Intermittent Fasting Calculator

Important: please read this first

Do not fast if you are pregnant, breastfeeding, under 18, diabetic, or if you have a history of eating disorders. Talk to a doctor first if any of these apply to you, or if you take medicine that must be taken with food.

How to Use This Calculator

  1. Read the warning above first, and skip fasting completely if any point applies to you.
  2. Choose a protocol, or enter your own fasting hours under Custom.
  3. Enter the time of your last meal, then click the button to see when your fast ends and when your eating window opens and closes.

Intermittent Fasting Calculator: 16:8, 18:6, 20:4 and OMAD Windows

This intermittent fasting calculator turns your last meal time into a clear daily plan, showing exactly when your fast ends and when your eating window opens and closes. Intermittent fasting changes when you eat rather than what you eat, and a clear clock plan removes most of the guesswork. However, fasting is not safe for everyone, so this page puts the safety warning first and the method second. Read the warning carefully, and then use the tool only if none of it applies to you.

Intermittent fasting calculator: woman in modest clothing checking the clock before a healthy lunch at noon

Who Must Not Use This Intermittent Fasting Calculator to Fast

Do not fast if you are pregnant or breastfeeding, because you and your baby need a steady supply of energy and nutrients. Similarly, do not fast if you are under 18, since growing bodies need regular meals. Also, do not fast if you have diabetes, because fasting can push blood sugar too low, especially with insulin or similar medicine. In addition, do not fast if you have a history of eating disorders, because set fasting rules can restart harmful patterns around food. Above all, talk to a doctor first if you take regular medicine, feel unwell when you skip meals, or feel unsure in any way. A calculator can count hours, but only a doctor knows your health history.

How the Intermittent Fasting Calculator Works

First, you choose a protocol, which is simply a split of the 24 hour day into fasting hours and eating hours. For example, 16:8 means 16 hours of fasting and an 8 hour eating window, while 18:6 and 20:4 shorten the eating window further. Next, OMAD, short for one meal a day, means a 23 hour fast with a 1 hour eating window, and it suits very few people. Then you enter the time of your last meal, because that moment starts the fast. As a result, the tool adds your fasting hours to that start time, and it shows the fast end time plus the full eating window for every protocol side by side. During the fast, water and unsweetened drinks are usually kept, while any food or sweet drink breaks the fast.

Intermittent Fasting Calculator Example: 16:8 From 8:00 PM

Say your last meal ends at 8:00 PM, and you choose the popular 16:8 protocol. First, add the 16 fasting hours to the start time: 8:00 PM plus 16 hours takes you through the night to 12:00 PM the next day. So your fast ends at 12:00 PM, and your eating window opens at noon. Next, the eating window runs for 8 hours, from 12:00 PM to 8:00 PM. Finally, at 8:00 PM the window closes, and the next fast begins at exactly the time you started today. This neat loop is why 16:8 fits daily life so well, because most of the fast happens while you sleep.

Tips for the First Two Weeks

Start gently, because a sudden jump to long fasts often ends in a headache and a quick quit. For instance, begin with a 12 hour overnight fast for a few days, and then move to 14 hours before you try 16:8. Also, eat proper meals inside your window, with enough protein, vegetables and water, rather than saving up for one huge meal. In addition, keep your window at the same times each day, since a steady rhythm helps hunger settle. If you feel dizzy, faint or properly unwell at any point, stop the fast, eat normally and talk to a doctor.

Frequently Asked Questions

Is 16:8 a good starting protocol?

Yes, because most of the fast falls overnight and the eating window still fits lunch and dinner. Most beginners find 16:8 easier to keep than 20:4 or OMAD. Start there, and change the protocol only after two steady weeks.

What can I drink during the fast?

Water is the main drink, and plain tea or black coffee without sugar are usually kept as well. Sweet drinks, milk and juice contain calories, however, so they break the fast. If plain water feels hard at first, add a slice of lemon or cucumber for flavour.

Does this intermittent fasting calculator suit women?

It suits healthy adult women who are not pregnant or breastfeeding and who have no eating disorder history. Some women do better with a gentler 14:10 split, especially at first. Listen to your energy, mood and cycle, and shorten the fast if your body protests.

What if I take medicine with food?

Talk to your doctor or pharmacist before you fast, because some medicines must be taken with a meal and some affect blood sugar. Never move or skip a dose just to fit a fasting window. Your medicine schedule comes first, and the window can move around it only with medical advice.

When should I stop fasting and seek help?

Stop at once if you feel faint, confused, shaky or very unwell, and eat something. Then talk to a doctor before you try again. Fasting should feel like a calm routine, not like a daily struggle you force yourself through.

Related Tools

Sources

The description of intermittent fasting and metabolic switching on this page follows Johns Hopkins Medicine, which explains how the body moves from sugar stores to fat burning after hours without food. The safety groups listed above follow the warnings given by Johns Hopkins Medicine and other major medical centres: pregnant or breastfeeding women, children and teenagers, people with diabetes and people with an eating disorder history should not fast without medical advice.