BYTETOOLS

Intermittent Fasting Calculator

Build a 16:8, 18:6, 20:4, OMAD, 5:2 or alternate-day fasting schedule from your first or last meal, with window times, a live countdown and a 7-day plan.

Your eating window

12:00 PM – 8:00 PM

8 hours of eating, then a 16 hour fast until the window opens again. The most popular time-restricted eating protocol.

12:00 PM
Window opens
8:00 PM
Window closes
8 h
Eating hours
16 h
Fasting hours

Live countdown

--:--:--

Starting the clock…

The next seven days

DayPlan
Day 1Eat 12:00 PM – 8:00 PM · fast 16 h
Day 2Eat 12:00 PM – 8:00 PM · fast 16 h
Day 3Eat 12:00 PM – 8:00 PM · fast 16 h
Day 4Eat 12:00 PM – 8:00 PM · fast 16 h
Day 5Eat 12:00 PM – 8:00 PM · fast 16 h
Day 6Eat 12:00 PM – 8:00 PM · fast 16 h
Day 7Eat 12:00 PM – 8:00 PM · fast 16 h

Times use your device's clock and time zone. A window that runs past midnight is labelled “next day” so an 8 pm to 4 am schedule reads correctly.

Informational only, not medical advice, and this tool diagnoses nothing. Intermittent fasting is a scheduling idea, not a magic mechanism — most trials find it works about as well as steady calorie restriction, and only because it usually reduces intake. It is not suitable for everyone: do not fast without medical supervision if you are pregnant or breastfeeding, under 18, underweight, have a history of disordered eating, or take insulin, sulphonylureas or any medication that must be taken with food. Speak to a doctor or dietitian first.

What is the Intermittent Fasting Calculator?

An intermittent fasting calculator turns a protocol like 16:8 and one anchor time into an eating window. Anchor a 16:8 schedule to a first meal at noon and the window runs from 12:00 to 20:00, with the 16-hour fast running overnight until noon the next day.

  • Eight protocols: 12:12, 14:10, 16:8, 18:6, 20:4, OMAD, 5:2 and alternate-day
  • Anchor the schedule to your first meal or your last, whichever you plan around
  • Correct handling of eating windows that cross midnight
  • Live countdown that tells you whether you are eating or fasting right now
  • Seven-day grid with weekday names and restricted days shaded
  • Uses your own device clock and time zone, entirely offline

How to use the Intermittent Fasting Calculator

  1. 1

    Pick your protocol from the dropdown — the window-based schedules run from 12:12 up to OMAD, and the day-based ones are 5:2 and alternate-day.

  2. 2

    For a window protocol, choose whether to anchor it to your first or your last meal, then set that time.

  3. 3

    For 5:2, set the calorie cap and pick which two days of the week are restricted.

  4. 4

    Read the eating window open and close times, the fasting hours, and the live countdown to the next boundary.

  5. 5

    Check the seven-day table to see the plan laid out day by day, with restricted days shaded.

About the Intermittent Fasting Calculator

The ByteTools Intermittent Fasting Calculator turns a protocol into actual clock times. Choose 12:12, 14:10, 16:8, 18:6, 20:4, OMAD, 5:2 or alternate-day fasting, anchor it to either your first or your last meal, and it returns the exact times your eating window opens and closes along with a live countdown to the next boundary.

Windows that cross midnight are handled properly — an 8 pm anchor on 16:8 correctly shows a window closing at 4 am the following day rather than wrapping to nonsense. The day-based protocols work differently: 5:2 lets you pick which two days are restricted and what calorie cap applies, and alternate-day fasting lays out the pattern across the week.

Everything runs in your browser using your device's clock and time zone, and nothing is uploaded or stored. This is informational only, not medical advice, and it diagnoses nothing. Fasting is not suitable for everyone — speak to a doctor or dietitian first, particularly if you are pregnant, under 18, have a history of disordered eating, or take medication that must be taken with food.

Frequently asked questions

What is the 16:8 intermittent fasting method?

You eat within an eight-hour window and fast for the other sixteen, usually every day. A common pattern is eating between noon and 8 pm, which effectively means skipping breakfast. It is the most popular time-restricted eating protocol because the fast is mostly spent asleep.

What time should I stop eating for 16:8?

That depends on when you want to start. Anchor the calculator to your first meal and it adds eight hours; anchor it to your last meal and it works backwards. If your first meal is at 11 am you stop at 7 pm; if you want to finish by 6 pm you start at 10 am.

Does intermittent fasting actually work for weight loss?

It works about as well as steady calorie restriction, and mostly for the same reason — a shorter eating window usually means eating less. Randomised trials comparing the two consistently find similar results, so the honest answer is that fasting is a scheduling tool that suits some people's appetite and routine better than counting calories every day.

What is the difference between 5:2 and alternate-day fasting?

On 5:2 you eat normally five days a week and restrict to roughly 500 to 600 kcal on two non-consecutive days. Alternate-day fasting restricts every other day, which is around three or four restricted days a week and considerably harder to maintain.

Can I drink coffee or water while fasting?

Water, black coffee and plain tea contain essentially no calories and are accepted on every common protocol. Anything with sugar, milk or cream breaks the fast in the sense most protocols intend, and the debate about zero-calorie sweeteners has no clear answer.

Who should not try intermittent fasting?

Anyone who is pregnant or breastfeeding, under 18, underweight, has a history of disordered eating, or takes insulin, sulphonylureas or any medication that must be taken with food. Fasting can also be unsafe with certain medical conditions, so talk to a doctor before starting rather than after.

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