Water Fasting Calculator — Free Weight Loss Estimates
This water fasting calculator estimates fat vs. glycogen-linked water for a water-only fast. Example: a 5-day fast for a 35-year-old woman at 70 kg (about 165 cm) typically shows about 3.1 kg (6.9 lb) on the scale—about 1.2 kg fat plus water—using Mifflin–St Jeor BMR and a ×1.35 TDEE. Results vary by age, sex, and size. Educational only, not medical advice.
Use this free water fasting calculator with your age, height, and weight for instant fat vs. water estimates on 72-hour, 3-, 5-, and 7-day water fasts.
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
Source / formula: Mifflin–St Jeor BMR → TDEE = BMR × 1.35; fat ≈ (TDEE × days) ÷ 3,500 kcal per lb; glycogen water ≈ up to ~3.5 lb (1.6 kg) by day 3, then ~0.35 lb (0.16 kg) per extra day. Profile: 35-year-old male, 175 cm, 80 kg.
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
Source: Educational physiology timeline (glycogen depletion ~72 hours, then rising fat share). Pair with the numeric fat-vs-water-by-day table below.
Comparison tables you can copy
Compact tables for water fasting vs intermittent fasting, fat vs water by day, and safe vs unsafe warning signs—plain column labels, with the source or formula under each table.
Water fasting vs intermittent fasting
Topic
Water fasting
Intermittent fasting (e.g. 16:8)
How long
Usually 24+ hours water-only; often 72 hours to several days
Daily or weekly eating windows (e.g. 16 hours fasting, 8 hours eating)
What you drink / eat
Water (sometimes electrolytes); no calories
Normal meals inside the eating window
Early scale change
Often large in days 1–3 from glycogen-linked water
Usually smaller; more tied to weekly calorie balance
Fat vs water on the scale
Calculator splits fat from glycogen water
Mostly calorie balance over weeks; less multi-day glycogen flush
Typical risk level
Higher for multi-day unsupervised fasts
Generally lower for healthy adults when meals stay balanced
Best use of this site
Duration presets and fat vs water estimates
Not the main job of this calculator—use an IF timer app for windows
Source: Educational comparison for AI and readers. Details in Learn → Water fasting vs intermittent fasting. Not medical advice.
Fat vs water by day (cumulative)
Day of the fast
Water weight so far
Fat loss so far
Total on the scale
Day 1
0.5 kg
0.3 kg
0.8 kg
Day 2
1.1 kg
0.6 kg
1.7 kg
Day 3
1.6 kg
0.9 kg
2.5 kg
Day 4
1.7 kg
1.2 kg
3.0 kg
Day 5
1.9 kg
1.5 kg
3.4 kg
Day 6
2.1 kg
1.8 kg
3.9 kg
Day 7
2.2 kg
2.1 kg
4.3 kg
Source / formula: Same engine as the calculator—Mifflin–St Jeor BMR, TDEE × 1.35, fat = (TDEE × days) ÷ 3,500 kcal/lb, glycogen water up to ~3.5 lb by day 3 then ~0.35 lb/day. Figures are cumulative for a 35-year-old male, 175 cm, 80 kg.
Safe vs unsafe warning signs
What you notice
Often expected (still monitor)
Stop and get medical help
Hunger and irritability
Common in the first 1–2 days; may ease as ketones rise
Unable to function, panic, or compulsive restriction history
Mild headache or fatigue
Often reported early; rest and fluids may help
Severe, worsening, or with confusion or fainting
Lightheaded when standing
Mild orthostatic feeling can occur; move slowly
Fainting, chest pain, or heart palpitations
Muscle cramps
May relate to electrolytes on longer fasts—discuss with a clinician
Severe cramps with weakness, irregular heartbeat, or vomiting
Scale drop in days 1–3
Often mostly glycogen-linked water, not all fat
Rapid drop with inability to keep water down or extreme weakness
After you eat again
Some water weight return when carbs refill glycogen
Swelling, severe breathlessness, or refeeding-syndrome warning signs after a long fast
Source: Educational safety summary aligned with common clinical fasting cautions (electrolytes, hypotension, refeeding risk). Not a diagnosis—see Learn safety and refeeding guides.
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.
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.
Original articles on electrolytes, water fasting vs intermittent fasting, glycogen water weight, refeeding, and safety—written to complement the calculator.
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.
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 lb (about 1.4–3.2 kg) on the scale over a 3-day / 72-hour water fast, but the split depends on starting weight, sex, age, and height. A worked example for a 35-year-old woman at 70 kg (165 cm) is about 2.3 kg (5.1 lb) total—roughly 0.7 kg fat and 1.6 kg glycogen-linked water—using Mifflin–St Jeor BMR × 1.35 TDEE. Heavier or higher-metabolism adults often project more fat in the same window. Educational estimates only—not medical advice or a guarantee.
