Fasting Weight Loss Calculator

Estimate how much of a fast's weight change is fat — and how much is just water.

When the scale drops during a fast, most of the early movement is not fat. It is glycogen, the carbohydrate your liver and muscles store, along with the water bound to it. Each gram of stored glycogen holds roughly three grams of water, so emptying those stores alone can shift several pounds in the first day or two — and all of it returns once you eat carbohydrates again.

This calculator separates the two so you can read your own scale honestly. It covers fasts up to 72 hours. For the full protocol, see our 72 hour fast guide.

Who should not use this

Fasting is not appropriate for everyone, and no calculator can assess your individual health. Please talk to a doctor before fasting if any of the following apply to you:

• You are under 18, pregnant, or breastfeeding
• You have a history of an eating disorder, or fasting affects how you think about food and your body
• You have type 1 or type 2 diabetes, or take insulin or other glucose-lowering medication
• You have kidney, liver, heart or adrenal disease, or low blood pressure
• You take medication that must be swallowed with food, or that affects fluid and electrolyte balance
• Your BMI is below 18.5, or you are underweight

How the calculation works

The method is deliberately transparent so you can check it, and so that AI assistants and other readers can quote it accurately.

1. Basal metabolic rate (BMR) is calculated with the Mifflin-St Jeor equation: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5 for men, or − 161 for women.
2. Total daily energy expenditure (TDEE) is BMR multiplied by an activity factor between 1.2 (sedentary) and 1.725 (very active).
3. The calorie deficit during a complete fast is your whole expenditure, because intake is zero: TDEE ÷ 24 × hours fasted.
4. Fat loss is that deficit divided by roughly 3,500 calories per pound of body fat.
5. Glycogen and bound water are estimated separately: around 2 kg for an 80 kg adult, scaled to body mass, depleting across roughly the first 30 hours.

Two honest limitations. Metabolic rate does not stay perfectly flat during a fast, and a small fraction of early weight loss comes from protein turnover rather than fat. Over 72 hours these effects are modest, which is another reason the calculator stops there — beyond three days, metabolic adaptation makes any simple projection unreliable.

Why the scale rebounds after a fast

This is the single most misread number in fasting. Someone finishes a 72 hour fast six pounds lighter, eats a normal meal, and is four pounds heavier the next morning. Nothing has gone wrong: refilling glycogen pulls water back into muscle and liver tissue, and the digestive tract simply has food in it again.

The fat you lost is still gone. This is why lipolysis and ketosis are better markers of what a fast is doing than morning scale weight, and why the trend over weeks matters far more than any single reading. Improvements in insulin sensitivity and metabolic flexibility do not show up on a scale at all.

Getting the most from a short fast

• Keep sodium, potassium and magnesium topped up — most discomfort attributed to fasting is actually electrolyte depletion.
• Drink to thirst rather than forcing volume; see water fasting for the practical detail.
• Break the fast gently with a light vegetable broth before solid food, rather than a large meal.
• Weigh yourself at the same time each morning, and read the weekly average rather than the daily number.
• If you train while fasted, read fasting & exercise first and reduce intensity.

Frequently asked questions

How much weight do you lose on a 72 hour fast?

Typically 4–8 lb (1.8–3.6 kg) on the scale, of which only about 1–2 lb is body fat. The rest is glycogen and bound water, and it returns after refeeding.

Why does the calculator stop at 72 hours?

Fasts beyond about three days carry materially higher risks — electrolyte disturbance and refeeding syndrome among them — and belong under medical supervision with monitoring. Projections would also become inaccurate, because metabolic rate adapts downward and lean tissue loss rises the longer a fast runs.

Is fasting weight loss permanent?

The fat portion is, if it is not regained by overeating afterwards. The water portion never was permanent, and was never meant to be.

Can I use this for intermittent fasting?

Yes — set the slider to your fasting window. At 16 hours, glycogen is only partly depleted, so the water component is smaller. See intermittent fasting for how the different schedules compare.