The 16:8 fasting window, and what the trials actually found
16:8 means you eat inside an eight-hour window and drink only water, tea or black coffee for the other sixteen. The arithmetic is the easy part, so here it is first.
Fasting window calculator
When your last meal was, and how long you want to fast.
What the window is not
It is worth being plain about this, because most pages about 16:8 are not.
In 2020 a randomised trial put 116 adults with overweight or obesity into either a 16:8 window (eating between 12:00 and 20:00) or three structured meals a day, and followed them for twelve weeks. The fasting group lost 0.94 kg. The three-meals group lost 0.68 kg. The difference between the groups was 0.26 kg, and it was not statistically significant. The authors' conclusion was that time-restricted eating, on its own, is not more effective for weight loss than eating throughout the day.
A larger and longer trial in 2022 asked a sharper question: if everybody is already counting calories, does adding a window help? 139 people with obesity were given the same calorie target for twelve months, and half of them were also told to eat only between 08:00 and 16:00. The window group lost 8.0 kg and the calorie-only group lost 6.3 kg — a difference of 1.8 kg that, again, was not statistically significant.
Shortening the window further does not appear to rescue it either. An eight-week trial comparing four-hour and six-hour eating windows found both groups lost about the same amount, with no advantage to the stricter one.
What the window is
None of that makes fasting useless. It makes it a method rather than a mechanism.
Read the trials again and the pattern is ordinary: people who ate inside a window ate somewhat less, and lost roughly what eating somewhat less would predict. Nothing special happened at hour sixteen. What a window does is remove decisions. There is no question about whether to have something after dinner, because the window is shut. For some people that is far easier to hold to than counting, and the easiest method you will actually keep is better than the optimal one you will not.
So the honest way to choose a window is by fit, not by physiology:
- If your extra eating happens late at night, closing the window early takes it away without you having to decide anything.
- If your appetite is worst in the morning, a late window costs you nothing.
- If eight hours leaves you eating past discomfort at each meal, the window is too short for you. That is not a failure of discipline.
Using the calculator above
Put in the time you finished your last meal and how many hours you want to fast. It tells you when the fast ends and how long the eating window that follows runs — including whether that lands the next day, which is where the mental arithmetic usually goes wrong after a late dinner.
Nothing is sent anywhere. The page does the sum in your browser.
What to watch instead of the window
If you are fasting to lose weight, the window is the input and your weight is the output — but daily weight readings are mostly noise, and reading them day by day will tell you almost nothing. How to read the fluctuation is a separate problem, with its own calculator.