Intermittent Fasting Timer Calculator
Calculate your eating window and fasting end time from last meal time and protocol.
Supports 16:8, 18:6, 20:4, and OMAD intermittent fasting schedules.
Intermittent fasting (IF) cycles between eating and fasting windows. The most common protocols limit when you eat — not necessarily what or how much — to naturally reduce total calorie intake and trigger metabolic benefits.
Common fasting protocols:
| Protocol | Fast Duration | Eating Window |
|---|---|---|
| 16:8 | 16 hours | 8 hours |
| 18:6 | 18 hours | 6 hours |
| 20:4 (Warrior Diet) | 20 hours | 4 hours |
| OMAD | 23 hours | 1 meal |
| 5:2 | 2 days/week at 500 cal | 5 normal days |
Eating window calculation:
Eating Window End = Eating Window Start + Eating Hours
Next Fast Start = Eating Window End
Calorie estimation during eating window:
For weight loss: Target Calories = TDEE − 500 kcal/day (roughly 1 lb/week loss)
Where TDEE = Total Daily Energy Expenditure (your maintenance calories)
Worked example — 16:8 protocol:
- First meal: 12:00 PM (noon)
- Eating window: 12:00 PM – 8:00 PM
- Fast: 8:00 PM until 12:00 PM next day (16 hours)
- TDEE: 2,200 kcal → Target during eating window: 1,700 kcal in 2–3 meals
Metabolic milestones during fasting:
- 8–12 hours: Glycogen stores deplete, body begins using fat for energy
- 12–16 hours: Ketosis begins; fat oxidation accelerates
- 16–18 hours: Autophagy (cellular cleanup) upregulates
- 24+ hours: Growth hormone spikes significantly
Who should NOT fast: Pregnant women, people with eating disorder history, those on certain medications, or anyone with diabetes — consult a doctor first.