Intermittent Fasting Calculator
Intermittent fasting is not about what you eat but when you eat. You split the day into a fasting period, when you only drink water, coffee, or tea, and an eating window, when you have all your meals. This calculator takes your chosen method — 16:8, 18:6, 20:4, or OMAD — and the time of your last meal, and works out the exact hour when your fast ends and you can eat again, plus when your eating window closes and the next fast begins.
Consult a healthcare professional before starting a fast, especially if you have a medical condition, are pregnant, or take medication.
You can eat again at
Eating window
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How it works
The two numbers in a method are the fasting hours and the eating hours: 16:8 means 16 hours fasting and 8 hours eating, 18:6 means 18 and 6, 20:4 means 20 and 4, and OMAD (one meal a day) means about 23 hours fasting and a single 1-hour eating window. The calculator adds the fasting hours to the time of your last meal to find when you can eat again, then adds the eating hours to find when the window closes. Everything is reactive: change the method or the time and the results update instantly.
The most common intermittent fasting protocols
The two numbers name the fasting hours and the eating hours. Beginners usually start with the gentler ratios and work up as they adapt. This table summarizes the most popular protocols and who each one tends to suit.
| Protocol | Fasting / eating | Best for |
|---|---|---|
| 14:10 | 14 h fast / 10 h eating | Beginners; the gentlest way to start. |
| 16:8 | 16 h fast / 8 h eating | The most popular and sustainable day to day. |
| 18:6 | 18 h fast / 6 h eating | Intermediate; for those already used to 16:8. |
| 20:4 (Warrior) | 20 h fast / 4 h eating | Advanced; a short window with one or two meals. |
| 5:2 | 2 days of ~500-600 kcal per week | Those who prefer restricting on set days, not daily. |
| OMAD | 23 h fast / 1 meal | Experienced fasters; a single meal a day. |
What breaks the fast and when NOT to do it
During the fasting window you can drink water, black coffee and unsweetened tea or infusions: they have no calories and do not break the fast. Anything with calories breaks it — milk, sugar, juice, soda, broth or a snack. Staying well hydrated helps control hunger and the headaches common in the first few days. Intermittent fasting is not for everyone: it is not advised during pregnancy and breastfeeding, in people with diabetes who use insulin or glucose-lowering medication, in those with a history of eating disorders, or in minors. In any of these cases, talk to a doctor before starting.
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Preguntas frecuentes
- What does 16:8 mean?
- It is the most popular intermittent fasting schedule: you fast for 16 hours and eat all your meals within an 8-hour window. For example, if your last meal is at 8 pm, you fast overnight and until noon the next day, then eat between noon and 8 pm.
- What is OMAD?
- OMAD stands for One Meal A Day. You fast for roughly 23 hours and eat all your calories in a single meal within about a 1-hour window. It is one of the most restrictive forms of intermittent fasting and is not recommended for beginners.
- Can I drink anything during the fasting window?
- Yes. Water, black coffee, and unsweetened tea are fine and do not break the fast. Anything with calories — milk, sugar, juice, soda, or a snack — starts your eating window and effectively ends the fast.
- Which method should a beginner start with?
- Most people start with 16:8 because it is the easiest to sustain: much of the fasting happens while you sleep. Once that feels comfortable you can try 18:6 or 20:4. OMAD is best left for experienced fasters.
- Does the time of my last meal really matter?
- Yes, because your eating window is anchored to it. The calculator adds the fasting hours to your last meal to tell you when you can eat again, so an earlier last meal means an earlier end to your fast the next day.