Intermittent fasting is not about what you eat but when. Getting the timing right for your schedule determines whether you stick to it long-term.
How IF Windows Work
Every IF protocol divides the 24-hour day into a fasting window and an eating window. During the eating window you consume all your calories; during the fasting window you consume only water, black coffee, or plain tea.
Eating Window = 24 โ Fasting Hours
Last Meal Time = First Meal Time + Eating Window Hours
The 16:8 Protocol (Most Popular)
Fast for 16 hours, eat within an 8-hour window.
How to calculate your schedule:
Choose your last meal time, then count back 8 hours for your first meal:
| Last Meal | First Meal | Fast Duration |
|---|---|---|
| 8:00 PM | 12:00 PM | 16 hours (8PMโ12PM) |
| 7:00 PM | 11:00 AM | 16 hours |
| 9:00 PM | 1:00 PM | 16 hours |
The overnight sleep period counts toward your fast โ so most of the fasting happens while you sleep.
Tip: If you finish eating at 8 PM and sleep at 10 PM, only 2 hours of fasting are "felt." You wake up 8 hours later at 6 AM โ 10 hours fasted โ and only need to hold out until noon.
The 18:6 Protocol
More aggressive variant โ 18-hour fast, 6-hour eating window.
Example schedules:
- Noon to 6 PM eating window
- 1 PM to 7 PM
- 2 PM to 8 PM
The 20:4 Protocol (Warrior Diet)
20-hour fast with a 4-hour eating window, typically in the evening. Allows small amounts of raw fruits and vegetables during the day.
The 5:2 Protocol
Eat normally 5 days per week. On 2 non-consecutive days, restrict to 500โ600 calories.
Weekly Calorie Deficit (5:2) = (Normal daily calories โ 500) ร 2
Example: 2,000 calorie/day normal intake:
- Fasting day deficit = 2,000 โ 500 = 1,500 calories ร 2 days = 3,000 weekly deficit
- This equates to roughly 0.86 lb (~0.4 kg) of fat loss per week from the restriction alone
The 24-Hour (Eat Stop Eat) Protocol
One or two 24-hour fasts per week. Dinner to dinner (or lunch to lunch). No calories during the fast period.
The Metabolic Timeline of Fasting
Understanding what happens in your body helps you commit to the window:
| Hours Fasted | What Happens |
|---|---|
| 0โ4 hours | Digestion and nutrient absorption |
| 4โ8 hours | Blood glucose normalises, insulin drops |
| 8โ12 hours | Liver glycogen begins depleting |
| 12โ16 hours | Glycogen nearly empty, fat oxidation increases, ketones begin |
| 16โ24 hours | Fat burning elevated, autophagy (cellular cleanup) accelerates |
| 24โ48 hours | Deep ketosis, significant autophagy |
Calculating Your IF Calorie Budget
IF doesn't change your calorie targets โ it changes the timing. Your TDEE (Total Daily Energy Expenditure) remains your guide:
Daily Calories = TDEE ร Goal Modifier
Weight loss: TDEE ร 0.80 (20% deficit)
Maintenance: TDEE ร 1.0
Muscle gain: TDEE ร 1.10โ1.15
All calories must fit within the eating window. This naturally limits snacking and late-night eating, which is why many people find IF effective even without tracking every calorie.
Choosing the Right Protocol for Your Lifestyle
| Protocol | Best For |
|---|---|
| 16:8 | Most people โ easy to maintain long-term |
| 18:6 | Those who adapted to 16:8 wanting more |
| 5:2 | People who prefer flexibility on most days |
| 20:4 | Experienced fasters, busy schedules |
| 24-hour | Occasional resets, advanced practitioners |
What Breaks a Fast
These break the fast and should be saved for your eating window:
- Any calories (even small amounts)
- Milk, cream, or sugar in coffee
- Juice, sports drinks, soda
- BCAAs or protein supplements
These typically do not break a fast:
- Water
- Black coffee
- Plain tea (no additives)
- Electrolytes without calories
Use our TDEE calculator to set your daily calorie target, then simply place those calories within your chosen eating window.