How to Start Intermittent Fasting: A Step-by-Step Plan

The simplest way to start intermittent fasting is to stretch the overnight gap you already have. A first schedule of 12 or 14 hours without food mostly means no snacks after dinner. The four steps below give the times to use and what the first weeks can bring.

What is intermittent fasting?

Intermittent fasting means you eat only within set hours and have only drinks with almost no calories the rest of the time. It sets when you eat. It doesn’t set what you eat, and it costs nothing.

Daily schedules are named by their hours of fasting and eating. On 16:8, for example, you fast for 16 hours and eat within an 8-hour window. The common daily schedules are 12:12, 14:10, 16:8, 18:6 and 20:4. One meal a day (OMAD) means about 23 hours of fasting. Weekly schedules such as 5:2 cut food on two days and leave the other days normal.

So the basic rules of intermittent fasting are short:

• Fast for your chosen number of hours.
• Stick to water, black coffee and plain tea during the fast.
• Eat normal, filling meals in the window.
• Keep the same times on most days.

Who should not start intermittent fasting

Fasting is not safe for everyone. Don’t start without your doctor’s advice if any of these apply:

• You are pregnant, trying to conceive or breastfeeding.
• You are under 18.
• You have, or have had, an eating disorder (we advise against fasting then).
• You are underweight (we advise against fasting then too), frail, or still getting over an illness or surgery.
• You have type 1 diabetes, or take insulin or another diabetes medicine that can cause a hypo (low blood sugar), such as a sulfonylurea.
• You take any medicine that has to go with meals.

Never skip or move a medicine to protect a fast. Before any fast of more than 24 hours, speak to a doctor. The medical disclaimer has the full list.

Step 1: choose a gentle first schedule

Start one step above what you already do. If you snack late in the evening, a 12-hour overnight fast is a real change. If you already go 12 hours, try 14.

What you do now	Schedule to try first	Example eating window
Eat from breakfast until a late snack	12:12	8 AM to 8 PM
Already go about 12 hours overnight	14:10	9 AM to 7 PM
Already fast 14 hours or more	16:8	11 AM to 7 PM

These are the windows our schedule chooser gives when you say you most want to eat in the middle of the day. Six questions move the window earlier or later to fit your day. Anyone in a group from the list above gets a “talk to a doctor first” result instead of a schedule.

Step 2: set your times with the calculator

The fasting calculator starts counting from the time you finish your last meal. It adds your fasting hours to that time to find when you eat again. The window then runs for the rest of the 24 hours.

On 16:8, a meal finished at 8:00 PM means you eat again at 12:00 PM the next day, and the window closes at 8:00 PM. On 14:10, a last meal at 7:00 PM gives a window from 9:00 AM to 7:00 PM.

The calculator shows a live countdown and which stage of the fast you are in. It also adds the window to your phone’s calendar, so you don’t have to remember the times.

Step 3: what to drink while you fast, and what to eat after

While you fast, drink water, black coffee or plain tea. Black coffee has about 2 calories in a 240 ml (8 fl oz) cup. Milk, sugar and syrups add more. Some people use a rough limit of about 50 calories during a fast. No official body sets that limit; it is only a rule of thumb. Our guide to drinks during a fast covers tea, sweeteners and diet drinks.

Roughly a fifth of a day’s water normally comes from what you eat. Fewer meals mean less of that water, so drink a bit more than usual and follow your thirst. In Europe the official adequate intake of total water is 2.0 litres for women and 2.5 litres for men each day. That counts drinks and food together.

In your window, eat normal meals with a good source of protein in each. Adults need at least 0.8 g of protein daily for each kg they weigh. If you exercise, it rises to about 1.4 to 2.0 g per kg. Our week of example meals shows how that looks on a plate.

Step 4: add an hour or two a week

Stay on your first schedule until it feels routine, then move your first meal later by an hour or two. For beginners, a typical path is 12:12, then 14:10, then 16:8. You don’t have to go further than the schedule that suits your day. The schedule chooser never suggests OMAD, because fitting a whole day’s food into one meal is hard.

If a week goes badly, go back a step. A shorter fast you keep doing is worth more than a longer one you drop.

When does intermittent fasting start working?

Part of any early drop on the scale is water. Glycogen, the carbohydrate your liver and muscles keep in reserve, binds roughly three to four times its own weight of water. A shorter eating day uses some of that reserve, and the water goes with it.

Longer-term results depend on how much you eat in total. When calories are matched, several human trials agree that intermittent fasting and ordinary calorie cutting give about equal weight loss. A 2025 review of 99 trials found the same, though its authors called it “some indication”, so the evidence is moderate. The fasting hours on their own don’t make up for eating more in the window. So whether you should do intermittent fasting depends on whether fixed hours make your eating easier to manage.

In human studies, fat burning rises and ketone production begins at some point between 12 and 36 hours into a fast. This change is called the metabolic switch. How soon it comes depends on the size of your previous meal and on your activity. A schedule can still be worthwhile without reaching it.

What the first weeks can feel like

Plan for hunger before your window opens. Water, black coffee and plain tea are the drinks to reach for, because they have almost no calories.

Headaches can happen too. Doctors recognise “fasting headache” as its own type. It comes on after at least 8 hours without food, is usually mild to moderate, and eases once you eat. Its cause is not yet known. Fluids, salt, caffeine and sleep are the usual things to check.

Drinks in the window count too. Two or three lattes, or wine with dinner, can wipe out the calorie gap that a 16:8 schedule opens up.

Stop fasting and get help if you have chest pain, faint, become confused, or have a racing or irregular heartbeat. In an emergency, call 911 in the US or Canada, 999 in the UK or 112 in the EU. Elsewhere, use your local number. Our guide to intermittent fasting side effects covers the milder problems and what to do about them.

How to start intermittent fasting today

Answer the six questions in the schedule chooser. It gives you a schedule, the eating window that fits your day, and a link that opens both in the calculator.