Build live 2026 estimates with USA, UK, metric, and imperial options for intermittent fasting timer.
An intermittent fasting (IF) timer tracks your fasting window, your eating window, and the biological milestones your body passes through as the fast extends. Whether you follow 16:8, 18:6, 20:4, or OMAD (one meal a day), this tool shows where you are in the cycle and what's happening metabolically.
IF has exploded in popularity in the US since 2019. Research from Harvard, Johns Hopkins, and NIH shows improvements in insulin sensitivity, triglycerides, blood pressure, and visceral fat loss. It is not superior to continuous calorie restriction for weight loss when calories are matched — but many people find time-restricted eating easier to stick to.
The 5:2 diet was popularised in the UK by The Fast Diet (Mosley & Spencer). NHS resources note that IF can be effective for weight loss but that the NHS doesn't endorse one specific pattern over another — any sustainable calorie deficit works. IF is not recommended for people with eating disorder history, pregnancy, breastfeeding, type 1 diabetes, or for most under-18s.
Fat oxidation begins ramping up around 12 hours into a fast as glycogen depletes. Meaningful fat loss from IF comes from the total weekly calorie deficit, not from any single fasted hour.
Mouse studies suggest autophagy accelerates after ~24 hours of fasting. Human data is thinner but broadly points to a similar window. The 16:8 pattern probably gives modest autophagy daily; longer occasional fasts give more pronounced effects.
Black coffee, plain tea, and water do not meaningfully break a fast. Adding milk, sugar, or sweeteners with insulin response does. For strict autophagy purposes, stick to water and electrolytes.
Prioritise protein (0.8–1.2 g per lb of lean body mass for active people), vegetables, fruit, whole grains, and healthy fats. IF doesn't override nutrition basics — eating junk in a short window still means eating junk.
Most people feel hungry during the first 7–10 days, then hunger smooths out as ghrelin adjusts to the new schedule. Headaches in week one are usually from dehydration or caffeine shifts — drink water and add electrolytes.
This tool provides planning estimates for informational purposes only and is not medical advice. Fasting tolerance varies by age, health status, medication use, activity level, and pregnancy status. Always consult a qualified healthcare professional before making major changes to eating patterns.
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