Intermittent Fasting Window Calculator

Intermittent fasting protocols: eating windows, fasting hours and scheduling

Intermittent fasting is a dietary pattern that cycles between periods of eating and periods of fasting within each day or week. Rather than prescribing what to eat, it focuses on when to eat. The most popular daily approach is time-restricted eating, in which all food consumption is confined to a set window of hours and the rest of the day is spent fasting. This calculator focuses on time-restricted eating and helps you identify your exact eating window, fasting period, and critical time markers based on your preferred protocol and first meal time.

The most widely practised protocol is 16:8, where 16 hours of fasting are paired with an 8-hour eating window. This is popular because the 8-hour window is broad enough to accommodate two or three comfortable meals and the fasting period mostly occurs overnight. A person who finishes eating at 8pm and does not eat again until noon the next day is effectively doing a 16-hour fast with no deliberate midday restriction, since much of the fast happens during sleep. The 16:8 structure has been studied in randomised trials and has shown modest effects on weight, fasting glucose, blood pressure and inflammatory markers compared to unrestricted eating.

The 18:6 protocol narrows the eating window to six hours and extends the fast to 18 hours. This requires more deliberate scheduling but is still workable with two meals or one main meal and a snack. The 20:4 protocol, sometimes called the Warrior Diet, compresses eating into four hours and typically involves a single large meal in the evening with optional small snacks of raw fruit or vegetables during the day. The 23:1 protocol (one meal a day, or OMAD) is the most extreme time-restricted pattern, with only one hour allocated for eating. Each narrower window increases the fasting duration and the metabolic effects associated with extended fasting, while also increasing the difficulty of meeting nutritional needs within the window.

Knowing your exact eating window and last meal time in advance makes the protocol easier to follow consistently. Without a clear cutoff time, it is easy to drift: one small snack before bed becomes two, and the fasting period shortens without the person realising it. This calculator gives you a concrete last meal time to aim for, removing the ambiguity and making compliance simpler.

The first meal time is the single most important scheduling choice because it determines everything downstream. If your first meal is at 8am on a 16:8 protocol, your eating window closes at 4pm, your fast begins at 4pm, and it ends at 8am the next day. If you shift your first meal to noon, your window closes at 8pm, which suits people who prefer to eat dinner with family but do not want a morning meal. Neither schedule is inherently superior: the best schedule is the one that fits your daily routine and that you can maintain consistently over weeks and months.

Choosing the right protocol for your lifestyle

The 16:8 protocol is the appropriate starting point for most people new to time-restricted eating. It is the least disruptive to normal social patterns around food, fits into most work schedules without difficulty, and has the most research support of the time-restricted eating formats. If you find 16:8 straightforward after two to four weeks, you can experiment with 18:6 if you want a longer fasting window. Moving to 20:4 or OMAD is a significant step that many people find unsustainable for nutritional and social reasons over the long term.

Social meals are one of the most common practical challenges. A fixed eating window that ends at 4pm makes dinner with family or friends difficult to navigate. Scheduling flexibility is important: choosing a first meal time that places your eating window over the evening allows you to participate in dinner while still maintaining the fasting structure. Conversely, if your work involves early-morning physical activity or if you train in the morning, an earlier first meal may be more appropriate for performance and recovery.

People with a history of disordered eating, those who are pregnant or breastfeeding, those with type 1 diabetes or certain other conditions, and anyone on medications that require food should consult a doctor or registered dietitian before starting any structured fasting practice. For most healthy adults, time-restricted eating within a 16 to 18 hour fasting window is considered safe, but individual medical context always takes precedence over general guidance.

What this calculator assumes

The calculator treats each fasting cycle as exactly 24 hours. It calculates the last meal time by adding the eating window to the first meal time, identifies the fast start as equal to the last meal time, and notes that the fast ends the following day at the same time as the first meal. Times are shown in 12-hour AM/PM format. The custom option allows any eating window between 1 and 23 hours. For preset protocols (16:8, 18:6, 20:4, 23:1), the eating window input field is disabled and the window is set automatically. Switching to the Custom option re-enables the field.