Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Intermittent Fasting Windows Are Calculated
Intermittent fasting is an eating pattern that alternates periods of fasting with periods of eating¹. Unlike most calculators on this site, this one does not need your weight, age or other body stats. It is a schedule, not a physiology problem, so this tool works from a protocol and a start time alone.
What This Calculator Measures
This is a scheduling tool, not a physiological calculator. Your eating and fasting windows come from the protocol you choose, not from your personal stats.
Step 1: Choose Your Protocol
16:8, 18:6, and 14:10 are daily protocols defined by the number of fasting hours to eating hours. 5:2 and alternate-day fasting instead work on a day-based weekly pattern rather than a daily clock-time window.
Step 2: Set Your Eating Window Start Time
For daily protocols, we take your chosen start time and the protocol's eating-hour count to calculate your exact eating and fasting windows in clock time.
How This Calculator Computes Your Result
Pick a protocol and, for daily protocols, an eating-window start time. We calculate the end of your eating window by adding the protocol's eating-hour count, and everything outside that window becomes your fasting period.
Step 3: Display the Result as a Timeline
Rather than plain text alone, we show your eating and fasting windows as a color-coded 24-hour timeline bar, making your schedule easy to read and share at a glance.
Common Schedules and Example Windows
The calculator offers three daily schedules and two weekly ones. Here is what each looks like with a typical start time. The fasting window always ends the next day, and a daily eating window that runs past midnight is also marked "(next day)".
Protocol | Fast and eat | Example eating window | Fasting window |
14:10 | 14 h fast, 10 h eating | 8:00 AM – 6:00 PM | 6:00 PM – 8:00 AM |
16:8 | 16 h fast, 8 h eating | 12:00 PM – 8:00 PM | 8:00 PM – 12:00 PM |
18:6 | 18 h fast, 6 h eating | 12:00 PM – 6:00 PM | 6:00 PM – 12:00 PM |
5:2 | 2 fast days a week | 5 normal days | 2 non-consecutive days |
Alternate-day | Fast every other day | Normal days | Fast or very-low-calorie days |
Daily schedules give you clock times and a timeline. The 5:2 and alternate-day options work by whole days, so they need no start time.
What the Research Shows
In head-to-head trials, intermittent fasting has worked about as well as ordinary calorie restriction, not better. In a 12-month trial of 139 adults with obesity, eating only between 8:00 a.m. and 4:00 p.m. on top of a calorie target led to 8.0 kg of weight loss, against 6.3 kg with the calorie target alone, a difference that was not statistically significant³.
In a 12-week trial of 116 adults, a 16:8 schedule with eating from 12:00 p.m. to 8:00 p.m. and no calorie target led to 0.94 kg of weight loss, which was not significantly different from eating 3 meals a day⁴. A large trial of alternate-day fasting found the same pattern², and a 2026 Cochrane review of 22 trials found that intermittent fasting may make little or no difference to weight loss compared with standard diet advice⁷. So the schedule is a tool for eating less, not a shortcut. The TDEE calculator and calorie deficit calculator can help you set a calorie target to pair with it.
Who Should Not Fast Without Medical Advice
A review of the evidence advises against intermittent fasting for pregnant and breastfeeding women, children, people with an eating disorder, and people with a BMI under 18.5⁵.
Diabetes needs extra care. Cutting calories while taking glucose-lowering medicine can cause severe low blood sugar⁵. A review of fasting in type 1 and type 2 diabetes concluded it can be done safely when it is supervised by your healthcare provider, with glucose monitoring⁶. If you take insulin or other diabetes medicine, talk to your doctor before you start.
Daily Fasting Schedules Compared
Each daily protocol on a 24-hour clock, with an example eating window that starts at noon.
- Fasting
- Eating window
Show data table
| Protocol | Fasting | Eating window |
|---|---|---|
| 16:8 | 12:00 AM to 12:00 PM, 8:00 PM to 12:00 AM | 12:00 PM to 8:00 PM (8 hours) |
| 18:6 | 12:00 AM to 12:00 PM, 6:00 PM to 12:00 AM | 12:00 PM to 6:00 PM (6 hours) |
| 14:10 | 12:00 AM to 12:00 PM, 10:00 PM to 12:00 AM | 12:00 PM to 10:00 PM (10 hours) |
A Note on Accuracy
This is a scheduling tool, not a physiological calculator, your window length is a protocol choice, not something derived from your personal stats. A large randomized trial comparing alternate-day fasting directly against daily calorie restriction found no significant difference in weight loss between the two approaches. Intermittent fasting is not appropriate for everyone: people with a history of disordered eating, pregnant or breastfeeding people, and those with certain medical conditions (such as insulin-dependent diabetes) should consult a doctor before trying it.
Frequently Asked Questions
Does intermittent fasting burn more fat than regular calorie restriction?
Research to date generally shows intermittent fasting produces weight loss comparable to standard calorie restriction at the same calorie deficit, not a distinct metabolic advantage¹, a finding also confirmed directly in a large randomized trial². Its main benefit for many people is that a shorter eating window can make it easier to stick to a calorie target.
Which protocol should I choose?
There's no single best protocol, it depends on what fits your schedule and lifestyle. 16:8 and 14:10 are common starting points with a daily eating window, while 5:2 and alternate-day fasting work on a weekly, day-based pattern instead.
Is intermittent fasting safe for everyone?
No. People with a history of disordered eating, pregnant or breastfeeding people, and those with certain medical conditions (such as insulin-dependent diabetes) should consult a doctor before trying intermittent fasting.
How do 5:2 and alternate-day fasting work differently from 16:8?
16:8, 18:6, and 14:10 are daily eating-window protocols, you eat and fast at the same times every day. 5:2 involves eating normally 5 days a week and significantly restricting calories on 2 non-consecutive days. Alternate-day fasting alternates between normal eating days and fasting or very-low-calorie days.
Sources
Clinical Application of Intermittent Fasting for Weight Loss: Progress and Future Directions, Varady et al., 2022
Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults, Trepanowski et al., 2017
Calorie Restriction With or Without Time-Restricted Eating in Weight Loss, Liu et al., 2022
Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters: The TREAT Randomized Clinical Trial, Lowe et al., 2020
Intermittent Fasting in Cardiovascular Disorders: An Overview, Malinowski et al., 2019
Clinical Management of Intermittent Fasting in Patients With Diabetes Mellitus, Grajower and Horne, 2019
Intermittent Fasting for Adults With Overweight or Obesity, Garegnani et al., Cochrane Database of Systematic Reviews, 2026