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5-day fast

5 Day Fast Weight Loss Calculator — Free

A 5-day water fast adds two more days of calorie deficit after the initial glycogen-related water drop. Total scale change is usually larger than a 3-day fast, but the proportion attributed to fat loss increases after day 3.

The 5-day preset is selected automatically. Enter your stats for a personalized estimate.

Five-day fasts carry real medical risks—discuss electrolytes, medications, and refeeding with a clinician before starting.

More fat, less water %

After day 3, water taper slows while fat loss accumulates daily.

Medical caution

Extended fasts are not appropriate for everyone.

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

What to know before you start

How five days changes the fat vs. water split

In our model, glycogen-linked water loss is concentrated in the first three days, with a smaller taper on days 4–5. Fat loss, by contrast, accumulates linearly from your daily TDEE deficit. Over five full days, that fat component becomes a larger fraction of the total estimate—especially for people with higher resting metabolism.

Use the results panel to compare fat and water lines side by side. If your goal is understanding true adipose change rather than temporary scale movement, focus on the fat-loss figure and expect some water weight to return after you refeed carbohydrates.

Supervision and stopping criteria

Clinical fasting programs often include daily check-ins, blood pressure monitoring, and lab work for fasts beyond 72 hours. At home, you should have a clear stop plan: persistent vomiting, fainting, chest pain, confusion, or heart palpitations mean end the fast and seek medical care.

Never use calculator output as justification to extend a fast beyond what you and your provider agreed is safe. The numbers here are educational projections based on population-average formulas, not individualized medical prescriptions.

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 5-day water fast?
By day five, most people have already passed the steepest glycogen-related water loss, so additional scale change increasingly reflects fat burned at your TDEE. In practice, a five-day fast often shows a larger total than 72 hours mainly from four to five days of calorie deficit, not from another big water flush. Use the 5 Days preset with your age, height, and weight for a personalized split. Extended fasts carry real medical risks—electrolytes, medications, and refeeding should be discussed with a clinician before you start.
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 exercise during a water fast?
Often yes, but intensity should match how long you have been fasting. During the first 24 hours, many people tolerate normal daily movement or moderate exercise if they already train regularly. Beyond 24 hours—and especially after 48–72 hours—favor low-intensity steady-state (LISS) work: walking, easy cycling, gentle yoga, or mobility. Avoid heavy lifting, sprints, or HIIT; without dietary protein, muscle repair and recovery are impaired and overexertion can worsen dizziness or electrolyte issues. This calculator’s TDEE baseline (×1.35) does not add workout calories, so heavy training would make real fat loss differ from the estimate.
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.