Enter your wake time, first meal, or last meal to get your exact eating and fasting windows for any IF protocol.
Choose Your Protocol
16:8 — Fast for 16 hours, eat within an 8-hour window. The most researched and beginner-friendly protocol. Simply skip breakfast or stop eating after dinner.
Your Schedule
Your Fasting Schedule
For 16:8, 18:6, 20:4, and OMAD protocols: The calculator derives your eating window from the anchor time you provide, then places the fasting window in the remaining hours of the 24-hour day.
Wake-up anchor: Eating start = wake time + your chosen delay (0, 1, or 2 hours). Eating end = eating start + eating window hours.
First meal anchor: Eating start = first meal time. Eating end = first meal time + eating window hours.
Last meal anchor: Eating end = last meal time. Eating start = last meal time − eating window hours. Fasting starts immediately after eating ends.
For 5:2: The recommended fast-day calorie target is 25% of estimated TDEE. A simplified formula uses body weight: approximately 500–600 kcal for most adults, derived from a generic TDEE estimate of ~2,000–2,400 kcal/day.
Current window status is determined by comparing the current local time against the calculated eating window start and end times.
No. Plain black coffee contains essentially zero calories and does not trigger an insulin response, so it does not break a fast. The same applies to plain water, sparkling water, and unsweetened black or green tea. What does break a fast is any calorie intake — including cream, milk, sugar, or flavored syrups in coffee. Even 50 calories is enough to end the metabolic fasted state for most purposes.
Yes. Fasted training — especially cardio — is common among intermittent fasting practitioners and is generally safe for healthy adults. Some research suggests it can increase fat oxidation during the session. That said, high-intensity or long-duration training on an empty stomach may feel harder, and muscle protein synthesis is best supported by eating protein near your workout. If performance matters, train during or just before your eating window. If fat loss is the priority, fasted morning cardio is a reasonable approach.
Most people notice reduced hunger and improved energy within 1–2 weeks as the body adapts to the new eating schedule. Measurable weight loss results typically appear within 2–4 weeks when a calorie deficit is maintained. Metabolic markers like fasting insulin and blood glucose may improve within 4–8 weeks. Results depend heavily on what you eat during your eating window — intermittent fasting is an eating schedule, not a free pass to eat anything.
Intermittent fasting does not prescribe specific foods, but the quality of what you eat still matters for body composition and health. Whole foods, adequate protein (0.7–1g per pound of bodyweight), vegetables, healthy fats, and complex carbohydrates support better results than the same calorie total from ultra-processed foods. Many people combine 16:8 with a lower-carbohydrate or Mediterranean-style diet. The fasting window handles when you eat; your food choices handle what you eat.
16:8 is considered safe for most healthy adults. It is simply a structured eating schedule that eliminates late-night eating and skips or delays breakfast. However, intermittent fasting is not appropriate for everyone: pregnant or breastfeeding women, people with a history of eating disorders, those with type 1 diabetes, children and teenagers, and anyone on medication that requires food should consult a doctor before starting. If you feel dizzy, extremely fatigued, or unwell during fasting, break the fast and speak with a healthcare provider.
Intermittent fasting (IF) is not a diet — it is an eating schedule. Rather than telling you what to eat, it tells you when to eat. You cycle between defined periods of eating and voluntary fasting, which shifts your body's metabolic state in ways that can support fat loss, improved insulin sensitivity, and reduced inflammation.
The core mechanism is straightforward: when you eat, blood glucose and insulin rise. During the fasting window, both levels drop, and your body gradually shifts from burning glucose to burning stored fat for fuel. This metabolic switch typically occurs 12–16 hours after your last meal, which is why longer fasting windows tend to produce stronger effects. Unlike calorie-restriction diets that require constant tracking, intermittent fasting just requires a clock.
16:8 is the most researched and most popular entry point. You fast for 16 hours — which includes sleep — and eat within an 8-hour window. For most people this means skipping breakfast and eating between noon and 8pm, or eating between 9am and 5pm after a short delay after waking. It is easy to maintain socially and produces reliable results for most people over 4–8 weeks.
18:6 compresses the eating window to 6 hours, extending the fasting period to 18 hours. This provides a longer metabolic fasting period each day, which some people find accelerates fat loss and appetite reduction once they have adapted to 16:8. A typical window might be 1pm–7pm or 12pm–6pm.
20:4 (Warrior Diet) is an advanced protocol with a 4-hour eating window. It was popularized by Ori Hofmekler and mimics the feeding patterns of ancient warriors — undereating during the day and consuming one large meal at night. It suits experienced fasters but is unnecessarily aggressive for beginners.
OMAD (One Meal a Day) takes the concept to its limit: 23 hours of fasting, 1 hour of eating. This is a high-commitment approach favored by some high-performers and serious dieters. Nutrient density within that single meal becomes critically important — getting adequate protein, micronutrients, and fiber in one sitting requires careful planning.
