This intermittent fasting calculator turns a protocol (16:8, 18:6, 20:4, OMAD, 5:2) and your first-meal time into a concrete daily schedule — eating window, fasting close, and weekly fasting hours.

Eating Window–
Fast Resumes–
Fasting Hours / Week–

Fasting works for weight mainly by making a calorie deficit easier to sustain — the window is a fence, not a metabolic loophole; pair it with the Calorie Deficit Calculator for targets. Not for pregnancy, breastfeeding, under-18s, anyone with a history of eating disorders, or people on glucose-lowering medication without clinician supervision. Ease in (12:12 → 14:10 → 16:8 across two weeks) beats white-knuckling day one.

The Window Is Arithmetic: Start + Eating Hours = Fast Resumes

Every daily protocol is two numbers summing to 24. The schedule's job is adherence design: anchor the window to your social and work life (most people find noon–8 pm easiest — breakfast is the most skippable meal socially and metabolically), keep wake/sleep consistent, and count weekly fasting exposure for the dose picture. Targets to eat within the window: our Calorie Deficit Calculator. Part of our Health Calculators collection.

Worked Examples (Verified)

  • 16:8, first meal noon (defaults): eat 12 PM – 8 PM, fast 8 PM → noon; weekly dose 112 fasting hours at 7 days
  • 18:6 anchored late: first meal 2 PM → window 2–8 PM — the dinner-with-family configuration
  • OMAD reality check: 23:1 reads as "23 fasting hours" but the eating hour must carry a full day's nutrition — the protocol where protein planning matters most
  • 5:2 is the alternate-day family member (2 very-low-calorie days weekly) — different arithmetic from daily windows; this page schedules daily protocols, and 5:2 practitioners use the "fasting days per week" field loosely at best — stated honestly

Choosing a Protocol Like an Adult

  • Start at 12:12 or 14:10 for a week — adherence data favors graduated compression over heroic starts.
  • Black coffee, tea and water during the fast are the standard allowances; anything with meaningful calories opens the window.
  • Training: schedule hard sessions inside or just before the window until adapted (2–3 weeks for most).
  • Hunger waves peak and pass on ghrelin's schedule (~20–30 min) — the second week is markedly easier than the first.

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Frequently Asked Questions

What is the best intermittent fasting schedule for beginners?

12:12 for a few days, then 14:10, then 16:8 if it suits your life — with the eating window placed over your most social meals (commonly noon–8 pm for 16:8). The best protocol is the most aggressive one you can sustain for months, not days; most long-term adherents settle at 16:8.

How many hours a week is 16:8?

112 fasting hours out of 168 when done daily (16 × 7). At 5 days a week it's 80. This calculator shows the weekly dose for any protocol and schedule, which is the figure that best matches how people actually practice fasting flexibly.

Does intermittent fasting burn more fat than regular dieting?

Controlled trials generally find equivalent fat loss at equal calories — IF's advantage is adherence for people who prefer time rules to calorie counting, plus possibly improved insulin sensitivity scheduling. The window doesn't repeal energy balance; it organizes it.

Can I drink coffee while fasting?

Black coffee (and unsweetened tea, water) is the standard fast-compatible drink — negligible calories, appetite-blunting for many. Cream, sugar or "fasting-approved" sweetened products reopen the metabolic question; purists and pragmatists argue, the arithmetic above doesn't.

Who should not fast?

Pregnant or breastfeeding women, minors, people with eating-disorder history, underweight individuals, and anyone on insulin or sulfonylureas (hypoglycemia risk) without medical supervision. If fasting makes you feel unwell beyond mild hunger, stop and reassess — the schedule serves you, not vice versa.

Is this intermittent fasting calculator free?

Yes — free, instant, no sign-up. Scheduling information, not medical advice.