Intermittent Fasting Calculator

Planning a fasting day takes three quick entries. The tool works out your eating window from your first meal time and your plan, then works out the fasting window that follows it.

1. Pick your fasting plan. 16:8 is the most common first plan, and 18:6, 20:4 and OMAD are longer fasts.
2. Pick your first meal time. This is when your eating window opens and your overnight fast ends.
3. Enter how many days a week you follow the plan, then press Calculate, or press Enter. The eating window appears first and large, with the fasting window and the weekly fasting hours below. Press Reset to clear the form and start over.

Intermittent fasting is a timing rule, not a menu. Each daily plan splits the 24 hour day into a fasting window and an eating window. On 16:8 you fast for 16 hours and eat during 8 hours, on 18:6 you fast for 18 hours and eat during 6 hours, and on 20:4 you fast for 20 hours and eat during 4 hours. OMAD means one meal a day, with about one hour to eat and 23 hours of fasting.

The math is simple. The eating window starts at your first meal and runs for the eating hours, and the fasting window runs from the end of the eating window to the first meal the next day. The 5:2 plan works differently, since it splits the week instead of the day, with 5 normal eating days and 2 low-calorie days of about 500 to 600 calories. Our calorie calculator works out how many calories you need in a day, and our TDEE calculator works out your total daily energy use.

A person follows 16:8 and eats the first meal at 12:00 PM, seven days a week. Here is exactly what the calculator does.

1. Eating window: 12:00 PM plus 8 hours = 12:00 PM to 8:00 PM.
2. Fasting window: 8:00 PM to 12:00 PM the next day = 16 hours.
3. Fasting hours per week: 16 x 7 = 112 hours.

As a second check, 18:6 with a first meal at 11:00 AM gives an eating window of 11:00 AM to 5:00 PM and 126 fasting hours a week. The same steps work for any plan, so try your own numbers in the tool above.

Start with 16:8. It is the most studied window, and the fast is mostly overnight, so you skip breakfast and eat from noon to 8:00 PM. Two solid meals and a snack fit comfortably in eight hours. Move to a longer fast only after your energy, your sleep and your meal quality stay stable for a few weeks.

Longer fasts are harder to keep up. On 20:4 you have four hours to eat, which suits one large meal and a snack, and OMAD suits one meal a day. The 5:2 plan suits people who prefer normal days with two hard days, rather than a daily window. Pick the plan you can repeat for weeks, not the hardest one you can manage for a day.

Fasting controls when you eat, not what you eat, so the food inside the window still decides the result. Trials find that weight loss from intermittent fasting is similar to ordinary calorie reduction, so total intake in the window matters most. Build meals around protein, fiber and vegetables, and keep the window free of constant snacking.

Drink water through the fast, since thirst often feels like hunger. Black coffee and plain tea have no calories, so they do not break the fast for most plans. Break the fast with a normal meal, not a large one, and keep the last meal at least an hour or two before bed. Our water intake calculator works out how much water you need in a day.

Intermittent fasting is not right for everyone. It is not advised for children and teens, during pregnancy or breastfeeding, for people with type 1 diabetes who take insulin, or for anyone with a history of eating disorders. People who take medicine with food or who manage a health condition should talk to a doctor first.

Stop fasting and talk to a doctor if you feel dizzy, faint or unwell, or if your sleep, mood or energy get worse. A plan that fits your day and your health is worth more than a longer fast. This calculator gives general information only, and it is not medical advice.