses your personal BMR and TDEE to estimate fat loss plus glycogen-linked water weight.
Rapid water loss
Expect the largest water-weight shift in days 1–3 as glycogen stores empty.
Modest fat loss
Fat loss still occurs from your daily calorie deficit, but water dominates early scale change.
Fill in your details and click Calculate Weight Loss to see your estimated breakdown.
More water fasting calculators
Each tool uses the same science with a preset fast length—or browse our main calculator for all options, guides, and FAQs.
Also see: reference table, trusted resources, FAQ
Medical disclaimer · Estimates only—not medical advice.