Water fasting weight loss calculator

This shows how much energy your body uses over a water fast of 24 to 72 hours, and the most fat that energy could add up to. It checks first whether a water fast is safe to estimate for you, and it never predicts what the scale will say, because most early weight loss is water that comes back.

First: does any of these apply to you?

Pregnant, trying to conceive or breastfeeding Diabetes treated with insulin or other glucose-lowering medicine An eating disorder now or in the past Any medicine that must be taken with food, or for blood pressure or heart conditions Kidney, liver or heart disease, gout, or recovering from illness Frail, or have lost a lot of weight without trying

Units

Sex at birth

Age

Weight (kg)

Height (cm)

Activity

Length of the fast

Energy your body uses over the fast

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The most fat that could equal

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a rough upper estimate, not a promise

The scale usually drops by more than the fat figure during a water fast. Most of that early loss is water and stored carbohydrate (each gram of stored carbohydrate holds about 3 to 4 grams of water), and it comes back when you eat again. The fat figure uses the common rule of about 7,700 calories per kilogram of fat, which is rough and tends to overstate the loss for leaner people.

How to use the water fasting weight loss calculator

Start with the six health questions at the top and tick any that apply. Then choose kg and cm or lb and feet, your sex at birth, age, weight, height and activity level. Last, pick a fast of 24, 36, 48 or 72 hours.

The result has two figures. The first is the energy your body uses over the whole fast, shown with your estimated maintenance calories for one day. The second is the most fat that energy could equal, in kilograms or pounds. For any fast over 24 hours, a note appears asking you to see a doctor first. It also links to the break-fast planner.

How the calculator works out the energy and fat figures

The calculator first estimates your maintenance calories, the energy you use in a normal day. It works out your resting needs with the Mifflin-St Jeor equation, then multiplies by an activity level. Those activity multipliers are common ones that many calculators use, not an official standard.

Next it scales that daily figure to the length of the fast, so a 48-hour fast uses two days’ worth. Dividing the total by 7,700 kcal, the rough energy in a kilogram of body fat, gives the fat figure.

That fat figure is a ceiling. Early in a fast, your liver draws on its glycogen (stored sugar) for fuel. Fat takes over as the main fuel only once the glycogen runs low, somewhere between 12 and 36 hours in. For leaner people, the 7,700 rule also gives a fat figure that is too high.

Worked example: a 90 kg man at four fast lengths

Here is the calculator’s own maths for a man of 40 with light activity, 90 kg (198 lb) and 180 cm. His resting needs come to 1,830 kcal a day. With the light-activity multiplier of 1.375, his maintenance is about 2,516 kcal.

Length of the fast	Energy used	The most fat that could equal
24 hours	2,516 kcal	0.33 kg (0.7 lb)
36 hours	3,774 kcal	0.49 kg (1.1 lb)
48 hours	5,033 kcal	0.65 kg (1.4 lb)
72 hours	7,549 kcal	0.98 kg (2.2 lb)

Three days without food account for under a kilogram of fat, even on the upper estimate.

Why the scale shows more than the fat figure

Scale weight usually falls faster than the fat figure on a water fast. Each gram of glycogen is stored with about 3 to 4 grams of water. When the glycogen is burned, its water leaves the body too, and both return once you eat again. So the calculator never forecasts what the scale will say.

The safety stops, and why each one is there

The calculator gives no estimate in these cases and asks you to talk to your doctor:

Age under 18. Guidance for teenagers puts healthy habits ahead of weight and dieting.
Pregnancy, breastfeeding or trying for a baby. Deliberate weight loss in pregnancy is not recommended. Breastfeeding is on the list as our caution.
Diabetes on insulin or other medicine that lowers glucose. Insulin and sulfonylureas, for example, can send blood sugar too low without food.
Any past or current eating disorder, as our caution.
Tablets that go with food, or blood-pressure and heart medicines. A water fast leaves no meal to take them with. Feeling dizzy while taking blood-pressure tablets means you should stop the fast.
Kidney, liver or heart disease, gout, or recovery from illness, as our caution.
Frailty or unplanned weight loss. Losing over 10% of your weight in 3 to 6 months without trying is a refeeding risk.
A BMI under 18.5, the adult underweight line, worked out from your height and weight.

It also offers nothing past 72 hours.

Refeeding after a water fast

Refeeding syndrome is a drop in blood phosphate, potassium and magnesium after food returns, and it can kill. The risk starts once someone has eaten little or nothing for over 5 days, and it is high after 10. Our own line is stricter. Get medical advice before you break any fast longer than 2 to 3 days.

End the fast and get help at once for faintness, confusion, chest pain or a racing heartbeat.

The guide to water fasting covers what happens hour by hour, what breaks a water fast and how it compares with daily intermittent fasting. For first meals after a fast of up to 72 hours, open the break-fast planner. Your maintenance calories on their own, with calorie targets, are in the TDEE calculator.

Frequently asked

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

The calculator shows the most fat the fast could account for, not a scale weight. Take a lightly active 40-year-old man of 90 kg (198 lb) and 180 cm. His 72-hour fast uses about 7,549 kcal, which is at most about 0.98 kg (2.2 lb) of fat. The scale usually falls further, but most of the extra is water and stored carbohydrate that return when you eat.

How much fat do you burn in a 24-hour water fast?

No more than one day's energy use divided by about 7,700 kcal per kilogram. A sedentary 40-year-old woman of 80 kg and 168 cm uses about 1,787 kcal in a day. That is at most 0.23 kg (about half a pound) of fat, and less in practice, because some of the energy comes from stored carbohydrate.

Why won't the calculator give me an estimate?

It stops if you tick any of the six health questions, if you are under 18, or if your BMI is under 18.5. For those groups a water fast is not safe to plan without a doctor. Instead of figures, you see a message and a link to the list of people who should not fast.

Is the 7,700 calories per kilogram of fat rule accurate?

It is a rough rule. It roughly fits people carrying a lot of body fat, but for leaner people it gives too high a figure. After you change how much you eat, weight takes about a year to move half the distance to where it will settle. A straight sum therefore overstates long-term loss.

Why does the calculator stop at 72 hours?

The site does not estimate fasts longer than 72 hours, because they need medical supervision. Talk to a doctor before any fast over 24 hours. Past 5 days of eating little or nothing, refeeding syndrome becomes a recognised risk when food returns.