3-Day Water Fasting Calculator: Fat vs. Water Weight Breakdown
This water fasting calculator uses a 3-day preset. For a 35-year-old woman at 70 kg it typically shows about 2.3 kg (5.1 lb) total—about 0.7 kg fat and 1.6 kg glycogen-linked water—via Mifflin–St Jeor BMR × 1.35 TDEE. Composition varies by body size. Educational only, not medical advice.
People search “3-day water fast” wanting a plan-length answer, not a blank calculator. Here is a concrete composition: for a 35-year-old woman at 70 kg and 165 cm, three water-only days project about 2.3 kg (5.1 lb) total—roughly 0.7 kg fat and 1.6 kg glycogen water. Many adults land somewhere near 1.5–3.5 kg total; sex, age, and size move the fat line.
The 3-day preset is selected below so you can swap in your own stats without hunting for the right duration button.
The homepage remains the general tool for “how much weight will I lose on a water fast?” across any length. This page is only about the three-day plan’s fat-vs-water mix.
Worked 3-day total
≈2.3 kg (5.1 lb) for 70 kg / 35F; ~70% of that example is water.
Plan-length focus
Answers “what does a 3-day fast look like?”—not every duration.
What to know before you start
What a 3-day water fast actually does
During the first three days without food, your body transitions from burning primarily glucose from recent meals to tapping liver glycogen, then increasing fat oxidation. Hunger often peaks around day 2 for many people, then may ease as ketone production rises. The scale can drop several pounds, but a meaningful portion is water released as glycogen stores shrink—not adipose tissue.
Fat loss still accumulates every hour you maintain a calorie deficit. Our calculator estimates that portion using your personal BMR (Mifflin-St Jeor) and a sedentary-to-lightly-active TDEE multiplier. Heavier, younger adults typically show larger fat-loss projections than lighter or older users with the same fast length.
Planning electrolytes and breaking the fast
Multi-day water fasting increases the risk of electrolyte imbalance. Many clinicians who supervise therapeutic fasts recommend supplemental sodium, potassium, and magnesium—especially after the first 24 hours. Symptoms like muscle cramps, heart palpitations, or severe dizziness are reasons to stop and seek care.
When you refeed after 72 hours, start with small portions of easily digestible food—bone broth, steamed vegetables, or a small serving of protein. Avoid a large carbohydrate-heavy meal immediately; rapid glycogen repletion can cause discomfort and temporarily restore water weight. See our refeeding guide for a phased approach.
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.
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.
- How much will I lose?
- kg & lbs calculator
- By starting weight
- Men vs women
- Day-by-day timeline
- Break a fast / refeed
- How estimates work
- 72-hour fast
- 3-day fast(current)
- 4-day fast
- 5-day fast
- 7-day fast
- Extended fasting
- By age
Also see: reference table, trusted resources, FAQ
Medical disclaimer · Estimates only—not medical advice.