Worked example · Fat vs. water

Water Fasting Calculator: How Much Weight Will I Lose?

Updated September 25, 2026

This water fasting calculator answers “how much weight will I lose on a water fast?” with a fat vs. water split. 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 glycogen-linked water—using Mifflin–St Jeor BMR × 1.35 TDEE. Your number depends on sex, age, size, and duration. Educational only, not medical advice.

Gender

Age

Height

Current Weight

Fasting Duration

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

“How much weight will I lose on a water fast?” has no single answer—total scale change is fat burned from your calorie deficit plus water released as glycogen stores empty. Duration calculators for 72 hours, 3, 5, or 7 days answer specific lengths; this page explains the question itself with a worked example, then lets you personalize any length.

The example below uses the same engine as our free calculator: Mifflin–St Jeor BMR, TDEE × 1.35 (sedentary–lightly active), fat loss ≈ deficit ÷ 3,500 kcal/lb, and a glycogen water model (~3.5 lb in the first 72 hours, then a small taper).

Results are educational estimates, not predictions or medical advice. Hydration, sodium, sleep, and health conditions can move the real outcome. Talk to a clinician before an extended water-only fast.

Two kinds of loss

Fat accumulates from your daily TDEE deficit; early water often dominates the scale.

Worked example

See 3-, 5-, and 7-day estimates for a 35-year-old woman at 70 kg, then adjust for you.

What to know before you start

Worked example: 35-year-old woman, 70 kg, 165 cm

Using Mifflin–St Jeor, her BMR is about 1,395 kcal/day and TDEE about 1,884 kcal/day (×1.35). Glycogen-linked water in our model is about 1.6 kg (3.5 lb) by day 3, then rises slowly. Fat loss scales with days of deficit.

3-day / 72-hour fast: about 2.3 kg (5.1 lb) total—roughly 0.7 kg fat and 1.6 kg water. 5-day fast: about 3.1 kg (6.9 lb) total—roughly 1.2 kg fat and 1.9 kg water. 7-day fast: about 3.9 kg (8.7 lb) total—roughly 1.7 kg fat and 2.2 kg water.

Notice how total scale change grows with length, but after day 3 most of the new loss is fat, not another large water flush. A heavier or younger person with a higher TDEE would project more fat in the same window; an older or lighter person would project less. Enter your own age, sex, height, and weight in the calculator above to replace this example with your estimate.

Why the scale lies in the first 72 hours

Glycogen is stored carbohydrate in liver and muscle; each gram binds roughly 3–4 grams of water. When you stop eating, those stores empty and bound water is excreted—so the scale can drop several pounds before much adipose tissue is gone. That is why “pounds lost on a water fast” without a fat vs. water split misleads almost everyone.

Our calculator separates the two lines so you can plan expectations: early dramatic drops are often mostly water; lasting fat loss tracks your energy deficit over the whole fast. After you refeed carbohydrates, some water weight usually returns as glycogen rebuilds—that rebound is expected, not proof the fast “did nothing.”

How to use this page vs. duration calculators

Use this page when your question is the general one—how much weight on a water fast, what the number means, and how fat and water split. Use the 72-hour, 3-day, 5-day, or 7-day calculators when you already know the length and want a dedicated preset plus length-specific guidance.

For age and sex effects on BMR, see the by-age calculator. For safety, electrolytes, and refeeding, read the Learn guides before any multi-day water-only fast. Calculator output never clears you medically for fasting.

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.

Common questions

Answers for this fast length—see the main calculator FAQ for the full FAQ.

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 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 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.

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.

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.