Periodic Fasting · The Fast Diet
5:2 Diet Calculator
Eat normally 5 days a week. Restrict to ~500 calories on 2 non-consecutive days. Calculate your targets for both types of days.
Step 1 — Choose Your Protocol
Fast 2 non-consecutive days/week, eat normally the other 5.
Step 2 — Personalise Your Schedule
Your eating window starts from this time
🎯 Your Goal
⏱ Time Checkpoints
Your Fasting Timeline
Elapsed-time checkpoints for a 0-hour schedule. These do not measure metabolic changes.
Fast Started
+0hThe clock measures time since your last calories. It does not measure digestion or metabolic changes.
📄 Mayo ClinicEarly Fasting
+2hDigestion and nutrient absorption vary with the meal and the individual.
📄 NIH — Metabolism OverviewBetween Meals
+4hYour body uses a mixture of fuels. Time alone cannot identify your blood glucose or insulin level.
📄 PubMed — Insulin & Glucose DynamicsOvernight Interval
+8hAn overnight interval may include sleep. Glycogen use varies with diet and activity.
📄 Cahill GF Jr — NEJM 197012 Hours Elapsed
+12hTwelve hours is a time checkpoint, not a diagnosis of ketosis. Ketone production varies between people.
📄 Cahill GF Jr — NEJM 197016 Hours Elapsed
+16hSixteen hours does not establish autophagy activation. There is no validated universal human autophagy timer.
📄 Levine & Kroemer — Cell 2008Time checkpoints are educational, not a recommendation to extend your fast.
How the 5:2 Diet Works
5 Normal Days
Eat your calculated TDEE (Total Daily Energy Expenditure) without restriction. No calorie counting required.
2 Restricted Days
Limit intake to ~500 calories (women) or ~600 calories (men) on 2 non-consecutive days per week (e.g. Monday + Thursday).
The 5:2 diet was popularised by Dr Michael Mosley. A 2021 BMJ systematic review found 5:2 to be comparable in effectiveness to continuous calorie restriction for weight loss.
Medical Disclaimer
This tool provides estimates for informational purposes only. Always consult a licensed healthcare provider before beginning any fasting protocol — especially if you are pregnant, have a history of eating disorders, diabetes, or any other medical condition.
Sources: Mayo Clinic · NIH · PubMed