Intermittent Fasting Calculator

Map a fasting window and eating window onto your chosen start time.

What it does

Intermittent fasting is a pattern of alternating fasting and eating periods. Common versions include 16:8, 14:10, and 18:6, where the first number is fasting hours and the second is eating hours. The pattern sets when you eat, not which foods are allowed.

This calculator turns those hours into clock times. Enter the length of the fasting period and the time you want to start eating. The tool returns the eating window, the fasting window, and the daily split.

The result is useful for planning around work, sleep, workouts, and family meals. A window that looks fine on paper can be hard to keep if it ends before dinner or starts before your first break. Adjust the start time and compare a few options before choosing one.

This page does not assess whether fasting is safe for you, count calories, or predict weight change. People who are pregnant or breastfeeding, take medication that requires food, have diabetes, or have a history of disordered eating should talk with a clinician before changing their eating schedule.

How to use

1. Enter the number of hours you plan to fast, such as 16.
2. Enter the time you want to start eating, using the 24-hour time field.
3. Read the eating window and fasting window.
4. Check whether the fast overlaps sleep, work, or medication times.
5. Adjust the start time or fasting hours and compare the result.
6. Keep water available and treat the schedule as a plan, not a medical prescription.

Formula

fasting hours

Hours per day without food.

eating hours

Hours per day inside the eating window.

eating start

Clock time when the eating window opens.

eating end

Clock time when the eating window closes.

The schedule repeats every 24 hours. If the eating window crosses midnight, the tool adds next day to the end time. A 16-hour fast starting at noon leaves an 8-hour eating window from noon to 8:00 PM, then a fast from 8:00 PM to noon the next day.

Worked example

Reference table

Limits

• The calculator does not count calories or predict weight loss.
• It does not assess whether a fasting schedule is medically safe for you.
• Sleep, work, training, and medication schedules may matter more than the clock math.
• A longer fast is not automatically better than a shorter one.

Common mistakes

• Confusing the eating window start with the fasting window start.
• Forgetting that a late eating window can run past midnight.
• Planning a long fast around a late dinner without checking the clock.
• Treating the eating window as permission to ignore food quality.
• Starting a long fast without asking a clinician about medication or health risks.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• Johns Hopkins Patient Guide to Diabetes: Intermittent fasting overview
• Harvard Health: Time to try intermittent fasting?

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