Metric · Imperial · Same engine

Water Fasting Calculator in kg and Pounds

Updated September 25, 2026

Use this water fasting calculator in kg or pounds: switch weight to kg or lb and height to cm or in. For a 35-year-old woman at 70 kg (154 lb) and 165 cm, a 5-day water fast typically shows about 3.1 kg (6.9 lb) on the scale—about 1.2 kg fat plus glycogen-linked water—via Mifflin–St Jeor BMR × 1.35. 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.

Searching for a water fasting calculator in kg or in pounds? This page is built for both. Enter weight in kilograms or pounds and height in centimeters or inches—the math is identical. Duration pages (72-hour, 3-, 5-, 7-day) lock a length; here the focus is unit choice with the same fat vs. water model.

The engine uses Mifflin–St Jeor BMR, TDEE × 1.35 (sedentary–lightly active), fat ≈ deficit ÷ 3,500 kcal per pound of fat, and a glycogen water model (~3.5 lb / ~1.6 kg in the first 72 hours, then a small taper). Results display in the unit you selected.

Estimates are educational, not medical advice. Hydration, sodium, sleep, and health conditions change real outcomes. Talk to a clinician before an extended water-only fast.

kg or lbs — your choice

Toggle weight and height units anytime; the estimate converts with your inputs.

Same numbers, both systems

Worked example shown in kilograms and pounds so you can cross-check.

What to know before you start

How to switch between kg and pounds

Above the weight field, tap kg or lbs. Above height, tap cm or in. When you switch, the displayed number converts (for example 70 kg ↔ about 154 lb; 165 cm ↔ about 65 in) so you do not re-enter from scratch. Age, sex, and fast length stay the same—only the unit labels change.

Pick the system you use on your scale at home. Mixing systems (weight in lbs, height in cm) is fine; the calculator converts internally before running Mifflin–St Jeor. After Calculate, fat and water lines appear in the same weight unit you chose.

Worked example in kg and in pounds

Profile: 35-year-old woman, 70 kg (154 lb), 165 cm (65 in). Mifflin–St Jeor BMR ≈ 1,395 kcal/day; TDEE ≈ 1,884 kcal/day (×1.35). Glycogen-linked water ≈ 1.6 kg (3.5 lb) by day 3, then a slow rise.

3-day / 72-hour: about 2.3 kg (5.1 lb) total—roughly 0.7 kg (1.6 lb) fat and 1.6 kg (3.5 lb) water. 5-day: about 3.1 kg (6.9 lb) total—roughly 1.2 kg (2.6 lb) fat and 1.9 kg (4.2 lb) water. 7-day: about 3.9 kg (8.7 lb) total—roughly 1.7 kg (3.7 lb) fat and 2.2 kg (4.9 lb) water.

Use the calculator above with your own age, sex, height, and weight in whichever unit you prefer. For length-specific pages, open the 72-hour, 3-day, 5-day, or 7-day calculators; for the general “how much will I lose?” question, see that landing page.

Why unit pages matter for search and clarity

Many people search “water fasting calculator kg” or “water fasting calculator pounds” because their scale or doctor uses one system. Showing both units in the answer and UI reduces mistakes—especially converting 70 kg to pounds by hand before estimating.

Remember: early scale drops are often mostly glycogen-linked water; lasting change tracks fat from your calorie deficit. After you refeed carbs, some water weight usually returns. Read Learn guides on safety, electrolytes, and refeeding before any multi-day water-only fast.

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 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.

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 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.

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.