FastingFibi

Intermittent fasting calculator

Pick your rhythm and your last meal. Fibi works out when your eating window starts and which fasting phases you pass through.

I’m keeping an eye on the clock ⏰

Your rhythm

I know when I …

You fast from 8 PM to 12 PM the next day. Your eating window: 12 PM – 8 PM.

The calculator needs JavaScript. The example shows 16:8 with your last meal at 8 PM.

24-hour clock: the purple band shows your fasting time (8 PM – 12 PM). 0 6 12 18 16:8 The classic 8 PM – 12 PM

What roughly happens in your body

  1. Digestion phase

    Your body is processing your last meal and mostly uses the sugar (glucose) from it.

  2. Glycogen use

    Your body starts to use stored glycogen reserves from your liver and muscles.

  3. Entering ketosis

    For many people, fat burning now gets going: your body starts to make ketones as an energy source. Exactly when is individual.

  4. Done! Time to eat

    Your eating window starts. Eat calmly and normally, there’s no need for a double portion.

The times are typical values, nothing is measured. When your body switches to fat and ketones is individual. It usually happens between 12 and 36 hours, depending on how full your stores were and how much you move.12

How the calculator works

With intermittent fasting, all that counts is the time between your last meal and your next one. With 16:8 you fast for 16 hours and eat within the remaining 8. Your sleep counts in full. Have dinner at 8 PM, and by noon the next day you’ve already fasted 16 hours.3

You can also work backwards. Switch to “want to eat again” and enter when you’d like breakfast or lunch. The calculator then shows you when to finish eating the evening before.

Which rhythm suits you?

The five rhythms at a glance
RhythmFastingEatingFor whom
12:1212 hours12 hoursGentle start
14:1014 hours10 hoursGreat to start
16:816 hours8 hoursThe classic
18:618 hours6 hoursExperienced
20:420 hours4 hoursAdvanced

Start small, for example with 12:12 or 14:10, and work your way up to 16:8 over a few weeks. It’s easiest if you push breakfast back little by little. Sticking with it matters more than long fasts.4

Three sample days for 16:8

What 16:8 can look like
TypeEatingFastingWhat you skip
Breakfast person8 AM – 4 PM4 PM – 8 AMDinner, or it becomes an early snack
The classic12 PM – 8 PM8 PM – 12 PMBreakfast
Night owl1 PM – 9 PM9 PM – 1 PMBreakfast, and you keep dinner with others

Fibi’s tip: fit your eating window around your life. A rhythm you enjoy sticking with beats one that only looks perfect on paper.

What can I drink while fasting?

Anything without calories is fine, so water, unsweetened tea and black coffee.35

Frequently asked questions

Does the night count?

Yes, your sleep counts in full. That’s why 16:8 fits into everyday life so well. Have dinner at 8 PM, and by noon the next day you’ve already fasted 16 hours.

Do I have to fast for the same length every day?

No. Many people fast on weekdays and keep the weekend flexible. Fixed times make it easier, though, because your routine gets used to them. In FastingFibi you set this up as a weekly plan with fasting days and days off.

I ate earlier than planned. Now what?

No problem, it happens to everyone. The time you managed still counts, and the next fast is a fresh start.

How do I break my fast?

After 12 to 18 hours a normal meal is fine. Eat calmly and don’t go for a double portion. The longer you’ve fasted, the more gently you start, for example with soup or steamed vegetables.

Sources

  1. Anton SD et al., Obesity 2018; 26: 254–268: When the body switches from sugar to fat (usually after 12 to 36 hours)
  2. Cahill GF Jr, Annu Rev Nutr 2006; 26: 1–22: Energy metabolism during long fasts
  3. AOK, Gesundheitsmagazin: Intermittent fasting: how it works (German)
  4. de Cabo R, Mattson MP, N Engl J Med 2019; 381: 2541–2551: Intermittent fasting, health and aging (review)
  5. Verbraucherzentralen: Fad diets under the microscope (German)

Last checked on 6 October 2026 · By Lukas Zimmermann

Fasting isn’t right for everyone, for example not during pregnancy, with an eating disorder or under 18. If you have health conditions or take medication, check with a doctor first. A quick word on safety