Fasting Weight Loss Calculator

It first estimates your basal metabolic rate (BMR) using the Mifflin-St Jeor equation from your age, sex, height, and weight, then multiplies that by an activity factor to get your total daily energy expenditure (TDEE). It then applies a typical daily calorie intake reduction associated with your chosen fasting protocol to estimate a daily deficit, and converts that deficit into a projected weekly weight loss rate and an estimated timeline to your goal weight.

16:8 and 18:6 restrict eating to an 8 or 6 hour daily window, with the rest of the day spent fasting. 20:4, often called OMAD or a variation of it, narrows that window to about 4 hours or a single meal. 5:2 involves eating normally five days a week and sharply restricting calories on two non-consecutive days. Alternate-day fasting alternates a normal eating day with a very low calorie or zero calorie fasting day. Each protocol tends to produce a different typical reduction in total weekly calorie intake, which is reflected in this calculator's estimate.

No. The percentages used here are general estimates based on commonly reported patterns of reduced calorie intake for each protocol, not a measurement of your personal eating habits. Some people naturally eat less within a shorter window, while others compensate by eating more during their eating period, which can significantly change actual results. Treat the output as a starting estimate rather than a guaranteed figure.

The 3,500-calorie estimate for one pound of fat is a long-standing approximation, not an exact biological constant. Actual fat loss also depends on factors such as water weight shifts, muscle mass changes, metabolic adaptation over time, and individual variation, so real-world results frequently differ from a simple linear projection, especially over longer timelines.

Activity level is used as a multiplier on your BMR to estimate your total daily energy expenditure, since someone who is sedentary burns meaningfully fewer calories per day than someone who exercises regularly, even with an identical BMR. A more accurate activity level selection produces a more realistic daily deficit and timeline estimate.

No. This tool provides a general educational estimate based on standard formulas and commonly cited averages. It is not a substitute for personalized medical or nutritional advice. Intermittent fasting is not appropriate for everyone, including some people who are pregnant or breastfeeding, have a history of disordered eating, have diabetes or another condition requiring regular food or medication timing, or are underweight. Speak with a doctor or registered dietitian before starting a new fasting routine, especially if you have an existing health condition.

Weight loss is rarely perfectly linear. Factors such as sleep quality, stress, hormonal changes, water retention, how closely you stick to your eating window, and whether calorie intake during meals stays consistent can all shift your actual pace above or below this calculator's projection. Many people see faster loss in the first one to two weeks due to water weight, followed by a slower and steadier rate afterward.

Regular physical activity, particularly resistance training, can help preserve lean muscle mass during a calorie deficit and may modestly increase your total daily energy expenditure, which this calculator accounts for through the activity level input. Adjusting your activity level selection to reflect a more active routine will produce a faster projected timeline, though extreme calorie deficits combined with intense exercise are generally not recommended without professional guidance.