Intermittent Fasting vs OMAD: 16:8, 18:6 and 20:4 Compared

Intermittent fasting vs OMAD compares a whole family of eating patterns with its strictest daily member. OMAD (one meal a day) means about 23 hours of fasting. The 16:8, 18:6 and 20:4 schedules leave 8, 6 or 4 hours to eat. In trials, the longer fasts did not lose more weight once calories were equal. The real differences are hunger, protein and how well each schedule fits your day.

Intermittent fasting vs time-restricted eating: the terms

Intermittent fasting is the umbrella term. It covers two kinds of schedule:

• Daily eating windows, called time-restricted eating in research (or time-restricted feeding in some studies). You eat within the same hours every day.
• Fast-day methods such as 5:2, 4:3 and alternate-day fasting, where two or more days a week are low-calorie. Our 5:2 and alternate-day guide covers those.

The ratio in a name such as 16:8 is hours of fasting, then hours of eating, and the two always add up to 24.

16:8 vs 18:6 vs 20:4 vs OMAD at a glance

Here are the different intermittent fasting schedules side by side. The example windows assume your last meal ends at 7 pm.

Schedule	Hours fasting	Eating window	Example window	Meals that fit
12:12	12	12 hours	7 am to 7 pm	Three
14:10	14	10 hours	9 am to 7 pm	Three
16:8	16	8 hours	11 am to 7 pm	Two or three
18:6	18	6 hours	1 pm to 7 pm	Two
20:4	20	4 hours	3 pm to 7 pm	One or two
OMAD (23:1)	23	1 hour	6 pm to 7 pm	One

The fasting calculator has all six as presets. Enter your last meal and it shows when your window opens, with a live countdown.

What trials found when schedules were compared

Few trials put one window against another, and most were short:

Comparison	Who and how long	Result
20:4 vs 18:6 (eating 3 to 7 pm or 1 to 7 pm)	Adults with obesity, 8 weeks	Same weight loss, about 3%, compared with a group with no time limit; both groups ate about 550 kcal a day less without counting
16:8 (noon to 8 pm) vs eating through the day	116 adults, 12 weeks, no calorie target	No extra weight loss from 16:8
8-hour window plus calorie cutting vs calorie cutting alone	139 adults with obesity, 12 months	8.0 kg vs 6.3 kg lost; the difference was not significant
One meal vs three meals, same calories	Healthy normal-weight middle-aged adults, two 8-week periods	More hunger, a little less fat mass, higher blood pressure and cholesterol on one meal

A 2025 review pooled 99 trials and found fasting diets and daily calorie cutting work about equally well.

The evidence that total calories matter more than timing is moderate. Many human trials point the same way, and the largest review calls it “some indication”. Evidence comparing one window length with another is early, from small or short trials.

Is 18:6 better than 16:8?

No trial in the results above compares 16 hours with 18 hours directly. The closest is the 8-week trial where 4-hour and 6-hour windows lost the same weight. So for intermittent fasting of 16 or 18 hours, the extra fasting time has not been shown to add much. The same goes for choosing a 6, 8 or 10 hour window.

The body’s switch to burning more fat and making ketones happens somewhere between 12 and 36 hours. When it happens varies with what you last ate and how much you move. Two extra hours may or may not cross it on a given day. No human study shows when autophagy (cell recycling) starts, so no window can promise it.

Intermittent fasting vs OMAD: protein and hunger

The biggest practical difference is how much each meal has to hold. Take a worked example of a 70 kg (154 lb) adult who exercises. At 1.4 to 2.0 g of protein per kg a day, the daily total is 98 to 140 g:

Meals a day	Protein per meal
Three (12:12 or 14:10)	about 33 to 47 g
Two (16:8 or 18:6)	49 to 70 g
One (OMAD)	98 to 140 g

On OMAD the whole amount lands on one plate. The one-meal trial also found people were hungrier when they ate once a day. Our guide to OMAD intermittent fasting covers how to plan that meal and who should skip it.

Is it better to skip breakfast or dinner?

There is limited evidence either way. One 4-day study had 11 adults with overweight eat between 8 am and 2 pm. Over 24 hours, their blood sugar averaged about 0.2 mmol/L (4 mg/dL) lower. The comparison was eating between 8 am and 8 pm. That is early evidence, from a small, very short study. The 12-week trial of a later window, noon to 8 pm, found no extra weight loss.

So the best time for intermittent fasting is mostly the one that fits your day. If dinner with family or friends matters, keep the window in the afternoon and evening. If you are hungriest in the morning, an earlier window may be easier to keep.

Which intermittent fasting schedule is best for you?

The schedule chooser asks six questions about your routine and how long you already go without food. It suggests one schedule and the eating window that fits it:

• If you have never fasted, it starts you at 12:12.
• If you already go 12 hours overnight, it suggests 14:10.
• If you already fast 14 hours or more, it suggests 16:8, or 18:6 if you eat two meals or fewer.
• It never suggests OMAD, and on nights or rotating shifts it sends you to the shift-work planner.
• If you tick a reason fasting needs medical guidance, such as pregnancy or insulin, it gives no schedule.

We found no trial showing that one schedule is best for diabetes, insulin resistance or autophagy. If you take insulin or sulfonylureas, talk to your doctor before fasting at all, because both can cause low blood sugar. Get medical advice first, too, if you are under 18, pregnant or breastfeeding. Whichever schedule you are weighing up, we advise against all three for anyone underweight or with an eating disorder behind them. We suggest a doctor’s view instead. Any fast over 24 hours also needs a doctor’s view first.

To move up from a shorter window, our guide on how to start intermittent fasting sets out the steps. To try a window now, open the schedule chooser, then send its suggestion straight to the calculator.