Intermittent Fasting Calculator (Phases)
See which physiological phase your body is in as you fast.
As you fast, your body moves through distinct hormonal and metabolic phases. Enter how long you have fasted and your target protocol to see where you are.
Enter your values above to see your result.
Formula
Progress = (hours fasted / target) × 100. Phases: 0–4h digestion · 4–12h glycogen · 12–18h lipolysis · 18–24h ketosis+autophagy · 24h+ robust autophagy.
How it is calculated
In the first hours after eating you digest and store. Around 4–12 hours you burn stored glycogen; after ~12h, lipolysis ramps up and you burn fat.
By 18–24 hours, ketosis and autophagy (cellular cleanup, Nobel 2016) begin. Fasting works by lowering insulin — a hormonal reset, not a calorie trick.
Note: fasting is not for everyone — pregnancy, breastfeeding, eating disorders or type 1 diabetes.
Frequently asked questions
When does autophagy start?
Meaningful autophagy generally begins around 18–24 hours of fasting and increases beyond that.
Scientific references
- de Cabo R, Mattson MP. NEJM 2019;381:2541-51
- Anton SD et al. Obesity 2018;26:254-68
Educational tool — does not replace medical advice. Discuss your results with your doctor.