Water Fasting Weight Loss Projection Calculator (Fat vs Water Split)
Every gram of stored glycogen binds roughly three grams of water, so the first two days of a water fast produce a dramatic scale drop that has almost nothing to do with fat. This projection separates the three components — bound water, oxidised fat and the small lean-tissue contribution — so the number you see on day four does not mislead you after the refeed.
Only about 26% of the scale drop is fat — the rest returns once glycogen refills.
- 1.
Starting weight = 85 kg - 2.
Fast length = 3 days - 3.
Activity during fast = 1.2 - 4.
Gender = male - 5.
Age = 35 years - 6.
Height = 175 cm - 7.
Scale weight lost = 2.74 kg - 8.
True fat loss = 0.70 kg - 9.
Glycogen water = 1.90 kg - 10.
Daily burn = 2,129 kcal
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
What happens hour by hour
Blood glucose falls for the first 12 hours while liver glycogen supplies the brain. Between 12 and 36 hours glycogen depletes and hepatic gluconeogenesis ramps up, drawing on glycerol and amino acids. From roughly 48 hours ketone production dominates, protein sparing improves, and measurable autophagy markers rise. Growth hormone climbs steadily and peaks around day three.
Why lean loss is not zero
Even in deep ketosis the body converts some amino acids to glucose for red blood cells and renal medulla. Modelling puts this near 15% of the energy deficit in the first three days, falling below 5% in adapted multi-day fasts, which is why extended fasts are less catabolic per day than short repeated ones.
Refeeding without rebounding
Break a fast longer than 48 hours with a small protein and fat meal, not carbohydrate. Rapid carbohydrate loading after several days without food shifts phosphate, potassium and magnesium into cells and can precipitate refeeding syndrome. Ramp to normal intake over two to three days.
Typical scale loss during a water fast (80 kg adult)
| Fast length | Scale loss | Of which water | Of which fat | Dominant fuel |
|---|---|---|---|---|
| 24 hours | 1.2-1.6 kg | ~0.9 kg | ~0.25 kg | Liver glycogen |
| 48 hours | 2.2-2.8 kg | ~1.4 kg | ~0.55 kg | Gluconeogenesis |
| 72 hours | 3.0-3.8 kg | ~1.7 kg | ~0.85 kg | Ketones |
| 5 days | 4.5-5.5 kg | ~2.0 kg | ~1.4 kg | Ketones + fat |
| 7 days | 6.0-7.0 kg | ~2.2 kg | ~2.0 kg | Deep ketosis |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: starting weight 66.3 kg, activity during fast "Bed rest"
On the lower / more conservative end. Only about 22% of the scale drop is fat — the rest returns once glycogen refills.
Example 2: starting weight 85 kg, activity during fast "Light"
A typical middle-of-the-road setup. Only about 26% of the scale drop is fat — the rest returns once glycogen refills.
Example 3: starting weight 110.5 kg, activity during fast "Moderate"
On the higher / more demanding end. Only about 30% of the scale drop is fat — the rest returns once glycogen refills.
Quick answers about the Water Fasting Weight Loss Projection
What exactly does the Water Fasting Weight Loss Projection work out?
Every gram of stored glycogen binds roughly three grams of water, so the first two days of a water fast produce a dramatic scale drop that has almost nothing to do with fat. You enter starting weight, fast length, activity during fast and gender (plus 2 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 6 fields: starting weight, fast length, activity during fast, gender, age and height. Nothing else is needed and nothing is stored.
What happens hour by hour?
Blood glucose falls for the first 12 hours while liver glycogen supplies the brain. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for water Fasting Weight Loss Projection?
Even in deep ketosis the body converts some amino acids to glucose for red blood cells and renal medulla. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Typical scale loss during a water fast (80 kg adult)" table on this page tell me?
It is the reference range this tool works against — 5 rows from "24 hours" (1.2-1.6 kg) up to "7 days" (6.0-7.0 kg). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which activity during fast should I pick?
The dropdown offers 3 choices — Bed rest, Light and Moderate. Pick the one that matches your real situation rather than the one you would like to be true; activity during fast usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Water Fasting Weight Loss Projection runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Water Fasting Weight Loss Projection leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
Roughly 25-35% on a three-day fast and up to 60% on a seven-day fast. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
Related Calculators & Tools
Fasting Water & Electrolyte Needs
Calculate daily water, sodium, potassium and magnesium needs during a fast based on body weight, fasting hours and climate, with a DIY electrolyte recipe.
TDEE & Macro Calculator
Free TDEE calculator using Mifflin-St Jeor and Katch-McArdle. Get maintenance calories, a cutting target, protein/carb/fat grams and the exact date you hit your goal weight.
Calorie Deficit Target Date
Enter current weight, goal weight and daily deficit to get the exact date you reach target, weekly milestone table and a plateau-adjusted realistic date.
Keto Net Carb & Macro Split
Work out ketogenic net carbs, protein and fat grams from your daily calorie budget. Includes total vs net carb maths, fibre and sugar alcohol subtraction, and a keto tier table.
Body Fat % (US Navy Tape Method)
Estimate body fat percentage with the US Navy circumference formula. Enter neck, waist and hip measurements to get body fat %, lean mass, fat mass and Navy standard pass/fail.
One Rep Max (1RM) Calculator
Estimate your one rep max from any set of 1-12 reps using Epley, Brzycki and Lombardi formulas, plus a full percentage training chart for bench press, squat and deadlift.