Published by MetaSys Limited · Page updated · Editorial policy. Educational information; not a medical review.
How This Fasting Window Calculator Works
Intermittent fasting protocols all boil down to the same simple idea: you compress your eating into a shorter window each day and fast for the rest. The confusing part is usually the math — if I eat lunch at noon, when exactly does my fasting window start and end, and how many hours am I actually fasting? This calculator answers that directly. Pick a protocol and a start time, and it maps out your exact eating and fasting windows on the clock, no mental math required.
The Math Behind Each Protocol
Every protocol here is defined by how many hours of the 24-hour day are open for eating. 16:8 means a 16-hour fast and an 8-hour eating window. 18:6 means an 18-hour fast and a 6-hour window. 20:4, sometimes called the "Warrior Diet" style, gives you just 4 hours to eat. OMAD (one meal a day) narrows that down to a single 1-hour window. The calculator takes your eating window length, adds it to your chosen start time to get your eating window end time, and then calculates the fasting window as everything else — starting the moment your eating window closes and running until your eating window opens again the next day. The total fasting hours are just 24 minus your eating window length, so a 16:8 schedule always gives you 16 fasting hours, regardless of what time you start eating.
Using Your Result in Practice
The main value of seeing your windows mapped out is that it turns an abstract rule ("fast for 16 hours") into a concrete daily schedule you can actually follow — "don't eat before 12:00 PM or after 8:00 PM." Most people find it easiest to set a start time that lines up with a meal they don't mind skipping. If you're not a breakfast person, starting your window at noon or 1 PM so it lands on lunch and dinner is usually the path of least resistance. If mornings are when you have the most willpower, an earlier window that ends by mid-afternoon can work well too. Try picking one start time and sticking with it for at least a week or two before adjusting — a consistent schedule tends to be easier to sustain than constantly shifting the window around.
Important Caveats
It's worth being clear-eyed about what intermittent fasting actually does and doesn't do. Meal timing is a strategy for structuring when you eat, not a metabolic shortcut — when calories and diet quality are matched, research comparing intermittent fasting to regular eating schedules generally shows similar results for weight and fat loss. The main practical benefit for most people is that a smaller eating window makes it easier to naturally eat less without counting every calorie, and some people find the structure easier to stick to than constant snacking. It is also not appropriate for everyone: people who are pregnant or breastfeeding, anyone with a current or past history of disordered eating, and people managing diabetes, low blood pressure, or other conditions affected by prolonged fasting should talk to a doctor before starting any fasting protocol. This calculator is a scheduling tool, not medical guidance.
Read Methodology & Sources and Calculator Limitations. Report an error.