Short answerIntermittent fasting helps you lose weight only by helping you eat less. Cut 400 calories a day and you'll lose about 0.8 pounds a week on any schedule. A 3-day water fast can drop the scale 5 to 6 pounds, but only about 1.7 pounds of that is body tissue for someone who burns 2,000 calories a day; the rest is water that returns when you eat.
Fasting window calculator
Pick a plan and the time of your first meal, and the fasting window calculator shows your eating hours on a 24-hour clock:
| Plan | Fasting | Eating | Example |
|---|---|---|---|
| 14:10 | 14 hours | 10 hours | Eat 9 am to 7 pm |
| 16:8 | 16 hours | 8 hours | Eat noon to 8 pm |
| 18:6 | 18 hours | 6 hours | Eat 1 pm to 7 pm |
| 20:4 | 20 hours | 4 hours | Eat 3 pm to 7 pm |
| OMAD | about 23 hours | 1 meal | Dinner at 6 pm |
| 5:2 | 2 low days a week | 5 normal days | About 500 calories on Mon and Thu |
Most people find a midday-to-evening window easiest, because it skips breakfast but keeps dinner with family or friends. An earlier window, like 8 am to 4 pm, fits your body clock a little better but is harder socially.
Intermittent fasting weight loss calculator
Here's the part most fasting apps skip. The clock doesn't burn fat; the calorie gap does. In a 12-week trial of 16:8 eating, people lost about the same small amount as a group eating at any time. Reviews comparing intermittent fasting with ordinary calorie restriction find similar results when calories end up similar. Our guide to different types of diets compares fasting with keto, Mediterranean and others.
So this free intermittent fasting calculator asks two numbers: your maintenance calories, which the TDEE calculator works out, and what you actually eat. Every 3,500 calories you don't eat is roughly a pound:
- 300 fewer calories a day: about 0.6 lb a week
- 500 fewer a day: about 1 lb a week
- 750 fewer a day: about 1.5 lb a week
Fasting helps if it makes that gap easier, because one less meal is one less decision. If you overeat in your window, the scale won't move. To see where a steady gap takes you over months, run it through the weight loss simulator.
OMAD weight loss calculator
One meal a day (OMAD) usually creates a big deficit simply because one plate holds fewer calories than three meals. Pick OMAD in the calculator and enter what your one meal adds up to. The catch: it's hard to fit enough protein, fiber and vitamins in one sitting, and many people binge at the end of a long fast. If OMAD works for you, make it a full, balanced meal, not a snack. The macro calculator shows how much protein that one meal needs to carry.
The 5:2 diet
On 5:2 you eat normally five days a week and about 500 calories (women) or 600 (men) on two non-consecutive days. With maintenance at 2,000 calories, those two days cut about 3,000 calories a week, close to a pound, if your normal days stay normal. Logging the two low days in the calorie counter keeps them honest.
Water fast weight loss calculator
A water fast means no food, only water, for a day or more. Plain water has no calories (see does water have calories), so every calorie you burn comes from your body's stores. The scale drops fast, which is why it's popular, but most of the early drop isn't fat. Your body stores carbohydrate as glycogen, about 400 grams, bound to 3 to 4 times its weight in water. In the first days that's used up, and 3 to 4 pounds of water and glycogen leave with it. They return when you eat again.
The body tissue you actually burn is about your daily calorie needs divided by 3,500 for each day, roughly half a pound a day if you burn 2,000 calories. Not all of it is fat: during a fast the body also breaks down some protein, including muscle. This free water fasting calculator shows the two parts separately.
Who should not water fast
- Anyone pregnant, breastfeeding, under 18, or with a history of eating disorders (if you're nursing, plan with the breastfeeding calorie calculator instead)
- People with diabetes, especially on insulin or other glucose-lowering medicine
- People with heart, kidney or liver disease, gout, or low body weight
- Anyone taking medicines that need food or affect blood pressure or blood salts
Fasts longer than 3 days should only be done under medical supervision.
Fasting FAQ
How much weight can you lose with intermittent fasting?
It depends on how many fewer calories you eat, not on the clock. Eating 400 calories a day under maintenance loses about 0.8 pounds a week, whether you fast or not. Studies find intermittent fasting works about as well as ordinary calorie cutting.
How much weight do you lose on a water fast?
The scale can drop 4 to 6 pounds in 3 days, but much of it is water and stored carbohydrate that comes back when you eat. Actual body tissue lost is roughly your daily calorie needs divided by 3,500 for each day, about half a pound a day for many people.
What is a fasting window?
The hours each day when you don't eat. On 16:8, you fast for 16 hours, for example 8 pm to noon, and eat within the other 8.
Is OMAD good for weight loss?
One meal a day can create a calorie deficit, but it's hard to keep up and makes it easy to miss protein and fiber. For most people a 16:8 or 14:10 window is easier to stick with.
Sources
- Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of time-restricted eating on weight loss and other metabolic parameters: the TREAT randomized clinical trial. JAMA Intern Med, 2020;180(11):1491–1499.
- Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. Am J Clin Nutr, 1992;56(1 Suppl):292S–293S.
- Hall KD, Chow CC. Why is the 3500 kcal per pound weight loss rule wrong? Int J Obes, 2013.