Water Fasting Weight Loss Calculator

Fasting diets are very popular at the moment. That’s probably because they’re such a simple concept. Basically, a fast is a period during which you either don’t eat or eat a very limited number of foods. Some fasts are short and designed to be done frequently, while others are longer and done much less often.

Popular fasting diets include:

• One Meal A Day (OMAD) Diet
• The 5:2 Diet
• The 16:8 Diet
• Intermittent Fasting Diet
• Eat, Stop, Eat Diet
• Alternate Day Diet
• 24-Hour Fasting Diet

Of all the protocols, water fasting is arguably the strictest and the hardest to stick to. It’s also the one where the scale moves fastest. With a water fast, you consume only water for the duration of your fast. No coffee, no juice, no tea, and no solids: just water.

But how much weight can you expect to lose during a water fast? The answer depends on the length of your fast, your starting weight, and your sex. Use our water fasting calculator to estimate how much weight you will lose during your next water fast.

What is the Water Fasting Weight Loss Calculator?

The water fasting calculator estimates how much weight you will lose during a water-only fast. During a water fast, you eat no food and drink only water.

To calculate weight loss, the water fasting calculator takes into account your current weight, your sex, and the length of your fast in days. It then estimates how much weight you are likely to have lost by the end of your water-only fast, and what you will weigh.

This is useful for predicting weight loss, and the result can also be motivating as you’ll be able to see how much weight you can lose within a few short days.

How the Water Fasting Calculator Works

The calculator does not guess from calories. It uses weight loss that was measured. In a study published in the British Medical Journal, researchers weighed 76 people with obesity (58 women and 18 men) every morning while they fasted in hospital on water only, for up to 30 days. (1)

The calculator follows the study’s average weight curve: the share of starting body weight lost by the end of each day, separately for men and women. It multiplies your starting weight by the share for the number of days you enter. Here are some of the values it uses:

Example: a 200-pound man who water fasts for 7 days would lose about 5.1% of his weight, or roughly 10 pounds, and weigh about 190 pounds at the end. A 160-pound woman fasting for 7 days would lose about 4.5%, or roughly 7 pounds.

Two patterns show up in the data:

• Weight comes off fastest in the first two weeks. After about day 14, the daily loss slows to a steady, smaller amount.
• Men lose a slightly larger share than women, and heavier people lose more pounds, because the calculation is a percentage of your starting weight.

How much of that weight is fat?

Less than the scale suggests, especially early on. In the first two weeks, much of the loss is water, sodium, and stored glucose (glycogen), and part of it comes from lean tissue rather than fat. (1)(2) After about two weeks, the loss slows and fat supplies almost all of the energy: 96% in the study. (1)

That is why a lot of the early weight comes back within days of eating again (see the FAQ below).

What the calculator does not account for

• Your body composition and age. The study fasted people with obesity, so your result may differ if you are leaner.
• Activity, sleep, and stress, which change how much energy you burn.
• Breaking the fast. Anything other than plain water changes the result.

Treat the result as an average estimate, not a promise.

How to Use the Water Fasting Weight Loss Calculator

Our water fast calculator takes a few seconds to use:

1. Choose your units: Imperial (lb) or Metric (kg).
2. Select your sex, male or female.
3. Enter your current weight.
4. Pick the length of your fast: 1, 3, 5 or 7 days, or choose Other and enter any number of days from 1 to 31.
5. Hit “Calculate Weight Loss”.

Interpreting your Results

After you hit Calculate, you’ll see:

• Estimated weight loss: how many pounds or kilos you can expect to lose by the end of your water fast, with what you would weigh at the end.
• Of your body weight: the same loss as a share of your starting weight.
• Average loss per day: the total loss divided by the days of your fast. The first days lose more and later days less, as the chart shows.
• Your weight by day: a chart of your estimated weight at the end of each day of the fast.
• A safety note for any fast longer than 3 days.

It’s important to note that these results are estimates only, and actual weight loss can vary depending on things like activity levels, age, metabolic rate, stress levels, and genetics.

Before You Start a Water Fast

A water fast longer than a day or two is hard on the body. Don’t do one without talking to your doctor first if you:

• Have diabetes or take medication that lowers blood sugar or blood pressure
• Have heart, kidney, or liver disease, or gout
• Are pregnant or breastfeeding
• Are under 18 or underweight
• Have, or have had, an eating disorder

If you are new to fasting, start with 24 hours. Fasts longer than three days are best done with medical supervision.

How to Get the Best Results Safely

Use the following strategies to stay safe and get the most from your water fast.

Do not break your fast!

Fasting means going without food, and a water fast means the only thing you can consume is plain water. Tea, coffee, soda, and juices are OFF the menu! Breaking your fast with snacks or beverages other than water will affect your results and reduce weight loss.

Drink enough water, but not too much

Drink steadily through the day and let thirst guide you. Most people need around 2 to 3 liters a day. Drinking far more than that does not burn more fat. Any extra weight you pass as urine is water, not fat, and drinking large amounts while you eat no salt can dilute the sodium in your blood, which is dangerous.

On fasts longer than a day, you also lose sodium, potassium, and magnesium. If you feel dizzy, weak, or get cramps or a racing heart, stop the fast. Use our fasting electrolyte calculator to see how much of each mineral you may need.

Stay lightly active

While you may not feel like exercising during a water fast, you should avoid being sedentary. Gentle walks and stretching are enough. Skip hard workouts and heavy lifting: with no food coming in, they raise the risk of dizziness and fainting.

