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Water Fasting Calculator — Free Weight Loss Estimates

How much weight will you lose on a water fast? Enter your age, height, and weight for instant 72-hour, 3-, 5-, and 7-day estimates—with fat loss vs. water weight separated.

Gender

Age

Height

Current Weight

Fasting Duration

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

Free Water Fasting Calculator Online

No signup · Instant fat vs. water breakdown

Water Fasting Calc is a free water fasting calculator—no signup, no paywall. Estimate total weight change, fat loss, and water weight before you start a 3-, 5-, or 7-day fast (or any custom length).

Results use the Mifflin-St Jeor equation for metabolism, a standard TDEE multiplier, and a glycogen-based model for early water loss.

How to Calculate Water Fasting Weight Loss

Four steps used by our calculator engine

Learning how to calculate water fasting weight change helps you set realistic goals. Our tool follows four established steps:

1BMR (by age & gender)

Mifflin-St Jeor with sex, age, height, weight.

2TDEE

BMR × 1.35 for sedentary–lightly active daily burn.

3Fat loss

Deficit ÷ 3,500 cal/lb—true adipose loss.

4Water weight

~2–5 lbs glycogen water in 72h, then taper.

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 length	Water weight	Fat loss	Total estimate	BMR / TDEE
3 days	1.6 kg	0.9 kg	2.5 kg	1,724 / 2,327 kcal
4 days	1.7 kg	1.2 kg	3.0 kg	1,724 / 2,327 kcal
5 days	1.9 kg	1.5 kg	3.4 kg	1,724 / 2,327 kcal
7 days	2.2 kg	2.1 kg	4.3 kg	1,724 / 2,327 kcal

What happens day by day

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

Phase	Hours	Water trend	Fat trend	Notes
Day 1	0–24h	Low–moderate	Minimal	Glycogen begins depleting; mild scale drop
Day 2	24–48h	Moderate–high	Low	Water loss accelerates as glycogen falls
Day 3	48–72h	Peak	Moderate	Most glycogen gone; largest water-weight window
Days 4–5	72–120h	Tapering	Increasing share	Fat oxidation becomes a larger part of total loss
Days 6–7	120–168h	Low	Dominant	Extended 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.

Water Fast Weight Loss by Duration

72-hour, 3, 5, and 7-day presets

Each fast length has its own page with a pre-selected duration. Open the dedicated calculator or use the main tool for custom lengths.

72 Hour Fasting Calculator

72 hours

Most glycogen water is gone by hour 72—the scale often drops quickly in the first three days.

Dedicated calculator →Use main tool

3 Day Water Fast Calculator

3 days

The most popular extended fast. See fat vs. water weight for a full 3-day water fast.

Dedicated calculator →Use main tool

4 Day Water Fast Calculator

4 days

Bridges a 3-day reset and a 5-day extended fast—extra fat loss after the initial water flush.

Dedicated calculator →Use main tool

5 Day Fast Weight Loss Calculator

5 days

Adds two more days of fat burn after the initial water flush.

Dedicated calculator →Use main tool

7 Day Fast Weight Loss Calculator

7 days

Models water taper after day 3 plus seven days of TDEE burn. Week-long fasts need medical clearance for many people.

Dedicated calculator →Use main tool

Water Fasting Calculator by Age & Gender

Personalized BMR for every profile

Age belongs in the BMR formula—a 25-year-old and a 55-year-old with the same weight burn calories differently at rest.

Try our water fasting calculator by age. Use the gender toggle for female (−161) or male (+5) Mifflin-St Jeor constants.

Water Fasting Schedule Calculator

Plan duration before you start

Compare 72-hour, 3-, 5-, and 7-day outcomes before you commit. Pair with electrolytes, rest, and a gradual refeed.

1Enter gender, age, height, and current weight.
2Choose duration (72 hours through 7 days, or custom).
3Click Calculate Weight Loss.
4Review fat vs. water split in the results panel.

Extended & Prolonged Fasting Calculator

Custom day counts beyond 7 days

Use the custom day field for longer estimates. Extended fasts raise medical risks—not a substitute for professional guidance.

Water Weight vs. True Fat Loss

Why the scale lies in the first 72 hours

💧 Water weight

Glycogen binds water; fasting drains stores in ~72 hours—often 2–5 lbs that may return after refeeding carbs.

🔥 Fat loss

TDEE × days ÷ 3,500—the standard approach for true adipose change.

Static tables cannot know your stats. Use the reference table for examples, then personalize above.

Water Fasting vs. Intermittent Fasting

Which calculator should you use?

Intermittent fasting uses daily eating windows (e.g. 16:8). This site focuses on extended water-only fasts of 24 hours or more.

	Water fast (this tool)	Intermittent fasting
Duration	24h–7+ days, water only	Daily eating window
Main scale drop	Water + fat (days 1–3)	Usually smaller, gradual
Best calculator	Water Fasting Calc	IF schedule / macro apps

Exercise & Fasting Estimates

What this calculator does not include

This tool uses a fixed sedentary-to-lightly-active TDEE multiplier (1.35). It does not add calories burned from workouts.

If you train regularly, your real deficit may be higher than shown. Discuss activity levels with a healthcare provider before combining intense exercise with extended fasting.

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

Water Fasting Guides

Original articles on electrolytes, water fasting vs intermittent fasting, glycogen water weight, refeeding, and safety—written to complement the calculator.

Browse all guides →

These guides are for educational purposes only—not medical advice. Consult a healthcare provider before extended fasting.

Trusted Resources & References

Water Fasting Calc is an educational tool—not medical advice. Explore these independent sources on fasting, metabolism, nutrition, and weight management.

