Health & Medicine · Dietetics
Intermittent Fasting Calculator
Calculate your intermittent fasting and eating windows based on your chosen fasting protocol and wake time.
Calculator
Formula
t_wake is your chosen wake-up time, t_fast,morning is the hours you fast after waking before your first meal, t_eating window is the total hours per day allocated to eating, and t_fasting hours is the total daily fasting duration.
Source: General intermittent fasting protocols as described in Longo & Panda (2016), Cell Metabolism.
How it works
The calculator takes your wake-up time and the total number of fasting hours per day. The eating window duration is simply 24 minus the fasting hours. Your first meal time is determined by adding the number of hours you prefer to fast after waking to your wake time — many people fast through the morning and break their fast at midday.
Common protocols include 16:8 (16 hours fasting, 8 hours eating), 18:6, and the more restrictive 20:4 or OMAD (23:1). The calculator outputs your eating window start and end times on a 24-hour clock, allowing you to plan meals precisely around your daily schedule.
Worked example
Suppose you wake at 7:00 AM, choose the 16:8 protocol (16 fasting hours), and prefer to fast for 4 hours after waking:
Step 1 — Eating window duration: 24 − 16 = 8 hours
Step 2 — Eating window start: 7:00 + 4 hours = 11:00 (11 AM)
Step 3 — Eating window end: 11:00 + 8 hours = 19:00 (7 PM)
You would eat between 11 AM and 7 PM, fasting from 7 PM until 11 AM the following day.
Limitations & notes
This calculator provides scheduling guidance only and does not account for individual metabolic differences, medical conditions, or caloric needs. Pregnant or breastfeeding individuals, those with a history of eating disorders, and people with diabetes or other metabolic conditions should consult a healthcare provider before beginning any fasting regimen. The calculator assumes a consistent sleep-wake schedule; results may vary for shift workers or those with irregular schedules.
Frequently asked questions
What is the most popular intermittent fasting protocol?
The 16:8 method — 16 hours of fasting and an 8-hour eating window — is the most widely practiced protocol due to its flexibility and alignment with normal sleep schedules. It is generally well-tolerated by beginners.
Does drinking water break a fast?
Plain water, black coffee, and plain tea do not meaningfully raise insulin levels and are generally considered safe during the fasting window. Beverages containing calories, sweeteners, or milk are typically avoided during the fast.
What is the difference between 16:8 and 18:6?
16:8 allows an 8-hour eating window while 18:6 restricts eating to 6 hours, making 18:6 a more demanding protocol. The additional fasting time may enhance metabolic benefits but can be harder to sustain long-term.
Can intermittent fasting help with weight loss?
Research suggests IF can support weight loss primarily by reducing overall calorie intake through a restricted eating window. However, caloric quality and total intake still matter — IF is not effective without attention to overall diet.
How long does it take to adapt to intermittent fasting?
Most people experience hunger and adjustment symptoms for the first 1–2 weeks before the body adapts to the new eating pattern. Gradually extending the fasting window over several weeks can ease the transition.
Last updated: 2025-01-15 · Formula verified against primary sources.