Intermittent Fasting Calculator Guide: How Windows Work
How eating and fasting windows are calculated from your first meal time and chosen protocol, with a worked example on the 16:8 schedule.
Every intermittent fasting schedule comes down to two numbers that always add up to 24: how many hours you eat, and how many hours you don't.
The math
Eating window = your first meal time, running for however many hours your protocol allows. Fasting window = everything else, from the end of your eating window until your next first meal.
Worked example
First meal at 12:00 PM on a 16:8 schedule: the 8-hour eating window runs from 12:00 PM to 8:00 PM, and the 16-hour fasting window runs from 8:00 PM to 12:00 PM the next day.
Why sleep does most of the work
A big chunk of any fasting window overlaps with sleep — on the example above, roughly half the 16 fasting hours pass while asleep, which is a large part of why 16:8 feels more manageable to most people than the raw hour count suggests.
The Intermittent Fasting Calculator calculates both windows instantly for any protocol and meal time.