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16:8 Intermittent Fasting Calculator (window times)
Work out your 16:8 intermittent fasting schedule from the time of your last meal. On 16:8 you fast for 16 hours and eat within an 8-hour window — the most beginner-friendly protocol. Enter your last-meal time and the calculator shows both windows as clock times: if your last meal is at 8:00 PM, you fast until 12:00 PM (noon) the next day, and your eating window is 12:00 PM–8:00 PM. It also overlays an approximate fasting-state timeline (fed under 12 hours, glycogen depleting 12–16 hours, mild ketosis 16–24 hours), so you can see roughly where a 16-hour fast lands. During the fast, black coffee, plain water, unsweetened tea and electrolytes are fine — cream, sugar, artificial sweeteners and BCAAs break it. Intermittent fasting is not for everyone: do not start without a clinician’s supervision if you have Type 1 diabetes or take insulin or sulfonylureas, if you are pregnant or breastfeeding, if you have a history of an eating disorder, or for children and adolescents. Lifestyle information, not medical advice.
16:8 → fast 16h, eat within an 8h window
- Last meal 8:00 PM → eat 12:00 PM–8:00 PM
- A 16h fast reaches the approximate mild-ketosis onset (16h)
Frequently asked questions
What is the 16:8 fasting window if my last meal is at 8 PM?
You fast for 16 hours, so the eating window opens at 12:00 PM (noon) the next day and stays open for 8 hours until 8:00 PM. The fasting window is 8:00 PM to 12:00 PM. Shift your last meal earlier or later and both windows move by the same amount.
Is 16:8 good for beginners?
Yes — 16:8 is usually the recommended starting point because a 16-hour fast is manageable (much of it overnight) and the 8-hour eating window still leaves room for two or three meals. If it feels easy after a few weeks, some people move to 18:6 or 20:4. Do not start any protocol without a clinician if you have a contraindication such as Type 1 diabetes, pregnancy, or a history of an eating disorder.