16 hours of fasting followed by an 8-hour eating window, repeated daily. It's the most common starting protocol because the fasting hours mostly overlap with sleep, so a large part of the fast passes without much effort.

How is the eating window calculated?

Your eating window starts at the time you enter for your first meal and runs for however many hours your protocol allows — 8 hours for 16:8, 6 for 18:6, and so on. Everything outside that window is the fasting period, and the two always add up to 24 hours.

Does the fasting window include sleep?

Yes — sleep counts as fasting time the same as any other hour without food. This is exactly why 16:8 feels manageable to most people: if you sleep 7-8 hours, more than half the fasting window is already covered before you're even awake for most of it.

One Meal A Day — a 23:1 schedule where all of a day's food is eaten within a single roughly one-hour window. It's one of the more restrictive common protocols, sitting at the far end of the fasting-length spectrum from 16:8.

Can I change my first meal time day to day?

Yes — this calculator just shifts both windows to match whatever first-meal time you enter, so you can recalculate for a different day's schedule any time your routine changes.

Does this tell me what or how much to eat?

No — this calculator covers timing only. For a daily calorie target to eat within your window, our TDEE & BMR Calculator estimates that from your height, weight, age, and activity level.

AI Insights

Intermittent Fasting Calculator

Find your exact eating and fasting window times for 16:8, 18:6, 20:4, OMAD, or a custom schedule.

Last updated September 6, 2026

Fasting protocol

16:8 — the most common starting protocol

First meal of the day

Eating window

12:00 PM – 8:00 PM

8 hours to eat

Fasting window

8:00 PM – 12:00 PM (next day)

16 hours of fasting

Skipping breakfast while working through the morning hours works well here because sleep takes care of a huge chunk of the fasting time. Having that first bite around midday means the remaining fast hours happen while people are usually busy with daily tasks, which helps keep focus away from food. Planning a warm cup of tea or black coffee during those late morning office hours can also make the transition much easier.

AI-generated insight

How to use the calculator

1
Pick a protocol — 16:8, 18:6, 20:4, OMAD, or enter a custom fasting length.
2
Enter the time of your first meal of the day.
3
Your eating window and fasting window times update instantly.

How the windows are calculated

The eating window and fasting window always add up to a full 24 hours — a longer eating window means a shorter fast, and vice versa.

Your eating window starts at the first-meal time you enter and lasts however many hours your protocol allows. On 16:8, an 8 hour eating window starting at noon runs until 8:00 PM. Everything from 8:00 PM until noon the next day is the 16 hour fasting window.

Worked example: first meal at 12:00 PM on 16:8. Eating window = 12:00 PM to 8:00 PM (8 hours). Fasting window = 8:00 PM to 12:00 PM the next day (16 hours).

Common fasting protocols

Protocol	Fasting hours	Eating hours
16:8	16	8
18:6	18	6
20:4	20	4
OMAD	23	1

Everyday uses for a fasting calculator

Planning around a work schedule: finding a first-meal time that keeps your eating window inside your actual free hours, not fighting your calendar.

Switching protocols: seeing exactly how your windows shift before committing to a longer fast like 18:6 or 20:4.

Planning what to eat: once you know your eating window, our TDEE & BMR Calculator estimates a daily calorie target to fit inside it.

Want to check your weight trend alongside your fasting schedule? Try the BMI Calculator →

Frequently asked questions

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