Health
BMI, calorie & wellness calculators
Fasting State Timeline Calculator (ketosis & autophagy hours)
See roughly where each fasting hour lands on the metabolic timeline. As a fast lengthens, the body shifts fuel: you are fed and running on glycogen for about the first 12 hours, glycogen depletes from around 12–16 hours, mild ketosis tends to set in around 16–24 hours, and deeper ketosis with more autophagy is associated with 24 hours and beyond. These hour markers are approximate and vary a lot by person, diet and activity — treat the timeline as a general map, not a guarantee that you hit ketosis or autophagy at an exact hour, and remember fasting ketosis is milder than ketogenic-diet ketosis. Pick a protocol (16:8, 18:6, 20:4 or OMAD) and a last-meal time, and the calculator both schedules your eating and fasting windows and highlights which state your chosen fast duration reaches. 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.
Fed <12h · glycogen depleting 12–16h · mild ketosis 16–24h · deeper ketosis + autophagy 24h+
- An 18h fast (18:6) reaches the approximate mild-ketosis band
- Timings are approximate and vary by person
Frequently asked questions
When does autophagy start during a fast?
Greater autophagy is generally associated with longer fasts — roughly 24 hours and beyond in the deeper-ketosis range — but the exact onset is not precisely known in humans and varies by person, diet and activity. Treat any single "autophagy hour" figure as approximate. Shorter daily fasts like 16:8 still shift you toward fat metabolism even if they do not reach the deepest autophagy range.
How long until I reach ketosis when fasting?
Mild ketosis typically sets in around 16–24 hours of fasting, after glycogen has largely depleted (about 12–16 hours). These are approximate ranges — how fast you get there depends on your last meal, activity and individual metabolism. Fasting ketosis is milder than the ketosis from a ketogenic diet.