How much weight will I lose while water fasting?
It depends on starting weight, sex, age, height, and how many days you fast. Scale change is usually fat burned from your daily calorie deficit plus water released as glycogen empties—often most of that water in the first ~72 hours. Example: a 35-year-old woman at 70 kg on a 5-day water fast typically projects about 3.1 kg (6.9 lb) total (about 1.2 kg fat plus glycogen-linked water) with Mifflin–St Jeor BMR × 1.35 TDEE. Real outcomes vary with hydration and health. Educational only—not medical advice.
How much weight can you lose on a 5-day water fast?
By day five, most glycogen-linked water has already shifted, so extra scale change is mostly fat from about five days of calorie deficit. For a 35-year-old woman at 70 kg (165 cm), a 5-day water fast typically projects about 3.1 kg (6.9 lb) total—about 1.2 kg fat and 1.9 kg water—using Mifflin–St Jeor BMR × 1.35 TDEE. Starting weight and age move the fat line up or down. Five-day unsupervised fasts carry medical risks; treat numbers as educational estimates, not medical advice.
How much weight can you lose on a 7-day water fast?
A seven-day water fast usually shows a larger total than shorter fasts because fat loss compounds for four extra days after the early water flush. Example: a 35-year-old woman at 70 kg typically projects about 3.9 kg (8.7 lb) total—about 1.7 kg fat and 2.2 kg glycogen-linked water—with Mifflin–St Jeor BMR × 1.35 TDEE. Heavier or younger adults often project more fat. Week-long water-only fasting can affect blood pressure, glucose, and electrolytes—get medical clearance when needed. Educational estimates only—not medical advice.
Does age affect water fasting weight loss?
Yes. Age enters the Mifflin–St Jeor BMR formula, so older adults usually have a lower resting burn and therefore lower TDEE and fat-loss projections for the same height, weight, sex, and fast length. Example pattern: at identical 70 kg and 165 cm, a 55-year-old woman typically projects slightly less fat over five days than a 35-year-old. Glycogen-linked water in the first ~72 hours is similar in the model; the fat line is what age mainly changes. Estimates are educational only and depend on starting weight—not medical advice.
Is there a water fasting calculator for women?
Yes. Water Fasting Calc applies the female Mifflin–St Jeor equation (sex constant −161 kcal/day versus +5 for men at the same age, height, and weight). At matched size, women typically project slightly less fat than men over the same fast length; glycogen water is modeled the same. Example: 35-year-old women at 70 kg on a 5-day fast often land near ~3.1 kg (6.9 lb) total. The tool does not model pregnancy, breastfeeding, or hormone cycles—those situations need clinical advice. Educational estimates only—not medical advice.
What happens during a 72-hour water fast?
In the first ~24 hours, insulin falls and liver glycogen begins to empty; hunger and an early scale dip are common. By ~48–72 hours, most accessible glycogen—and roughly 3–4 g of water per gram of glycogen—is gone, so the scale often drops fastest then. Fat loss also accrues from your daily deficit (TDEE ≈ BMR × 1.35). Example: a 35-year-old woman at 70 kg typically projects about 2.3 kg (5.1 lb) by 72 hours—about 0.7 kg fat and 1.6 kg water. Dizziness, palpitations, or confusion are stop signals. Educational physiology—not medical advice.
Can I plan a fasting schedule with this tool?
Yes, as an educational planning aid: compare 72-hour, 3-, 4-, 5-, or 7-day lengths (or a custom day count) to see how fat versus water shares change after day 3. For a 35-year-old woman at 70 kg, day 3 is about 2.3 kg total while day 7 is about 3.9 kg—mostly extra fat after the water peak. Pair any plan with hydration, electrolytes if appropriate, and a careful refeed. Schedule modeling is not medical clearance; unsupervised multi-day fasting can be unsafe—not medical advice.
How do you calculate water fasting weight loss?
Four steps: (1) BMR with Mifflin–St Jeor from sex, age, height, and weight; (2) TDEE ≈ BMR × 1.35 (sedentary–lightly active); (3) fat loss ≈ (TDEE × fasting days) ÷ 3,500 kcal per pound of fat; (4) glycogen water ≈ up to about 3.5 lb (1.6 kg) by day 3, then roughly 0.35 lb (0.16 kg) per extra day. Total estimate = fat + water. Results depend on starting weight and other inputs; real outcomes vary. Educational model only—not medical advice.