Get plenty of sleep

Sleep deprivation can make it harder to lose weight. Improve your water fast results by getting 7-9 hours of sleep per night. This may mean you have to go to bed earlier, but getting more sleep can have a significant impact on weight loss. It’ll give you more energy, too.

Avoid stress

Stress causes your body to release a hormone called cortisol. Cortisol limits fat burning and promotes fat storage, which is the opposite of what you want during a water fast. Try to avoid stress to maximize your weight loss results.

Good options for reducing stress include:

• Better time management
• Learning to prioritize
• Breathing and meditation
• Spending time in nature
• Avoiding stressful situations and people
• Getting plenty of sleep
• Delegating when possible
• Not taking on more work than you can handle
• Learning how to say no
• Learning to unplug from technology, especially an hour or so before bedtime

Water Fasting Weight Loss Calculator FAQ

1. How accurate is the water fasting calculator?

It gives the average result measured in a real study of supervised water fasts, so it is a reasonable estimate for most adults. Your own result can be higher or lower. The study fasted people with obesity, and the calculator does not know your age, body fat, activity level, stress, or sleep.

2. Are water fasts safe?

A short water fast of 24 to 72 hours is usually tolerated by healthy adults, but it carries risks, and longer fasts carry more. Common side effects include:

• Mineral imbalances
• Hunger
• Headaches
• Nausea
• Dizziness and low blood pressure when you stand up
• Impaired kidney function
• Foul taste in the mouth
• Bad breath
• Insomnia
• Fatigue and weakness

These symptoms tend to be worse at the start of a water fast and diminish after a couple of days. Short water fasts are less impactful than longer fasts. See the list of people who should talk to a doctor first, above.

However, the biggest risk associated with water fasting is not the result of fasting but when you start eating again. This is called refeeding syndrome and is especially true for anyone who fasts for five days or more.

Refeeding syndrome is a potentially fatal condition caused by eating too much food after a lengthy fast. Symptoms of refeeding syndrome include:

• Weakness
• Confusion
• Difficulty breathing
• High blood pressure
• Abdominal pain
• Seizures
• Heart failure
• Coma
• Death

To avoid refeeding syndrome, you should break your fast with a small, light meal and avoid binging. Fresh fruit, vegetable soups, and natural yogurt are all good choices. Subsequent meals should also be light and easy to digest. Take several days to gradually return to eating normally.

3. Will I regain weight after a water fast?

In all likelihood, you WILL regain some weight when you end your water fast and return to eating normally. That’s because, during your fast, you lost fat but also some muscle and a substance called glycogen. Glycogen is glucose stored in your liver and muscles.

Glycogen is chemically bound to water, so as your glycogen levels return to normal, you’ll also store more water. When you break your fast, glycogen levels are replenished, leading to rapid weight regain.

However, providing you break your water fast with small meals and don’t eat to excess, you should not regain fat after water fasting.

4. How long should I water fast for?

For most people, one to three days is plenty. If you are entirely new to fasting, start with a 24-hour water fast to see how your body responds. Longer fasts should be done with medical supervision. The calculator accepts up to 31 days so you can see the trend. That does not make a fast that long safe to do on your own.

If you want a gentler way to fast regularly, a daily fasting window like 16:8 is easier to keep up.

5. How often can I do a water fast?

As a general rule, the longer you do a water fast, the less often you should do it. For example, you could do a two-day water fast once a month. However, fasting more often than this could be harmful, causing eating disorders and other long-term health issues.

However, there are other fasting protocols that are better suited to more frequent use, such as OMAD, 16:8, and the 5:2 diet.

6. What are some of the benefits of fasting?

People have been fasting for centuries. Benefits that studies have found or are still testing include:

• Weight loss
• Fat loss
• Lower blood pressure
• Lower blood glucose
• Improved blood lipid profile
• Reconnection with appetite

You may also read that fasting “detoxes” the body. It doesn’t: your liver and kidneys clear waste whether you eat or not. Fasting does increase autophagy, the process cells use to break down and recycle old parts. Researchers are studying whether that helps with ageing and disease, but there is no proof yet that water fasting prevents cancer, heart disease, or Alzheimer’s disease in people.

Other related calculators:

• Fasting Electrolyte Calculator
• Extended Fasting Calculator
• TDEE Calculator
• Calorie Deficit Calculator
• Intermittent Fasting Calculator
• Calories Per Meal Calculator

Wrapping Up

Water fasting makes the scale drop quickly: about 5% of your body weight in the first week for an average man and a little less for a woman, based on measured data. Much of that early loss is water and glycogen, and some of it returns when you eat again.

Water fasting is also challenging, and most people will find it too restrictive for anything other than short and occasional use.

Short water fasts are usually tolerated by healthy adults, but side effects such as severe hunger, headaches, dizziness, and fatigue are common. It’s also crucial to break your fast with small meals to avoid potentially life-threatening refeeding syndrome.

Use our water fasting calculator to estimate how much weight you can lose during a water fast.

References:

1. Runcie J, Hilditch TE. Energy provision, tissue utilization, and weight loss in prolonged starvation. British Medical Journal. 1974;2(5915):352-356.
2. Kerndt PR, Naughton JL, Driscoll CE, Loxterkamp DA. Fasting: the history, pathophysiology and complications. Western Journal of Medicine. 1982;137(5):379-399.