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Prolonged fasting

Extended Fasting Calculator — Free Weight Loss Estimates

Extended fasting means going without food for longer than about 24 hours—and often several days. This free calculator estimates how much of your projected weight loss is burned fat versus temporary glycogen water, so a prolonged fast's headline number doesn't mislead you.

A 10-day duration is pre-selected to model a longer fast, but you can set any number of days. Enter your age, height, weight, and gender for a personalized fat-loss and water-weight breakdown.

Prolonged fasting is not a casual weight-loss shortcut. Multi-day water fasts carry real medical risks and often warrant clinical supervision—use these figures to set expectations, not as a target.

Multi-day modeling

Compounds daily fat loss at your TDEE across the full fast length.

Fat vs. water split

Separates lasting fat loss from the early glycogen water flush.

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

What to know before you start

What counts as extended fasting?

Most clinicians describe fasts beyond 24 hours as extended or prolonged fasting, distinct from intermittent fasting windows like 16:8. As a fast lengthens, the body shifts from burning liver glycogen toward sustained fat oxidation and ketone production. That is why the first 72 hours show a large water drop on the scale while later days reflect steadier fat loss driven by your total daily energy expenditure.

This calculator uses the Mifflin-St Jeor BMR equation and a sedentary–lightly active TDEE multiplier to project fat loss for each day of the fast, while modeling early glycogen water separately. Because BMR falls with age and lower body mass, two people fasting the same number of days can see very different projected outcomes.

Safety during a prolonged fast

Extended water fasting can affect blood pressure, blood sugar, electrolytes, and heart rhythm. Pregnancy, breastfeeding, being underweight, eating disorders, type 1 diabetes, kidney disease, and many cardiovascular conditions are contraindications for unsupervised prolonged fasting, and several medications may need adjustment.

If you are considering a fast of several days, do it with medical guidance so a clinician can monitor vitals, electrolytes, and safe refeeding. Treat every estimate here as educational—not medical advice or a recommendation to fast longer.

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.

What is extended fasting?
Extended fasting (sometimes called prolonged or water fasting) usually means going without caloric intake beyond a single overnight fast—often 24, 48, 72 hours, or several days of water only (sometimes with electrolytes). It is distinct from intermittent fasting, which cycles eating windows within each day. Longer fasts raise questions about muscle loss, electrolyte balance, blood sugar, and safe refeeding syndrome on the way out. Use the custom day field here only for rough educational estimates, and involve a healthcare provider for multi-day fasts, especially beyond 72 hours or if you have chronic conditions.
How much weight can you lose on a 7-day water fast?
A seven-day water fast can produce a visibly larger total estimate because fat loss compounds daily while early water weight has largely already shifted. After roughly 72 hours, day-to-day scale changes often look smaller even when fat loss continues steadily. Our 7 Days preset models that pattern; it is not a target or a safe default for everyone. Week-long water-only fasting can affect blood pressure, glucose, and electrolytes—get medical clearance first, especially if you take prescriptions, are pregnant, or have a history of eating disorders.
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.