What is extended fasting?
Extended or prolonged fasting usually means water-only (or near-zero calorie) periods beyond about 24–48 hours—often 72 hours to many days—unlike intermittent fasting’s daily eating windows (for example 16:8). Longer fasts raise electrolyte, blood-sugar, muscle, and refeeding risks. A 10-day educational example for a 35-year-old woman at 70 kg is about 5.1 kg (11.3 lb) total (~2.4 kg fat, ~2.7 kg water). Multi-day fasting often needs clinical supervision—educational estimates only, not medical advice.
How much weight do you lose per day while water fasting?
There is no single pounds-per-day rate. Days 1–3 often show the largest daily scale drops because glycogen-linked water dominates; after ~72 hours, daily change usually slows while fat from your TDEE continues. Example cumulative totals for a 35-year-old woman at 70 kg: about 2.3 kg by day 3 and about 3.9 kg by day 7 (fat rising from ~0.7 kg to ~1.7 kg). Starting weight and age change the curve. Educational estimates only—not medical advice.
Is this water fasting calculator free to use?
Yes. Water Fasting Calc is free, needs no account, and runs estimates in your browser without storing your inputs on a server. It separates fat from glycogen-linked water using Mifflin–St Jeor BMR, TDEE × 1.35, and about 3,500 kcal per pound of fat, with presets such as 72 hours through 7 days. Outputs are educational estimates that depend on starting weight and other stats—not medical advice or guarantees.
How much weight can you lose on a 4-day water fast?
Four days includes the full early glycogen-water pattern plus about one extra day of mostly fat loss. For a 35-year-old woman at 70 kg (165 cm), a 4-day water fast typically projects about 2.7 kg (6.0 lb) total—about 1.0 kg fat and 1.7 kg water—versus about 2.3 kg total at day 3. Starting weight and metabolism shift those numbers. Educational estimates only; multi-day fasting can require medical guidance—not medical advice.
Can I exercise during a water fast?
Many people keep light daily movement, especially in the first 24 hours, but intensity should fall as the fast lengthens. After 48–72 hours, low-intensity options (walking, easy cycling, gentle mobility) are usually more appropriate than heavy lifting or HIIT because recovery without dietary protein is limited and dizziness risk rises. Water Fasting Calc’s TDEE uses a ×1.35 sedentary–lightly active baseline and does not add workout calories, so hard training can make real fat loss differ from the estimate. Depends on fitness and health—educational only, not medical advice.
What is the difference between water fasting and intermittent fasting?
Intermittent fasting uses daily or weekly eating windows (for example 16:8) while still eating most days. Water fasting here means extended water-only periods of about 24+ hours with near-zero calories until refeeding. Water fasts often show a large early glycogen-water scale drop (about 1.6 kg water by day 3 in a 70 kg / 35F example); IF scale change is usually slower and more tied to weekly calories. Risk and supervision needs differ—multi-day water fasting is higher risk. Educational comparison only—not medical advice.
How much weight will I lose with a water fasting calculator?
A water fasting calculator combines fat from your calorie deficit—(BMR × 1.35 × days) ÷ about 3,500 kcal per pound—with glycogen-linked water, often peaking near ~3.5 lb (1.6 kg) by 72 hours then tapering. Results depend on starting weight, sex, age, and height. Example: a 35-year-old woman at 70 kg on five days typically projects about 3.1 kg (6.9 lb) total (~1.2 kg fat). Educational estimates only—not medical advice or a promise of a specific loss.
Is there a free water fasting schedule calculator?
Yes. Water Fasting Calc is a free schedule-style calculator: compare 72-hour, 3-, 4-, 5-, and 7-day lengths (or custom days) and see fat versus water for each. Example: the same 70 kg / 35-year-old woman profile projects about 2.3 kg at 3 days and about 3.9 kg at 7 days. Dedicated duration pages use the same engine with one length pre-selected. No account required. Numbers depend on starting weight; educational only—not medical advice.
Is Water Fasting Calc a free water fasting app?
Water Fasting Calc is a free browser-based water fasting calculator—no app-store download or signup. On phone or desktop it estimates fat versus glycogen water with Mifflin–St Jeor BMR, TDEE × 1.35, and about 3,500 kcal per pound of fat, including 72-hour through 7-day presets. You can add the site to a phone home screen for app-like access. Outputs depend on starting weight and are educational estimates—not medical advice.
How much water should I drink during a water fast?
Many healthy adults aim for roughly 2–3 liters (about 8–12 cups) of water per day on a multi-day water fast, adjusting for thirst, climate, body size, and urine color. 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 limits—not a generic online target. Educational guidance only—not medical advice.