⏰ Intermittent Fasting Calculator — Your Eating & Fasting Schedule
Choose your IF protocol and get your exact eating window, fasting start time, and a personalised daily schedule.
5:2 Protocol Information
On your 2 fasting days, limit intake to 500–600 calories. Choose non-consecutive days (e.g. Monday and Thursday). Eat normally on the other 5 days.
5:2 Weekly Fasting Plan
Fasting days: Limit to 500 kcal (women) or 600 kcal (men) on 2 non-consecutive days.
Suggested fasting days: Monday & Thursday
Eating days (5): Eat normally at maintenance calories — no restriction needed.
Quick IF Facts
Sources: Obesity Reviews, Cell Metabolism, NEJM
How Intermittent Fasting Works: Metabolic Switching Explained
When you eat, your body digests carbohydrates into glucose and stores excess as glycogen in the liver and muscles. In a fed state, insulin is elevated and fat burning is suppressed — your body is in storage mode.
After roughly 12 hours of fasting, liver glycogen depletes and insulin drops. Your body switches its primary fuel source from glucose to fatty acids and ketone bodies — a process called metabolic switching. This is when IF's metabolic benefits begin.
At around 16–18 hours, autophagy (cellular "self-cleaning") ramps up significantly. At 24 hours, growth hormone spikes to preserve muscle mass. These cascading hormonal changes — not calorie restriction alone — are why IF produces different outcomes to simply eating less throughout the day.
Fasting timeline milestones
Frequently Asked Questions
16:8 Intermittent Fasting Schedule — Best Eating Window Times
The 16:8 protocol is the most researched and widely adopted form of intermittent fasting, and for good reason: a 16-hour fast is long enough to trigger meaningful metabolic benefits (glycogen depletion, fat oxidation, early autophagy) while still allowing an 8-hour eating window that fits most social and work schedules. The most popular window is 12:00 PM to 8:00 PM — this lets you skip breakfast and eat a late lunch, afternoon snack, and dinner without conflicting with evening social events. For morning exercisers, a 10:00 AM to 6:00 PM window allows a post-workout meal. For night owls, 2:00 PM to 10:00 PM works well. The specific hours matter less than consistency — your circadian rhythm adapts to the pattern within 1–2 weeks, reducing hunger outside the window substantially.
5:2 Fasting Calculator — How Many Calories on Fast Days?
The 5:2 protocol, popularised by Dr Michael Mosley's research, involves eating normally 5 days per week and restricting calories to 500 (women) or 600 (men) on 2 non-consecutive fasting days. The original research, published in the International Journal of Obesity (2011), compared 5:2 to continuous calorie restriction and found equivalent weight loss with better improvements in insulin sensitivity. On fasting days, spreading calories across 2 small meals (e.g., 250 kcal breakfast and 250 kcal dinner) is easier than OMAD for most people. Choosing non-consecutive days — Monday and Thursday or Tuesday and Friday — prevents two consecutive difficult days and allows recovery. Unlike daily IF protocols, 5:2 does not require watching eating window times on non-fasting days.
Intermittent Fasting for Women — Is It Different?
Women can absolutely benefit from intermittent fasting, but the approach may need to be more conservative, particularly at first. Female hormones — including oestrogen, progesterone, and luteinising hormone — are sensitive to caloric restriction and fasting stress. Extended fasts (18+ hours) can temporarily disrupt the hypothalamic-pituitary-ovarian axis, affecting menstrual cycles and fertility in some women. This is more likely with very long fasts, very low calorie intake within the eating window, and high exercise volume simultaneously. The evidence-based approach for women new to IF: start with 12:12, establish that for 2–4 weeks, then optionally progress to 14:10 or 16:8. Listen to cycle changes — if periods become irregular, shorten the fasting window. Women with PCOS may find IF particularly beneficial for insulin regulation, as PCOS often involves insulin resistance.
Intermittent Fasting: What the Research Actually Shows
Intermittent fasting has shifted from fringe practice to mainstream nutrition strategy, backed by a growing body of peer-reviewed evidence. A landmark 2019 review in the New England Journal of Medicine by Dr Mark Mattson concluded that IF produces benefits beyond simple caloric restriction: improvements in blood pressure, resting heart rate, triglycerides, and LDL cholesterol, as well as reductions in inflammatory markers.
IF vs. Continuous Calorie Restriction
Multiple randomised controlled trials have compared IF to traditional continuous calorie restriction (CCR) for weight loss. The overall finding: both produce similar weight loss when calories are matched. However, IF shows advantages in adherence (people find it easier to maintain), improvements in insulin sensitivity even without weight loss, and in some studies, greater preservation of lean muscle mass — particularly relevant for older adults.
Who Should Avoid IF
Intermittent fasting is not appropriate for everyone. People with type 1 diabetes (risk of hypoglycaemia), those with a history of eating disorders, children and adolescents, pregnant or breastfeeding women, and people who are underweight should not follow fasting protocols without medical supervision. If you take medications that require food, consult your doctor before starting IF.
Note: This calculator is for general wellness guidance. Intermittent fasting is not suitable for everyone. Consult a healthcare professional before starting any fasting protocol, especially if you have diabetes, a history of eating disorders, or take prescription medications.