Fasting Window Calculator

16:8 is by far the most popular: fast for 16 hours and eat within an 8 hour window, usually noon to 8 pm. 18:6 tightens that to a 6 hour window for people who tolerate longer fasts well. 20:4 is the Warrior Diet, a 4 hour evening eating window built around one large meal plus a smaller one. OMAD (One Meal A Day) pushes that further to a single meal in roughly a 1 hour window, which works out to a 23 hour fast. Longer protocols (24, 36, 48 hours) are done less frequently, typically once or twice per week rather than daily.

Human research does not establish a universal hour when autophagy starts or peaks during fasting. Timing varies by tissue, energy balance, activity, health, and measurement method. Treat the calculator's milestones as illustrative labels, not measured biological events or a reason to extend a fast.

Tolerance and risk depend on health, energy needs, pregnancy or breastfeeding status, medications, eating-disorder history, and other individual factors. The calculator cannot choose a safe window from sex alone. Discuss fasting with a clinician if you have symptoms or a condition that could be affected.

After an extended fast, restart eating according to medical guidance and your health context. Refeeding syndrome risk is not determined by a simple 48-hour cutoff and can be serious in people who are malnourished or medically vulnerable. Do not use the calculator to plan prolonged fasting or electrolyte intake.

Fasting is contraindicated during pregnancy and breastfeeding, for anyone with a current or past eating disorder, for people with type 1 diabetes (severe hypoglycemia risk), for children and teenagers still growing, and for adults who are underweight or recovering from illness or surgery. People with type 2 diabetes, gout, kidney disease, or who take blood-sugar or blood-pressure medication should only fast under medical supervision because dosing usually needs to be adjusted. This calculator is an educational tool, not medical advice; check with a clinician before starting any extended fast.