20:4 fasting schedule

On a 20:4 fasting day you finish your last meal, then have only water, black coffee or plain tea for 20 hours. The next 4 hours are for eating, usually two meals close together. Enter your usual last meal below and the calculator shows when your window opens and closes.

How the 20:4 schedule works

The numbers are hours: 20 hours of fasting, then a 4-hour eating window, which adds up to a full day. You may also see intermittent fasting 20 4 written as 20/4, 20 x 4 or 4/20.

The calculator counts from the moment you finish eating and adds 20 hours to find your next meal. The window then stays open for 4 hours. So on 20:4 your window always closes at the same clock time as yesterday’s last meal.

20:4 sample days with real times

Each row starts from a last meal and adds 20 hours.

Last meal finished	Window opens	Window closes	Suits a day with
2:00 pm	10:00 am	2:00 pm	brunch and lunch, and no food in the evening
4:00 pm	12:00 noon	4:00 pm	lunch and an early dinner at work
7:00 pm	3:00 pm	7:00 pm	the window used in the 8-week trial below
8:00 pm	4:00 pm	8:00 pm	a late lunch, then dinner at home

This is how a day with a 3 pm to 7 pm window can run:

Time	What happens
7:00 pm (day before)	Last bite. The 20-hour fast starts.
Morning to early afternoon	Water, black coffee or plain tea.
3:00 pm	Meal one, with a protein food at its centre.
5:00 pm	A small snack, if you want one.
6:30 pm	Second meal, finished by 7:00 pm.

What does 20 hour fasting do in the body?

By hour 20 the calculator shows the “metabolic switch” stage, which starts at 12 hours. In human studies, the shift to burning more fat and producing ketones comes somewhere between 12 and 36 hours. The timing depends on how big your last meal was and how much you move. So 20 hours may cross it on one day and not on another.

Autophagy (cell recycling) has no known start hour in people, so 20:4 cannot promise it. The fasting timeline goes through each stage hour by hour.

20:4 results: what the trials tested

One trial tested this exact window. Adults with obesity ate only between 3 pm and 7 pm for 8 weeks. They lost about 3% of their body weight compared with a group who ate at any time. A second group on a 6-hour window, 1 pm to 7 pm, lost the same amount. Neither group counted calories, yet each ate roughly 550 kcal less per day. For 20:4 itself the evidence is early, from one short trial.

As a worked example, 3% of 100 kg (220 lb) is 3 kg (6.6 lb). In the first days, part of any drop is water. Each gram of glycogen, the carbohydrate kept in liver and muscle, is stored with about 3 to 4 g of water. A short eating day uses some of that store.

Longer trials give a wider view. In a 12-month trial with calories matched, an 8-hour window plus calorie cutting lost no more weight than calorie cutting alone. A 2025 review that pooled 99 trials put fasting diets and daily calorie cutting roughly level. That evidence is moderate, resting on a set of human trials. The trials checked for this site did not run 20:4 for longer than 8 weeks or test it against 16:8 with calories held equal.

Protein in a 4-hour window

Protein is the hard part, because two meals have to carry the whole day. Take a worked example of a person weighing 80 kg (176 lb):

Who	Daily protein	Each of two meals
Adult, minimum (0.8 g per kg)	64 g	32 g
Exercises regularly (1.4 to 2.0 g per kg)	112 to 160 g	56 to 80 g
Aged 65 or over (1.0 to 1.2 g per kg)	80 to 96 g	40 to 48 g

Plan the first meal before the window opens, so hunger does not choose it for you. Start each plate with the protein food. Protein is linked to feeling full. Some people cannot eat 56 g or more at one sitting. For them, 18:6 or 16:8 leaves more room between meals.

Our example meal plans show protein-led meals you can adapt to two sittings.

Who 20:4 suits, and who should skip it

20:4 can suit someone who already keeps 18:6 without trouble and prefers two larger meals to three small ones. The schedule chooser stops at 18 hours, so 20:4 is a step you take by choice after 18:6.

Check with a doctor before trying 20:4 if you are breastfeeding or pregnant, under 18 or frail. A low body weight, or any eating disorder in your history, rules it out in our advice. Check first, too, if you take insulin, a sulfonylurea or a medicine that has to go with meals. On a 20-hour gap without food, insulin and sulfonylureas can push blood sugar too low. If you take blood-pressure tablets and the fast leaves you dizzy or faint, end it and contact your doctor.

20:4 stays under 24 hours. Stretching it into one meal a day or a full-day fast is a bigger step. Before any fast longer than 24 hours, talk to a doctor.

How to step up to 20:4, or back down

Start from a schedule you already keep. From 18:6, push your first meal one hour later for a few days, which gives a 19-hour fast. Then move it one more hour to reach 20:4.

Step back down if you get headaches most days, feel dizzy, sleep badly or keep missing your protein. A fasting headache usually comes after 8 hours or more without food and eases once you eat. Going back to 18:6 or 16:8 is a normal choice. Some people keep 20:4 for weekdays and use a wider window at weekends.

A realistic first week of 20:4 fasting

This plan is our suggestion for someone moving up from 18:6.

Day	Plan
1 and 2	Keep your 18:6 times, but open the window one hour later (19 hours).
3 and 4	Full 20 hours. Write down both meals the night before.
5	Add up the protein in both meals once, using the label or a food database.
6	If a dinner falls outside your window, run 16:8 that day and restart tomorrow.
7	Check your fasting timer history to see how many fasts reached the 20-hour goal.

The fasting timer has 20:4 as a goal, works offline and keeps a history and streak in your browser. The printable schedule puts your week on one sheet. For the full range of windows, see the full list of schedules or the side-by-side guide to 16:8, 18:6 and OMAD.

To start, set your usual last meal in the calculator at the top of this page. When you finish that meal, press start on the fasting timer.