Intermittent fasting (IF) is an eating pattern that focuses on when you eat, rather than what you eat, by cycling between periods of voluntary fasting and non-fasting. Successfully implementing any IF protocol requires accurately calculating the length of the fasting window to achieve the desired metabolic shift. This involves applying specific timing to a 24-hour cycle or a weekly schedule.
Calculating Daily Time-Restricted Schedules
Daily time-restricted eating protocols are the most common form of IF, relying on a fixed 24-hour cycle where the fasting period is always longer than the eating period. The calculation involves a simple ratio, such as 16:8, where the first number represents the hours of fasting and the second number represents the hours in the eating window. This 16:8 schedule, for example, means you must fit all calorie consumption into an 8-hour window each day.
To plot this onto your day, integrate your sleep time into the fasting window, which makes the schedule more manageable since 7 to 9 hours of the fast occur while you are asleep. If you finish your last bite of food at 8:00 PM, add 16 hours to that time to determine your fast-breaking point. In this example, your fasting window would end at 12:00 PM (noon) the following day.
The eating window calculation is the inverse. If your fast ends at noon, your 8-hour eating window concludes at 8:00 PM. Adjusting the window is a matter of shifting both the start and end times equally. If you need to eat earlier, for instance, ending your eating at 7:00 PM would mean your fast ends at 11:00 AM the next morning.
Protocols like 14:10 or 18:6 are calculated using the same time-addition method, simply changing the length of the hours added to the last meal time.
Calculating Non-Daily and Calorie-Restricted Protocols
Other forms of intermittent fasting do not rely on a daily time-restricted window but instead use a weekly frequency or a defined calorie limit for calculation. The 5:2 method, for example, calculates five days of regular eating and two non-consecutive days of calorie restriction within a seven-day week.
5:2 Method
On these “fasting” days, intake is typically restricted to 500 calories for women or 600 calories for men, which is approximately 25% of a typical daily energy requirement. The choice of which two days to restrict calories is flexible, but they must be separated by at least one day of normal eating.
Alternate Day Fasting (ADF)
Alternate Day Fasting (ADF) involves calculating a cycle of alternating 24-hour periods between “fasting” and “feasting.” A modified ADF schedule often allows for a small calorie intake on the fasting day, typically up to 500 calories. The calculation is a simple one-to-one alternation: a full day of eating is followed by a full day of calorie restriction, and then the cycle repeats.
Determining When the Fast Starts and Ends
Accurately calculating the duration of a fast requires knowing the precise moment the fast begins and what substances will end it. The fast begins immediately after the last consumption of a calorie-containing food or beverage. Logging this exact minute is necessary for accurate time tracking.
The most common source of calculation error involves consuming items during the fasting window that contain calories. While a strict definition of fasting means zero caloric intake, many people follow the practical guideline that a caloric intake of less than 50 calories will generally not significantly interrupt the metabolic benefits of the fast. This small threshold is a concession for sustaining the schedule, but it is not a guarantee that the fast is technically unbroken.
Items like black coffee, unsweetened tea, and plain water are generally considered safe because they contain negligible or zero calories. However, adding cream, milk, or sugar immediately adds calories that can quickly exceed the 50-calorie threshold, effectively ending the fasting period. Even zero-calorie sweeteners and flavored waters are viewed with caution by purists, as some evidence suggests they may still elicit a metabolic response. Therefore, for the most accurate calculation of fasting time, the safest method is to restrict intake to only water, black coffee, or unsweetened tea.