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72 hours · 3 days

72 Hour Fasting Calculator — Free

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.

A 72-hour fast is often used as a metabolic reset, but it is not appropriate for everyone. Read our safety guide and speak with a clinician if you have any medical conditions or take prescription medications.

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.

What to know before you start

Why 72 hours matters for water weight

Glycogen is stored carbohydrate in your liver and muscles. Each gram of glycogen binds roughly 3–4 grams of water. When you stop eating, insulin falls and your body begins burning through those stores. By the end of 72 hours, most people have depleted the majority of accessible glycogen, which is why the steepest scale drop usually happens in this window—not because you lost several pounds of body fat overnight.

Our calculator models up to about 3.5 lbs of glycogen-linked water across the first three days, then tapers additional water loss on longer fasts. That matches what many people report on the scale during a short water fast, but your hydration, sodium intake, and starting carb intake can shift the number up or down.

Who should be cautious about a 72-hour fast

Extended water fasting is not safe or appropriate for everyone. Pregnant or breastfeeding women, people with a history of eating disorders, those on insulin or blood-pressure medications, and anyone with kidney disease, heart arrhythmias, or type 1 diabetes should not attempt a multi-day water fast without direct medical supervision.

Even healthy adults may experience dizziness, headaches, or fatigue—especially on days 2–3. Plain water plus adequate electrolytes (sodium, potassium, magnesium) is standard advice in clinical fasting literature, but this page does not replace personalized medical guidance. Use the calculator to understand mechanisms, then decide with a qualified provider whether a 72-hour fast fits your situation.

Water Fasting Weight Loss Reference Table

Sample estimates for a 35-year-old male, 175 cm, 80 kg. Your results will differ—use the calculator for personalized numbers.

Fast lengthWater weightFat lossTotal estimate
3 days1.6 kg0.9 kg2.5 kg
4 days1.7 kg1.2 kg3.0 kg
5 days1.9 kg1.5 kg3.4 kg
7 days2.2 kg2.1 kg4.3 kg

What happens day by day

General patterns during a water fast—useful alongside reference breakdowns.

PhaseHoursWater trendFat trendNotes
Day 10–24hLow–moderateMinimalGlycogen begins depleting; mild scale drop
Day 224–48hModerate–highLowWater loss accelerates as glycogen falls
Day 348–72hPeakModerateMost glycogen gone; largest water-weight window
Days 4–572–120hTaperingIncreasing shareFat oxidation becomes a larger part of total loss
Days 6–7120–168hLowDominantExtended fast; medical oversight often advised

Estimates use Mifflin-St Jeor BMR, TDEE × 1.35, 3,500 cal/lb fat conversion, and glycogen water modeling. Not medical advice.

Common questions

Answers for this fast length—see the main calculator FAQ for the full FAQ.

How much weight can you lose on a 3-day water fast?
Many adults see roughly 3–7 lbs on the scale over 72 hours, but the composition matters more than the headline number. Days 1–3 usually include a large glycogen-linked water drop—stored carbohydrate binds water in muscle and liver—while fat loss accumulates from your daily calorie deficit. A taller, heavier person with a higher TDEE often projects more fat loss in the same window than a lighter, older adult. Select the 3 Days or 72 Hours preset above with your stats to see fat versus water weight separately; real outcomes still depend on hydration, sleep, and health status.
What happens during a 72-hour water fast?
Hour 0–24: Insulin falls and the body begins tapping liver glycogen; you may notice hunger, lower energy, or an early scale dip. Hour 24–72: Glycogen stores shrink further and bound water is released—often the fastest “weight loss” on the scale—while fat oxidation ramps up as the deficit continues. By roughly 72 hours, much of the glycogen-linked water shift has occurred, so later days usually look calmer on the scale even though fat loss continues. Electrolytes (sodium, potassium, magnesium) and plain water remain essential; symptoms like dizziness, palpitations, or confusion are reasons to stop and seek care.
How much weight do you lose per day while water fasting?
There is no fixed “X lbs per day” that applies to everyone. Days 1–3 frequently show the largest daily scale drops because glycogen depletion releases water; days 4+ often look flatter on the scale while fat loss from your TDEE continues in the background. A 180-lb active male and a 140-lb older female can diverge sharply even on the same calendar. This calculator models that curve—water taper plus steady fat from your personal BMR/TDEE—rather than repeating a generic blog number. Weigh yourself at the same time of day if you track, and remember that hydration shifts the scale independently of fat.
Is this water fasting calculator free to use?
Yes. Water Fasting Calc is free, requires no account, and runs calculations locally in your browser—your inputs are not stored on our servers. It supports metric and imperial units, separates estimated fat loss from glycogen-linked water weight, and offers presets for common fast lengths plus custom days. Results are grounded in published BMR equations and standard calorie-to-fat conversions, with clear disclaimers that outputs are estimates, not medical advice. Use it to understand mechanisms and ballpark ranges; confirm any real fasting plan with a qualified professional.

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.