Intermittent Fasting Calculator
Pick an intermittent-fasting method and the time you want to start eating, and this scheduler works out your exact eating and fasting windows.
Intermittent-fasting numbers split a 24-hour day — 16:8 means 16 hours fasting with an 8-hour eating window, and beginners usually start gentler at 12:12 or 14:10.
How the schedule works
Intermittent fasting splits your day into an eating window and a fasting window. The shorthand — 16:8, 18:6, 20:4 — is simply the fasting hours to eating hours, and the two always add up to 24. This scheduler takes the method you choose and the time you want to start eating, then works out exactly when your eating window closes and how long your fast runs.
There is no metabolic magic in the clock itself. Fasting windows mostly help by trimming the hours in which it is easy to overeat, which can make a modest calorie deficit easier to hold. If you eat the same calories in a shorter window, weight will not necessarily change — pair this with the TDEE and macro tools to know your numbers.
Choosing a method
Beginners usually do best starting gentle — 12:12 or 14:10 — before tightening to the popular, well-studied 16:8. Longer protocols like 20:4 or OMAD are far more restrictive and suit experienced fasters; they also make it harder to get enough protein and micronutrients. During the fast, water, black coffee and unsweetened tea are fine; anything with calories opens the eating window. Staying hydrated helps — our water intake calculator gives a daily baseline.
This is general lifestyle guidance, not medical advice. Intermittent fasting is not suitable for everyone — including people who are pregnant or breastfeeding, under 18, have a history of disordered eating, or take medication for diabetes — so check with a healthcare professional first.
Common fasting protocols
| Method | Fasting | Eating | Typically suits |
|---|---|---|---|
| 12:12 (starter) | 12 h | 12 h | Absolute beginners; easiest to sustain |
| 14:10 | 14 h | 10 h | Easing into time-restricted eating; low-stress |
| 16:8 | 16 h | 8 h | Most popular and most-studied daily routine |
| 18:6 | 18 h | 6 h | Comfortable with 16:8 and wanting a tighter window |
| 20:4 (Warrior) | 20 h | 4 h | Experienced fasters; one main meal plus snacks |
| OMAD (23:1) | 23 h | 1 h | Advanced only; higher risk of nutrient gaps |
| 5:2 (not a window) | 2 low-cal days/wk | Normal 5 days | ~500 kcal (women) / 600 kcal (men) on 2 days |
Hours are the fasting-to-eating split of a 24-hour day. 5:2 is calorie-based rather than a daily window. Lifestyle guidance only.
Background: de Cabo & Mattson, New England Journal of Medicine (2019); NIH, Johns Hopkins Medicine and NHS. Not medical advice.
Frequently asked questions
What do the numbers like 16:8 mean?
They describe how a 24-hour day is split between fasting and eating. In 16:8 you fast for 16 hours and eat only during an 8-hour window; the two numbers always add up to 24. So 18:6 is 18 hours fasting / 6 hours eating, and OMAD (23:1) is 23 hours fasting with a roughly 1-hour eating window. The calculator takes the method and the time you want to start eating, then works out when your eating window closes and your fast begins.
Which method should a beginner start with?
Gentler windows such as 12:12 or 14:10 are the usual starting points because they cause the least disruption and are easiest to keep up. Many people then progress to 16:8, which is the most common and most-studied daily approach. Longer windows like 20:4 or OMAD are far more restrictive and are generally better suited to experienced fasters. This is general guidance, not a recommendation for your specific situation.
Does drinking water, black coffee or tea break the fast?
For a time-restricted eating window, plain water is fine and staying hydrated is encouraged. Black coffee and unsweetened tea are essentially calorie-free and are commonly consumed during the fasting window. Anything with calories — milk, sugar, sweetened drinks, or snacks — is treated as eating and would fall inside the eating window.
How is 5:2 different from a 16:8-style window?
5:2 is not a daily clock. Instead you eat normally on five days and cap intake at roughly 500 kcal (women) or 600 kcal (men) on two non-consecutive days of the week. Because there is no fixed daily eating window, the start-time calculation used for 16:8 or 18:6 does not apply to it — it is a calorie-based schedule rather than a time-based one.
Will intermittent fasting on its own make me lose weight?
Not automatically. Fasting windows tend to help mainly by making it easier to eat fewer total calories, since there is less time to eat. If you consume the same or more calories inside the window, weight may not change. A steady, sustainable rate of about 0.5–1 kg (1–2 lb) per week is the widely recommended target for healthy adults. To see your daily calorie needs, use the TDEE or macro calculator alongside this scheduler.
Sources
- de Cabo & Mattson — Effects of Intermittent Fasting on Health, Aging, and Disease (NEJM, 2019) — Foundational review of fasting regimens
- Gabel et al. — 8-hour time-restricted feeding pilot (Nutrition & Healthy Aging, 2018) — 16:8 time-restricted eating trial
- Harvie et al. — Intermittent vs continuous energy restriction (Int J Obesity, 2011) — 5:2 two-day restriction origin study
- MedlinePlus (NIH) — Safe rate of weight loss — Safe ~1-2 lb per week rate
The thresholds and formulas used by this calculator follow the primary sources above. This tool provides general information, not personalised medical advice.