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Intermittent fasting calculator

Tell us when you last ate and which protocol you follow. We'll tell you when to break your fast, your eating window, and the phase your body is in right now.

Your fast

Fasted00:00Digestion
Break your fast at13:00mañana
Your eating window13:00 – 21:00
Time left16 h 00 min

Want an alert at every phase?

How the fasting calculator works

Intermittent fasting is counted from the moment you finish your last meal. The calculator adds your protocol's hours to that time and gives you when you can eat again, your eating window for the day, and the approximate metabolic phase you're in right now.

The phases are a guide: when glycogen runs out or ketones show up depends on what you ate, how active you are and your metabolism. Treat them as a map, not a precise measurement.

Intermittent fasting schedules based on your last meal

If you want a quick reference, these tables show when you break your fast on the most common daily protocols.

Fasting schedule 12:12
Last mealYou break your fastWindow
19:0007:0012 h
20:0008:0012 h
21:0009:0012 h
22:0010:0012 h
Fasting schedule 14:10
Last mealYou break your fastWindow
19:0009:0010 h
20:0010:0010 h
21:0011:0010 h
22:0012:0010 h
Fasting schedule 16:8
Last mealYou break your fastWindow
19:0011:008 h
20:0012:008 h
21:0013:008 h
22:0014:008 h
Fasting schedule 18:6
Last mealYou break your fastWindow
19:0013:006 h
20:0014:006 h
21:0015:006 h
22:0016:006 h
Fasting schedule 20:4
Last mealYou break your fastWindow
19:0015:004 h
20:0016:004 h
21:0017:004 h
22:0018:004 h

What happens in your body at each hour of fasting

0 h

Digestion: Your body is using your last mealGlucose and insulin are high while you digest. Energy on tap.

4 h

Insulin dropping: Hunger spikes settle downInsulin falls and your liver starts releasing its stored sugar (glycogen).

12 h

Fat burning: Fat becomes your fuelWith glycogen low, your body pulls from stored fat for energy.

16 h

Light ketosis: Steady energy and a clear headYour liver makes ketones, a fuel your brain uses well. Hunger usually drops.

18 h

Ketosis: More ketones, less hungerKetones keep climbing. Good moment for water with a pinch of salt.

24 h

Autophagy: Cellular recycling under wayStudies pick up autophagy markers from 16-24 h. Hydration and electrolytes matter here.

36 h

Extended fast: Glycogen stores at their lowestDeep ketosis. You may feel tired: rest and skip hard workouts.

48 h

Long fast: Only if you already have experienceElectrolytes are essential. Break the fast slowly and with a small amount.

72 h

Sensible limit: Don't go longer without medical supervisionRisks climb from here. Reintroduce food gradually.

More detail in the guide What the science says.

Which protocol to choose

If you've never fasted, start with 12:12 for a week, move to 14:10 and then 16:8: it's the most studied protocol and the one most people keep up. 20:4, OMAD and 24 to 72-hour fasts are occasional tools for people who already have experience. Reading path · The essentials

Frequently asked questions

How do you calculate an intermittent fasting schedule?
You count from the end of your last meal. On 16:8, if you finish dinner at 9 p.m., your fast ends at 1 p.m. the next day and your eating window runs from 1 p.m. to 9 p.m. The calculator does it for you on any protocol.
What time do I break a 16:8 fast if I eat dinner at 10 p.m.?
At 2 p.m. the next day. Your eating window would be 2 p.m. to 10 p.m. If you'd rather eat earlier, move dinner up: finishing at 8 p.m. lets you eat from noon.
Does sleep count as fasting time?
Yes. Sleep is the easy part of the fast: with 8 hours of sleep, a 16:8 only asks for about 8 waking hours without food, split between the evening and the morning.
Which protocol should I pick if I've never fasted?
Start with 12:12 for a week and move to 14:10, then 16:8. Protocols like 20:4, OMAD or 24 to 72-hour fasts don't give proportionally better results for most people and take more experience.
What can I drink during the fasting hours?
Water, sparkling water, black coffee, tea and unsweetened herbal infusions. Milk, juice, sweetened drinks, alcohol and supplements with calories break the fast. Past 18 hours, add a pinch of salt or electrolytes.
Are 48 or 72-hour fasts safe?
Only for healthy people with previous experience, well hydrated and using electrolytes. They are not recommended during pregnancy, breastfeeding, with medicated diabetes, low body weight or a history of eating disorders. Talk to your doctor first.

The calculator gives you the time. The app walks you through it.

Ayúname tells you when each phase starts, tracks your streaks and answers your questions with an AI coach. Free and ad-free.

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This is a guide, not a diagnosis. Phase timing varies from person to person. It is not medical advice: if you have a health condition, take medication, are pregnant or have a history of eating disorders, talk to your doctor before fasting.