5:2 operates on a weekly cycle rather than a daily one. You eat normally five days per week and restrict calories to approximately 500–600 on two non-consecutive days (e.g., Monday and Thursday). It is less of a time-restricted approach and more of a periodic calorie-restriction protocol, but it achieves similar metabolic benefits for many people and is easier to fit into a social schedule.
| Protocol | Fasting Hours | Eating Hours | Difficulty | Best For |
|---|---|---|---|---|
| 16:8 | 16 h | 8 h | Beginner | Starting out, daily adherence |
| 18:6 | 18 h | 6 h | Intermediate | More aggressive fat loss |
| 20:4 | 20 h | 4 h | Advanced | Experienced fasters, Warrior Diet |
| OMAD | 23 h | 1 h | Advanced | Maximum simplicity, one meal |
| 5:2 | 2 low-cal days/wk | 5 normal days | Intermediate | Flexible schedule, social eaters |
The most common 16:8 setup uses wake-up time as the anchor. Here is how the math works for someone who wakes at 7am and chooses to delay eating by 2 hours:
When you know what time you want to stop eating — say, because you have a regular dinner time — the "last meal" anchor is the most intuitive input:
Most people fail at IF not because it is metabolically hard, but because they try to jump straight to an aggressive protocol. Start with 16:8 and give your body at least two weeks to adapt before evaluating results. Here is the practical playbook:
1. Delay breakfast, do not skip dinner. It is psychologically easier to push your first meal later than to cut off eating earlier in the evening. Start with a 10am first meal, then gradually push to noon over two weeks.
2. Black coffee and tea are your best friends. Plain black coffee in the morning blunts hunger, provides caffeine, and does not break your fast. Unsweetened herbal teas and still or sparkling water are also completely fine during the fasting window.
3. Stay busy in the morning. Hunger is largely habitual. If you keep your morning occupied with work or exercise, you will often forget you have not eaten. The hunger signal usually peaks around your normal meal time and then passes within 20–30 minutes.
4. Eat enough during your window. IF is not about restricting calories to dangerous levels — it is about compressing your eating into a shorter window. Eat real meals with adequate protein and calories. Undereating will cause muscle loss and make the fast feel unsustainable.
5. Keep consistent days. Fasting on Monday and eating with no window on weekends will not produce the same results as consistent daily adherence. Pick an eating window and stick to it seven days a week for at least four weeks before evaluating whether IF is working for you.
The practical rule is simple: calories break a fast. Zero-calorie inputs do not. Here is a quick breakdown:
| Item | Breaks Fast? | Notes |
|---|---|---|
| Black coffee | No | Zero calories, may suppress hunger |
| Water (still or sparkling) | No | Essential — stay hydrated |
| Unsweetened herbal tea | No | Zero calories |
| Coffee with cream/milk | Yes | Dairy contains fat and calories |
| Diet soda / artificial sweeteners | Technically no / debated | No calories, but may increase cravings |
| Bulletproof coffee (butter/MCT oil) | Yes | High in fat calories |
| Gum / mints | Generally no | Trace calories usually fine |
| Bone broth | Yes | Contains protein and calories |
For most people using IF for weight loss or metabolic health, the strict rule is: if it has calories, it breaks the fast. Some practitioners allow up to 50 calories (e.g., a small splash of cream in coffee) without meaningful metabolic disruption, but this is a grey area and varies by individual.
Does black coffee break a fast?
No. Plain black coffee contains essentially zero calories and does not trigger an insulin response, so it does not break a fast. The same applies to plain water, sparkling water, and unsweetened black or green tea. What does break a fast is any calorie intake — including cream, milk, sugar, or flavored syrups. Even 50 calories is enough to end the fasted metabolic state for most purposes.
Can you work out while fasted?
Yes, and many people do deliberately. Fasted cardio (walking, cycling, jogging) at moderate intensity is well tolerated and may increase fat oxidation during the session. Weight training fasted is also possible but may feel harder, particularly for heavy compound movements. If muscle building is your goal, eating protein near your training window — before or after — supports better muscle protein synthesis. Start with lower-intensity fasted workouts and assess how you feel before attempting heavy lifting on an empty stomach.
How long until I see results from intermittent fasting?
Hunger adaptation typically happens within 1–2 weeks. Most people feel noticeably less hungry in the mornings after 7–10 days as ghrelin (the hunger hormone) adjusts to the new schedule. Weight loss results, assuming a calorie deficit, usually appear within 2–4 weeks. Improvements in fasting blood glucose and insulin sensitivity typically show up in bloodwork at 4–8 weeks. The critical variable is what you eat during your eating window — overeating will cancel out the benefits of the fasting period.
What should I eat during my eating window?
Intermittent fasting does not dictate food choices, but quality still matters. Prioritize adequate protein (0.7–1.0g per pound of bodyweight) to preserve muscle mass. Fill the rest of your calories with vegetables, whole grains, healthy fats (olive oil, avocados, nuts), and fruit. Limit ultra-processed foods, added sugars, and refined carbohydrates — not because they are "forbidden," but because they make it harder to stay full within a compressed eating window and tend to drive overconsumption.
Is 16:8 intermittent fasting safe for everyone?
16:8 is safe for most healthy adults. However, it is not appropriate for pregnant or breastfeeding women, people with a history of eating disorders, those with type 1 diabetes or hypoglycemia, children and teenagers, or anyone on medication that requires food. People with type 2 diabetes who are on insulin or sulfonylureas need medical supervision because fasting affects blood glucose management. If you feel consistently dizzy, lightheaded, or unwell during your fasting window, break the fast and consult a healthcare provider before continuing.