Government & public health

Medical institutions

Peer-reviewed research

Nutrition & clinical practice

External links are provided for educational purposes. We are not affiliated with these organizations. Always consult a qualified healthcare provider before starting or extending a water fast.

Frequently Asked Questions

Top questions about our free water fasting calculator.

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.

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 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.

Does age affect water fasting weight loss?

Yes, meaningfully. Resting metabolism typically declines with age, which lowers BMR and therefore TDEE in standard formulas. This calculator embeds age directly in the Mifflin-St Jeor equation (along with sex, height, and weight), so a 55-year-old and a 25-year-old of the same size do not receive identical fat-loss projections. Age does not change the glycogen water model much by itself—the early scale drop is still driven by carbohydrate stores—but older adults often see slower fat-loss curves over the same fast length. Treat any estimate as educational, not a clinical prediction.

Is there a water fasting calculator for women?

Yes. Choose Female to apply the women’s Mifflin-St Jeor constant (−161 kcal/day in the sex term versus +5 for men at the same age, height, and weight). That adjustment reflects average differences in lean mass and resting expenditure—not hormone cycles, pregnancy, or menopause, which this tool does not model. For the most useful estimate, enter current weight and height accurately, pick your fast length (72 hours through custom days), and review both fat and water lines in the results. Anyone who is pregnant, breastfeeding, or underweight should not use fasting estimates as planning advice without a doctor.

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.

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.

How do you calculate water fasting weight loss?

Step 1: Basal metabolic rate (BMR) via Mifflin-St Jeor from sex, age, height, and weight. Step 2: Total daily energy expenditure (TDEE) = BMR × 1.35, a sedentary–lightly active baseline without added exercise calories. Step 3: Fat loss (lbs) = (TDEE × fasting days) ÷ 3,500 kcal per pound of fat. Step 4: Glycogen-related water weight—modeled as up to ~3.5 lbs across the first three days, then a smaller daily taper for longer fasts. Total estimated loss = fat + water. The math runs in your browser; individual hydration, activity, and health can move real results above or below the projection.

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 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.

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.

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.

What is the difference between water fasting and intermittent fasting?

Intermittent fasting (IF) alternates feeding and fasting windows within a 24-hour cycle—common patterns include 16:8 or one meal a day—while still eating most days. Water fasting here means extended, water-only periods (typically 24+ hours) where calories are near zero until you refeed. IF tools focus on meal timing and weekly adherence; this calculator estimates scale change across multi-day water fasts using BMR, TDEE, and glycogen water modeling. Neither approach is inherently “better”; risks, sustainability, and medical suitability differ. Choose the method that matches your goal and get professional guidance for anything beyond short, occasional fasts.

How much weight will I lose with a water fasting calculator?

A water fasting calculator estimates total scale change by combining two parts: fat burned from your daily calorie deficit (BMR × 1.35 TDEE × days ÷ 3,500 cal/lb) and glycogen-linked water weight released in the first ~72 hours. Results depend on your age, sex, height, weight, and fast length—a heavier 30-year-old male typically projects more than a lighter 60-year-old female. Enter your stats above and choose 72 hours, 3, 5, 7 days, or a custom length to see your personalized fat vs. water split. These are educational estimates, not guarantees.

Is there a free water fasting schedule calculator?

Yes—Water Fasting Calc works as a free water fasting schedule calculator. Compare presets (72 hours, 3, 4, 5, or 7 days) before you start, or enter a custom day count for longer plans. Each duration also has a dedicated page with the preset pre-selected: 72-hour, 3-day, 5-day, and 7-day calculators. The tool separates projected fat loss from early water weight so you can plan refeed timing and set realistic expectations. No account, no paywall—calculations run in your browser.

Is Water Fasting Calc a free water fasting app?

Water Fasting Calc is a free, browser-based water fasting calculator—no download or app store required. It works on mobile and desktop, requires no signup, and runs all calculations locally in your browser. Features include metric and imperial units, age- and gender-adjusted BMR (Mifflin-St Jeor), presets for 72-hour through 7-day fasts, custom day counts, and a clear fat vs. water weight breakdown. Add it to your home screen on mobile for quick access like an app.

How much water should I drink during a water fast?

Many healthy adults aim for roughly 2–3 liters (8–12 cups) per day during a multi-day water fast, sipping steadily and adjusting for thirst, urine color, and symptoms. Needs vary with body size, climate, and activity. Electrolytes (sodium, potassium, magnesium) often matter as much as volume after the first 24 hours. People with kidney disease, heart failure, or fluid restrictions need individualized medical guidance—not generic online targets. See our hydration guide for educational details.

Fast length

Water weight

Fat loss

Total estimate

3 days

1.6 kg

0.9 kg

2.5 kg

4 days

1.7 kg

1.2 kg

3.0 kg

5 days

1.9 kg

1.5 kg

3.4 kg

7 days

2.2 kg

2.1 kg

4.3 kg

Phase

Hours

Water trend

Fat trend

Notes

Day 1

0–24h

Low–moderate

Minimal

Glycogen begins depleting; mild scale drop

Day 2

24–48h

Moderate–high

Low

Water loss accelerates as glycogen falls

Day 3

48–72h

Peak

Moderate

Most glycogen gone; largest water-weight window

Days 4–5

72–120h

Tapering

Increasing share

Fat oxidation becomes a larger part of total loss

Days 6–7

120–168h

Low

Dominant

Extended fast; medical oversight often advised

Water fast (this tool)

Intermittent fasting

Duration

24h–7+ days, water only

Daily eating window

Main scale drop

Water + fat (days 1–3)

Usually smaller, gradual

Best calculator

Water Fasting Calc

IF schedule / macro apps