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3-day fast

3 Day Water Fast Calculator — Free

A 3-day water fast is one of the most common extended fast lengths. It is long enough to deplete most glycogen stores but short enough that many people attempt it without medical supervision—though supervision is still recommended if you have any health conditions.

The 3-day preset is already selected—enter your details and click Calculate to see a personalized fat vs. water weight breakdown.

For a deeper explanation of why the scale moves so quickly in the first 72 hours, read our guide on glycogen and water weight.

Typical range

Many adults see roughly 3–7 lbs on the scale, though results vary widely.

Not all fat

A large share in 72 hours is water; our tool separates both components.

Fill in your details and click Calculate Weight Loss to see your estimated breakdown.

What to know before you start

What a 3-day water fast actually does

During the first three days without food, your body transitions from burning primarily glucose from recent meals to tapping liver glycogen, then increasing fat oxidation. Hunger often peaks around day 2 for many people, then may ease as ketone production rises. The scale can drop several pounds, but a meaningful portion is water released as glycogen stores shrink—not adipose tissue.

Fat loss still accumulates every hour you maintain a calorie deficit. Our calculator estimates that portion using your personal BMR (Mifflin-St Jeor) and a sedentary-to-lightly-active TDEE multiplier. Heavier, younger adults typically show larger fat-loss projections than lighter or older users with the same fast length.

Planning electrolytes and breaking the fast

Multi-day water fasting increases the risk of electrolyte imbalance. Many clinicians who supervise therapeutic fasts recommend supplemental sodium, potassium, and magnesium—especially after the first 24 hours. Symptoms like muscle cramps, heart palpitations, or severe dizziness are reasons to stop and seek care.

When you refeed after 72 hours, start with small portions of easily digestible food—bone broth, steamed vegetables, or a small serving of protein. Avoid a large carbohydrate-heavy meal immediately; rapid glycogen repletion can cause discomfort and temporarily restore water weight. See our refeeding guide for a phased approach.

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.
How much weight will I lose while water fasting?
Total scale change is the sum of two very different processes: fat burned for energy and water released as glycogen stores empty. Fat loss scales with your metabolism (sex, age, height, weight) and how many days you fast; water weight can move quickly in the first 72 hours and then slow. This calculator uses the Mifflin-St Jeor BMR equation, a sedentary–lightly active TDEE multiplier (×1.35), and a glycogen water model so you can see what may be temporary versus adipose tissue—not a single “pounds per day” guess.
Can I plan a fasting schedule with this tool?
You can compare presets side by side—72 hours, 3, 4, 5, 7 days, or a custom length—before committing to a schedule. That helps answer practical questions: whether an extra two days meaningfully changes projected fat loss after water weight tapers, or whether a shorter reset fits your goal. Pair any plan with electrolytes during the fast, light activity only if tolerated, and a structured refeed (small, bland meals first) afterward. This page models weight change only; it does not replace supervision for medical, therapeutic, or prolonged fasting.
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.