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Oct 18, 2026

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Intermittent Fasting Calculator

Plan fasting and eating-window times for common daily time-restricted eating schedules.

This tool only performs schedule arithmetic. It does not recommend fasting for you or assess whether fasting is appropriate with your medications, medical conditions, pregnancy, breastfeeding or eating-disorder history.

How Intermittent Fasting Schedules Work

Intermittent fasting (IF) is not a diet that tells you what to eat — it is a schedule that tells you when. You alternate between a fasting window (water, black coffee, and unsweetened tea only) and an eating window (all your day's food). This calculator turns any protocol and start time into exact clock times for your fast and your eating window.

ProtocolFastingEating windowBest for
12:1212 hours12 hoursBeginners — essentially an overnight fast with no late snacking
14:1014 hours10 hoursA gentle step up from 12:12
16:816 hours8 hoursThe most popular schedule; a good long-term default
18:618 hours6 hoursExperienced fasters who want a shorter eating window
20:420 hours4 hoursAdvanced only — hard to fit enough nutrition into 4 hours

Worked Example: 16:8 Starting at 8:00 PM

If you finish dinner at 8:00 PM and follow 16:8, your fast runs 16 hours to 12:00 PM the next day, and your eating window runs from noon to 8:00 PM. In practice that means skipping breakfast, eating lunch and dinner, and stopping after dinner. Enter any start time in the calculator to get your own exact windows for every protocol.

Choosing a Protocol

Start with 12:12 for a week, then move to 14:10, then 16:8 if it feels sustainable. There is no prize for jumping to 20:4 — the schedule you can keep for months beats the aggressive one you quit after five days. Most people find 16:8 the sweet spot: long enough to simplify eating (two meals instead of three plus snacks) without making social meals or training difficult.

Intermittent fasting is not suitable for everyone. Skip it, or talk to your doctor first, if you are pregnant or breastfeeding, take medication that must be taken with food (including many diabetes medications), have a history of disordered eating, or are under 18. If you feel dizzy, faint, or unwell while fasting, eat — no schedule is worth pushing through warning signs.

What to Eat in the Window

Fasting does not erase calories. Weight loss still requires a calorie deficit; the schedule just makes the deficit easier for some people by removing an entire meal or snacking period. Within your window, build meals around protein (aim for a palm-sized portion or more per meal), fiber-rich foods, and vegetables, and drink plenty of water. A 16:8 schedule filled with oversized, high-calorie meals will not produce weight loss. Use our calorie deficit calculator to find the daily target your eating window should hit, and our macro calculator to split it into protein, carbs, and fat.

Common Mistakes

  • Calorie-loaded drinks in the fast: juice, milk in coffee, and sugary sodas break the fast. Black coffee, plain tea, and water do not.
  • Overeating the window: treating the eating window as an all-you-can-eat period cancels the deficit.
  • Quitting after three days: hunger is strongest in the first week as your body adjusts. Most people report it easing significantly by days 7–10.
  • Ignoring sleep: poor sleep increases hunger hormones and makes fasting far harder. Fix sleep first.
  • Skipping protein: with fewer meals, each one must carry more protein to protect muscle while losing fat.

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Frequently Asked Questions

Water, black coffee, plain tea, and sparkling water are all fine. Anything with calories — juice, milk, soda, bone broth, or sweetened drinks — breaks the fast. Zero-calorie sweeteners are a gray area; plain water is the safest choice.

Short fasts do not meaningfully slow metabolism. Metabolic adaptation happens with prolonged calorie restriction of any kind, fasting or not, and it is driven mostly by weight loss itself. Eating enough protein and keeping some resistance training in your routine protects against muscle loss during a cut.

Yes, most people train fine in a fasted state, especially light-to-moderate cardio. Hard sessions near the end of a long fast can feel worse; scheduling tough workouts near the start of your eating window, or shortly before it opens, usually works best.

If the schedule produces a real calorie deficit, expect the scale to move within 2–4 weeks. The first few days mostly reflect water shifts. Judge a protocol after at least a month of consistent eating-window targets, not after a few days.

Whichever one you can sustain. A shorter eating window only helps if it actually reduces your total calories. If 18:6 leaves you ravenous and overeating at dinner, 16:8 with a controlled target will beat it.

Disclaimer: This content is for general information only and is not medical advice. Intermittent fasting is not suitable for everyone, including people who are pregnant or breastfeeding, people taking medication that must be taken with food, and people with a history of disordered eating. Talk to your doctor or a registered dietitian before starting a fasting schedule.

Sources: NIDDK, Intermittent Fasting (niddk.nih.gov).

Maintained by Live Fitness Check. Last updated: August 13, 2026. How we test our calculators .

About This Calculator

The Intermittent Fasting Calculator is a schedule planner for daily time-restricted eating patterns. Choose when your fast starts and a fasting-to-eating ratio, and the calculator performs the clock arithmetic for the end of the fast and the following eating window.

Methodology

Each supported schedule divides a 24-hour day into fasting and eating hours. For example, a 16:8 schedule represents 16 fasting hours followed by an 8-hour eating window. The calculator adds the selected fasting duration to the entered start time and then adds the eating-window duration.

Sources

NIDDK — Intermittent fasting and time-restricted eating overview: https://www.niddk.nih.gov/health-information/professionals/diabetes-discoveries-practice/patients-intermittent-fasting
National Institute on Aging — Calorie restriction and fasting diets: https://www.nia.nih.gov/news/calorie-restriction-and-fasting-diets-what-do-we-know
Disclaimer
This is a scheduling tool, not a recommendation to fast. Fasting may be inappropriate for some people, including during pregnancy or breastfeeding and for people with certain medical conditions, medications or eating-disorder histories. Seek individualized professional advice when relevant.

Frequently Asked Questions

What does 16:8 mean?

It describes 16 fasting hours followed by an 8-hour eating window within a 24-hour schedule.

Does this calculator tell me which fasting plan is best?

No. It only calculates times for the schedule you select and does not determine whether fasting is appropriate for you.

Does the calculator track calories?

No. It performs time-window arithmetic only and does not prescribe calories or foods.

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