Intermittent Fasting Calculator — Fasting Schedule Planner
Plan your intermittent fasting schedule. Choose from 16:8, 18:6, 20:4, or OMAD and get your exact eating and fasting window times.
Quick Answer
Intermittent fasting is an eating pattern that cycles between fasting and eating periods, not a diet specifying what to eat. The most popular protocols are 16:8 (16 hours fasting, 8 hours eating), 18:6, 20:4, and OMAD. Weight loss occurs primarily because a shorter eating window leads to naturally lower calorie intake. Start with 16:8 — it is the most researched and sustainable protocol.
What Intermittent Fasting Actually Does
Intermittent fasting (IF) is not a diet in the traditional sense — it does not specify what to eat, only when. The primary mechanism behind its weight loss effects is straightforward: a shorter eating window leads most people to eat fewer total calories without consciously tracking them. A 2020 meta-analysis in the New England Journal of Medicine confirmed that when total calories are matched, IF produces comparable results to continuous caloric restriction.
Beyond calorie reduction, fasting periods produce several physiological changes. As fasting extends beyond 12–16 hours, glycogen stores in the liver deplete and the body shifts to fat oxidation as its primary fuel source. Insulin levels fall, glucagon rises, and fatty acid mobilisation increases. Beyond 16–18 hours, cellular autophagy (the recycling of damaged cellular components) increases — a process associated with anti-ageing and disease prevention benefits in animal models, though human evidence remains early-stage.
Choosing Your Protocol
The 16:8 protocol is the correct starting point for virtually everyone. It is the most researched, most sustainable, and least disruptive to daily life and social eating. Most people already fast 10–12 hours overnight — 16:8 simply extends this by 4–6 hours by delaying breakfast or advancing dinner. Start by delaying breakfast by 1 hour per week until you reach your target window. Adaptation typically takes 2–4 weeks before hunger normalises.
18:6 and 20:4 are appropriate only after full adaptation to 16:8. OMAD (one meal a day) carries a meaningful risk of nutritional inadequacy — meeting adequate protein, fibre, and micronutrient needs in a single meal is genuinely difficult and warrants guidance from a registered dietitian.
What to Eat During Your Eating Window
The eating window is not a licence to consume anything in unlimited quantities. The evidence base for IF's health benefits is strongest when the eating window contains nutritious, whole-food meals adequate in protein (1.6–2.2g/kg bodyweight), fibre (25–30g/day), and micronutrients. Use our Macro Calculator to set your targets and our Shifa Score calculator to evaluate the quality of the foods you are choosing.
Practical tip: within your eating window, front-load protein at your first meal. This maximises muscle protein synthesis, reduces hunger through the remainder of the window, and makes hitting your daily protein target easier when fewer meals are available.
Who should not fast
IF is not appropriate for people with eating disorder history, pregnant or breastfeeding women, children and adolescents, type 1 diabetics, or anyone on medications that require food. Always consult a healthcare professional if you have an underlying health condition.