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4-day fast

4 Day Water Fast Calculator

A four-day fast sits between a short 3-day reset and a longer 5-day extended fast. By day 4, most glycogen-linked water has already shifted, so additional scale change increasingly reflects fat burned at your TDEE.

Duration is pre-set to 4 days—adjust your stats and click Calculate.

Compare this page with our 3-day and 5-day calculators to see how an extra day changes the fat-to-water ratio in your estimate.

After the water flush

Day 4 contributes mostly additional fat loss.

Compare plans

See our 3-day or 5-day calculators to compare durations.

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

What to know before you start

Why day 4 looks different on the scale

If you have fasted before, you may notice that days 1–3 produce the most dramatic scale movement, while day 4 often looks quieter—even though fat loss continues. That is because glycogen depletion and the associated water release are largely complete by hour 72. From day 4 onward, a greater share of estimated total loss in our model comes from accumulated fat deficit rather than another large water flush.

This is one reason a 4-day fast can be a useful comparison point: it captures the full early water-weight pattern plus one additional day where fat contribution dominates. Enter your age, height, and weight to see how that split applies to your metabolism specifically.

Medical considerations for day 4 and beyond

Crossing beyond 72 hours raises the importance of electrolyte monitoring and medical awareness. Some people feel stable; others develop fatigue, orthostatic hypotension, or difficulty concentrating. Extended fasting can interact with blood pressure medications, diuretics, and diabetes drugs.

This calculator provides educational estimates only. If you are considering a 4-day or longer water fast, discuss your medications, health history, and refeed plan with a healthcare provider before you begin.

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 4-day water fast?
Four days sits between a short 72-hour reset and a five-day extended fast: you still capture the full first-three-day glycogen water pattern, plus one additional day where fat loss at your TDEE adds to the total without another large water spike. For many users, day four is when the scale “slows down” even though body fat contribution is still climbing—exactly why splitting fat and water matters. Choose Custom, enter 4, or compare the reference table’s sample profile, then run the main calculator with your own age and weight for numbers that match your metabolism.
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.
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.