Intermittent Fasting Calculator

What this calculator does

Find out exactly when your fast ends and when your eating window opens and closes each day. Fasting schedules (also called time-restricted eating) are usually written as two numbers that add up to 24. For example, 16:8 means sixteen hours without food, then an eight-hour window for meals. The math is easy, but working out clock times across midnight is where people slip. That's especially true with longer fasts or a late dinner.

Choose a pattern and the time your last meal ends. The calculator gives the time the fast ends, your daily eating window, and a table comparing the common patterns from the same start. It's a scheduling tool only. It makes no promise about weight or health results. Talk to a doctor before changing how you eat if you're pregnant, have diabetes or another medical condition, take medication, or have a history of disordered eating.

Where it's used

Why this math matters outside the classroom:

• Planning a 16:8 day. Someone who finishes dinner at 8:00 PM adds 16 hours and gets noon the next day for their first meal.
• Elapsed time practice. Students count forward across midnight: from 7:30 PM, 14 hours later is 9:30 AM the next morning.
• Shift work meals. A night-shift nurse finishing a meal at 6:00 AM sees a 14-hour fast ends at 8:00 PM the same day.
• Comparing schedules. With dinner at 8:00 PM, an 18:6 window opens at 2:00 PM while a 12:12 window opens at 8:00 AM.
• Before a blood test. Patients told to fast 12 hours before an 8:00 AM appointment count back to an 8:00 PM last meal the night before.

Grade level and standards: Grades 3–4; Common Core State Standards for Mathematics 4.MD.A.2, 3.MD.A.1. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

1. Choose a schedule: 12:12, 14:10, 16:8, 18:6, 20:4, 23:1, or a custom fast length.
2. Enter the time your last meal of the day finishes. The fast is counted from then.
3. Read when the fast ends and the eating window that follows. You can compare other patterns in the table.
4. To plan backwards from a breakfast time instead, adjust the last-meal time until the fast ends when you want.

What the inputs mean

Fasting schedule

The first number is fasting hours and the second is eating hours. 23:1 is often called one meal a day.

Fast length

Any length from 1 to 23.5 hours works, and half hours such as 15.5 are fine. Some patterns skip whole days, such as 5:2 or alternate-day plans. Those are weekly calorie plans, not daily clock schedules, so they aren't covered.

Last meal finishes at

Use the time you finish eating. Many schedules allow water, plain tea or black coffee during the fast. If you're not sure what applies to you, check with a professional.

Formula and method

eating window (h) = 24 − fast length (h)
fast ends = last meal time + fast length
eating window closes = fast end + eating window = last meal time + 24 h

Here, h means hours. Times are counted in minutes after midnight on the day of your last meal. Adding the fast length can carry the time past midnight. The calculator wraps it back onto a 12-hour clock and tells you whether it lands on the same day or the next day.

The fast and the eating window together fill exactly 24 hours. So the window always closes at the same clock time your last meal ended, and the schedule repeats each day.

Worked example

You follow a 16:8 schedule and finish dinner at 8:00 PM.

1. Eating window = 24 − 16 = 8 hours.
2. Fast ends = 8:00 PM + 16 hours = 12:00 PM the next day.
3. Eating window = 12:00 PM + 8 hours, closing at 8:00 PM.

Result: Your fast ends at 12:00 PM (next day), and your eating window runs 12:00 PM – 8:00 PM each day.

Understanding the result

The eating window is the span when your meals fall, not a target for how much to eat. The schedule says nothing about the amount or quality of food.

If the window lands at an awkward time, shift your last-meal time rather than the pattern. For example, on 16:8, finishing dinner at 7:00 PM moves your first meal to 11:00 AM.

For planning, sticking with it matters more than the exact minute. Round to times you can keep on work days and weekends alike.

Assumptions and limitations

• The schedule repeats every 24 hours with the same last-meal time.
• Times are ordinary clock times, with no time-zone or daylight-saving change during the fast.
• The fast starts when the last meal ends.
• This is a clock calculator, not health advice. It doesn't judge whether fasting suits you.
• Weekly patterns such as 5:2 and alternate-day fasting aren't modelled.
• A daylight-saving change overnight shifts the clock time your fast ends by an hour.

Frequently asked questions

How this calculator is checked

Method: Clock arithmetic: fast end = last meal time + fast length, wrapped onto a 24-hour day; eating window = 24 h − fast length. Pattern names (16:8 and so on) follow the common fasting:eating hours convention. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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