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.

Autophagy (the cellular cleanup process that recycles damaged proteins and organelles) begins to ramp up around 16 to 18 hours of fasting in humans, after liver glycogen has been depleted and the body has shifted toward fat oxidation. It picks up meaningfully at 24 hours alongside a growth hormone surge, and reaches its strongest measurable rates between 48 and 72 hours. The exact timing varies with body composition, training status, and last meal size, so the milestones in this calculator are typical ranges rather than precise personal cutoffs.

Many women do better with shorter fasting windows, typically 12 to 14 hours rather than 16 or longer. The female hypothalamic-pituitary-gonadal axis is more sensitive to energy restriction, and aggressive fasting can disrupt menstrual cycles, thyroid function, and sleep in some women. Pre-menopausal women in particular often report better results with gentler windows, eating in the morning, and avoiding extended fasts during the luteal phase. Post-menopausal women generally tolerate longer fasts more easily. If menstrual changes, hair loss, or sleep disruption show up, shorten the window.

Start with a small portion of easy-to-digest protein and a source of healthy fat: a few ounces of cooked eggs, fish, or chicken with avocado or olive oil is a classic choice. Avoid breaking a long fast with a large carbohydrate-heavy meal, which can trigger a sharp insulin spike, GI distress, and in rare cases (after a 48 hour plus fast) refeeding syndrome. Wait 30 to 60 minutes, then have a normal-sized meal. Hydrate well throughout and add a pinch of salt if you have been fasting more than 24 hours.

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.