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About this tool
Place your 8-hour eating window and 16-hour fast around work, training and bedtime, with meal times and sleep-overlap checks.
The 16:8 Fasting Window Planner places an 8-hour eating window and its matching 16-hour fast on a real clock, anchored either to when you want your first meal or to the time you need to stop eating. Because eating hours and fasting hours always sum to 24, fixing one end fixes everything else — so the planner works out meal times inside the window, how many of the 16 fasting hours you are actually asleep for, and whether your training session lands fed or fasted. It also flags a last meal too close to bedtime, using the usual guidance of finishing about 3 hours before sleep.
Open 16:8 Fasting Window Planner on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Set the first meal or the last bite — the planner derives the other and the whole fast.
Shows how many fasting hours you sleep through, which is what decides whether 16 hours is manageable.
Flags a session with no meal within about two hours afterwards, or a meal eaten too close to training.
All food for the day goes inside an 8-hour window and the other 16 hours are a water-only fast. The two numbers always add up to 24, so a window from 12:00 to 20:00 means fasting from 20:00 until 12:00 the next day — and around 8 of those 16 hours are usually spent asleep.
Water, plain black coffee and plain unsweetened tea, all of which have essentially no calories. Milk, sugar, cream, honey, juice and sweetened protein drinks all supply calories and end the fast; some flavoured or diet drinks contain a few calories per serving, so the label is worth checking.
Either works, but they suit different sessions. Easy or moderate training is generally fine fasted; hard or long sessions usually go better with a meal roughly 90 minutes to 3 hours beforehand. Whichever you choose, try to have a protein-containing meal within a couple of hours of finishing, which means the session should not fall right at the end of your window.
Speak to a doctor before starting if you are pregnant or breastfeeding, under 18, taking insulin or other glucose-lowering medication, have or have had an eating disorder, are underweight, or take medication that has to be taken with food. This planner handles timing only and is not medical advice.