Intermittent Fasting Calculator

16:8 is the gold standard for beginners. It's the easiest to maintain — most people are already fasting 10-12 hours overnight, so 16:8 just means pushing breakfast back to noon. Start there, give it 2-3 weeks for your body to adapt, then experiment with tighter windows like 18:6 if you want. Jumping straight to OMAD is a recipe for binging and failure.

Yes — black coffee, plain tea (green, black, herbal), and water are all fine and won't break a fast. Avoid anything with calories: no cream, no milk, no sugar, no honey, no juice. Even diet sodas are debated; if your goal is autophagy or insulin response, stick to water and unsweetened drinks. A splash of milk is technically calories but probably won't derail weight loss — purists disagree.

Short fasts (under 24 hours) do not slow metabolism — research actually shows a small increase in metabolic rate during 16-48 hour fasts due to elevated norepinephrine. Multi-day fasts (3+ days) can suppress metabolism, but that's not relevant to standard IF. The metabolic-slowdown myth comes from chronic severe calorie restriction, not from IF itself.

Either works — it's personal preference. Fasted training may improve fat oxidation and is fine for low-to-moderate cardio. Fed training (within your eating window) typically allows higher intensity and better strength performance. Many people prefer training near the end of their fast so they can refeed with a big post-workout meal. If you're lifting heavy or doing intervals, fed training generally produces better results.

The first 1-2 weeks are the hardest. Common side effects: headaches (usually dehydration or caffeine timing — drink more water and salt), hunger pangs (peak around your usual meal times, fade after 30 min), low energy or irritability (your body is adapting to using stored fat for fuel), trouble sleeping (eating too late or too close to bed). Most people fully adapt within 14 days. If symptoms are severe or persist past 3 weeks, consult a doctor — IF isn't for everyone.