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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.