Choose a start time and eating-window length to calculate a repeating daily schedule. A fasting schedule alone cannot predict weight loss. If you want to describe weight change you have already observed, use the weight-loss percentage calculator.
Worked schedules, including midnight
| Start | Eating window | End | Time outside window |
|---|---|---|---|
| 08:00 | 12 hours | 20:00 same day | 12 hours |
| 10:00 | 10 hours | 20:00 same day | 14 hours |
| 12:00 | 8 hours | 20:00 same day | 16 hours |
| 18:00 | 8 hours | 02:00 next day | 16 hours |
The tool adds the eating duration to the first meal time and uses 24 minus that duration for the time outside the window. It uses a repeating local clock, not calendar dates or daylight-saving transitions. All three choices are examples, not recommendations to lengthen fasting.
Why this is not a fasting weight-loss prediction
Clock hours do not tell us your food intake, energy use, health status or whether a schedule is sustainable. A 2026 Cochrane review found little or no meaningful weight-loss advantage over standard dietary advice. This calculator therefore does not turn hours fasted into calories burned or pounds lost.
Plan adequate meals within a routine that fits you. The seven-day macro meal plan provides food assemblies and portions; the macro calculator explores daily targets. Neither makes a shorter eating window necessary.
When to get professional advice first
Fasting can be inappropriate for children, pregnancy or breastfeeding, an eating-disorder history, or conditions and medicines that require regular food. Diabetes and glucose-lowering medicines need particular care: NIDDK describes hypoglycemia and medication risks during fasting. Do not change a medicine or its timing using this tool. Ask your clinician whether fasting is suitable before trying it.
If a schedule causes dizziness, weakness, distress or difficulty eating enough, stop and seek appropriate advice. A meal log is optional and cannot determine whether fasting is safe for you.