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.

How much weight will I lose? Worked examples

The table below runs the same maths the calculator uses, for a moderately active adult at three common body weights. Scale loss is the figure you will see in the morning; fat loss is the part that stays once you eat again.

Note the pattern: from 48 to 72 hours the scale barely moves even though fat loss continues, because the water component is already spent. That is the opposite of what most people assume, and it is the main reason day three of a fast feels discouraging.

What the scale is actually measuring, hour by hour

• Hours 0–12. Blood glucose falls, insulin drops, and liver glycogen starts supplying fuel. Most early scale movement here is food leaving the digestive tract, not tissue.
• Hours 12–24. Liver glycogen is largely gone and lipolysis accelerates. Each gram of glycogen released takes about three grams of water with it, which is why the first day shows the biggest drop. See the 24 hour fast.
• Hours 24–48. Muscle glycogen depletes, ketosis deepens, and fat becomes the dominant fuel. Detail in the 48 hour fast.
• Hours 48–72. Growth hormone rises to protect lean tissue, water loss has flattened, and the scale slows to roughly the true rate of fat loss — around half a pound a day. See the 72 hour fast.

Why the fat number is a range, not a figure

Four assumptions sit behind every fasting weight loss calculator, including this one, and each carries error. Mifflin-St Jeor predicts basal metabolic rate within roughly 10% for most adults, but it does not know your lean mass — two people at the same weight can differ by 300 calories a day. Activity factors are broad brackets. The 3,500 calories per pound of fat figure assumes pure adipose tissue. And glycogen stores vary widely with training history and recent carbohydrate intake.

In practice that means treating the fat estimate as a band of about ±20%, and confirming real change with something that measures tissue rather than mass. On this site the live dashboard shows what that looks like over months: scale weight bouncing several pounds week to week while the underlying trend, lean mass and body composition readings move far more slowly and far more honestly.

Frequently asked questions

How much weight will I lose in a 3 day fast?

For a typical 180 lb moderately active adult, around 5.5–7 lb on the scale, of which roughly 2 lb is body fat. Lighter people lose less and heavier people more, because both daily calorie burn and the glycogen pool scale with body mass.

Is a fasting weight loss calculator accurate?

Accurate as an estimate, not as a measurement. Expect the fat figure to be within about 20% for most people, and use weekly averages or a DEXA scan to confirm what actually changed.

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.