7-Day Water Fasting Calculator: Fat, Water, and Limits
This water fasting calculator is locked to 7 days. For a 35-year-old woman at 70 kg it typically shows about 3.9 kg (8.7 lb)—about 1.7 kg fat and 2.2 kg glycogen-linked water—using Mifflin–St Jeor BMR × 1.35 TDEE. Week-long fasts need medical clearance for many people. Educational only, not medical advice.
A full week compounds four extra fat-loss days after the early water flush. For the familiar 35-year-old woman at 70 kg and 165 cm, seven water-only days project about 3.9 kg (8.7 lb) total—about 1.7 kg fat and 2.2 kg glycogen-linked water. That is more than 1.5× the day-3 fat line (~0.7 kg), while water only rises modestly past the 72-hour peak.
This page is built for week-long intent: numbers, fat-vs-water share, and clear limits. The 7-day preset is pre-selected.
Unsupervised seven-day fasts carry significant risk. Treat the calculator as an expectation model, and use the homepage when you want a flexible tool rather than a week-long answer.
7-day example
≈3.9 kg (8.7 lb); fat ~1.7 kg—more than double the 72-hour fat line.
Not a casual target
Week-long water fasts often need clinical clearance.
What to know before you start
What a 7-day estimate really shows
Week-long water fasts appear in some clinical and wellness contexts, but they are not a casual weight-loss shortcut. Our 7-day preset compounds seven days of estimated fat loss from your TDEE while modeling early glycogen water separately. The total can look impressive on paper; in practice, muscle preservation, electrolyte balance, and safe refeeding matter far more than the headline number.
Older adults and lighter individuals often see smaller fat-loss projections than younger, heavier users—even over the same seven days—because BMR declines with age and lower body mass. That is why age belongs in the equation, and why two people following identical fast lengths can experience very different outcomes.
When a 7-day fast is not appropriate
Pregnancy, breastfeeding, underweight status, active eating disorders, type 1 diabetes, kidney disease, and many cardiovascular conditions are contraindications for unsupervised prolonged fasting. Medications for blood pressure, blood sugar, and mental health may need adjustment or make fasting unsafe altogether.
If you are exploring a 7-day fast for health reasons, work with a clinician who can monitor vitals, labs, and refeeding. Our printable 7-Day Fast Planner includes educational checklists, but it does not replace professional supervision.
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
- 4-day fast
- 5-day fast
- 7-day fast(current)
- Extended fasting
- By age
Also see: reference table, trusted resources, FAQ
Medical disclaimer · Estimates only—not medical advice.