Extended Fasting Calculator: 14-Day and 30-Day Estimates
This extended fasting calculator models prolonged water fasts (default 10 days) and accepts custom lengths such as 14 or 30 days. For a 35-year-old woman at 70 kg, the same Mifflin–St Jeor BMR × 1.35 TDEE model shows about 5.1 kg (11.3 lb) at 10 days, about 6.8 kg (14.9 lb) at 14 days, and about 13.2 kg (29.1 lb) at 30 days. Educational only; fasts this long need medical supervision—not medical advice.
How much weight would you lose if you fasted for 2 weeks, or on a 30 day fast? Extended fasting means going well beyond a day. This page opens on a 10-day model: for a 35-year-old woman at 70 kg and 165 cm, about 5.1 kg (11.3 lb) total—roughly 2.4 kg fat and 2.7 kg water. The same formula, changed to 14 days, projects about 6.8 kg (14.9 lb); at 30 days, about 13.2 kg (29.1 lb). Change the day count to match the length you are researching. These are educational projections for one example profile, not a plan to fast that long.
Use this page for multi-day / prolonged intent. It is not the homepage’s open “how much will I lose?” tool, and it is not a single fixed 3- or 7-day answer page.
Prolonged unsupervised fasting can be dangerous. These figures set expectations; they are not a protocol.
10-day example
≈5.1 kg (11.3 lb); fat ~2.4 kg as deficits compound.
Any length
Custom days model prolonged fasts beyond the fixed presets.
What to know before you start
What counts as extended fasting?
Most clinicians describe fasts beyond 24 hours as extended or prolonged fasting, distinct from intermittent fasting windows like 16:8. As a fast lengthens, the body shifts from burning liver glycogen toward sustained fat oxidation and ketone production. That is why the first 72 hours show a large water drop on the scale while later days reflect steadier fat loss driven by your total daily energy expenditure.
This calculator uses the Mifflin-St Jeor BMR equation and a sedentary–lightly active TDEE multiplier to project fat loss for each day of the fast, while modeling early glycogen water separately. Because BMR falls with age and lower body mass, two people fasting the same number of days can see very different projected outcomes.
Safety during a prolonged fast
Extended water fasting can affect blood pressure, blood sugar, electrolytes, and heart rhythm. Pregnancy, breastfeeding, being underweight, eating disorders, type 1 diabetes, kidney disease, and many cardiovascular conditions are contraindications for unsupervised prolonged fasting, and several medications may need adjustment.
If you are considering a fast of several days, do it with medical guidance so a clinician can monitor vitals, electrolytes, and safe refeeding. Treat every estimate here as educational—not medical advice or a recommendation to fast longer.
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
- Extended fasting(current)
- By age
Also see: reference table, trusted resources, FAQ
Medical disclaimer · Estimates only—not medical advice.