Tool
Live tracker · 9 protocols · Free
Water Fasting Calculator.
Pick a fasting protocol — 16:8, OMAD, 24h, 48h, 72h, or custom — set when you last ate, and watch live as your body moves through ketosis, autophagy and stem cell regeneration. With water intake and calorie estimates.
New to water fasting? Read this first ▾
Water fasting means consuming only water (and usually electrolytes) for a defined period. As the fast progresses, your body shifts through several metabolic states — burning glucose, then glycogen, then fat (ketosis), and eventually triggering cellular repair (autophagy) and stem cell renewal.
Generally safe for
- Healthy adults
- Beginners doing 16:8 or 24h fasts
- People not on medication
- Non-pregnant, non-breastfeeding
Talk to a doctor first if you
- Have diabetes or take blood-sugar meds
- Are pregnant, breastfeeding, or under 18
- Have a history of eating disorders
- Plan a fast longer than 48 hours
- Take prescription medication
Important: This calculator is for information only and is not medical advice. Stop fasting and seek help if you feel unwell — dizzy, faint, irregular heartbeat, severe weakness.
Common questions
Water fasting explained.
What is autophagy and when does it start?
Autophagy is the body's cellular cleanup process — old, damaged proteins are broken down and recycled. Research suggests it begins ramping up around 16–18 hours into a fast, becomes significant by 24 hours, and peaks during 48–72 hours. The process slows again once you eat.
When does ketosis start during a water fast?
For most people, mild ketosis begins around 12–16 hours of fasting (faster if you're already low-carb). Deeper ketosis with stable energy and mental clarity tends to set in at 18–24 hours. Blood ketone meters or urine strips can confirm it.
Can I drink coffee or tea during a water fast?
Strict water fasts are water-only. "Clean" intermittent fasts often allow black coffee, plain tea and electrolytes — these have negligible calories and don't spike insulin meaningfully. Avoid milk, sugar, juice and anything with calories. This calculator assumes a water-only or "clean" fast.
How much water should I drink while fasting?
A common guideline is 30–35 ml per kg of body weight per day, which the calculator estimates for you. During longer fasts you also need electrolytes — sodium, potassium and magnesium — because you're not getting them from food. A pinch of pink salt and an electrolyte mix go a long way.
Will I lose weight during a water fast?
Yes — but most weight lost in the first 24–48 hours is water and stored glycogen, not fat. Real fat loss begins around 24h and accelerates from 48h onward. Expect ~0.3–0.5 kg of true fat per day during extended fasts. Some weight returns once you eat and rehydrate.
Is fasting safe?
For most healthy adults, short fasts (up to 24h) are well-tolerated. Risks rise with duration. Common side effects: hunger, fatigue, headaches, irritability, cold hands. Stop fasting and seek help if you feel faint, have heart palpitations, severe dizziness, or significant weakness. Don't fast if pregnant, breastfeeding, under 18, or with diabetes/eating-disorder history without medical supervision.
How do I break a long fast safely?
Start small — bone broth, a few berries, a small handful of nuts, or a bit of cooked vegetables. Wait 30–60 minutes, then a slightly larger meal. Avoid sugar, processed carbs, big portions and alcohol for the first meal. The longer the fast, the slower you should refeed — refeeding syndrome is real after 5+ day fasts.
How does this calculator work?
It takes your protocol's target hours and your last-meal time, computes elapsed and remaining time, identifies your current metabolic phase based on standard physiology references, and estimates calories burned (your weight × activity × hours fasted), fat loss (calories ÷ 7700 kcal/kg) and water needs (~30 ml/kg). Everything runs in your browser — no data is sent anywhere.
Reset the calculator?
Your current fast and saved preferences will be cleared. This can't be undone.